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The flame decomposition of some substitutedethyl nitrates
The Flame Decomposition of Some Substituted Ethyl Nitrates J. PawniNG, W. A. W. 5r~nm and J. TrTY'sNp. Explosives Research and D.~velopment Establishment. W a l t h a m A b b e y , Essex (Received d:mtt 1P$91
l)~tailed reaction product concentration ~rofilem have bee)~ obta{ned far the d,eeompoMt~on flames of 2.h!tdro~e?leth!fl nitrate, 2-methox*dethyl nitrate and ~-ethoxyeHi,yl Mtrate nt~)ilized in a simple fiat flame bp~rner. The reactions irt the flames at atrno~pheri~ preasu.re do not proceed to eqttil~brtrtm, and the temperatures (meamtred in 34 mm d~ameter tubes) ju,,t behind the flame frottt~ are 910", ~,900 and 450°G, re~peet~mbd, for the abram esters, the 2-ethoxyethfl ),.¢trate req.uiring about 60"U preheat for stable burnl)~3. ~he rates of bara~nff of the l~quids are .q.~venfor pressare~ u?~ to 1300 to 2000 lb/~n ~. Up to about 800 lblin~ o.methoxyethyl nitrate b~rns faster titan 2.hydroxyethyl ~dtrate, 'in. ~pite oJ a coru~ide~ably I~we~ heat of ez'plonlon and flame front tempera.lure. The primary ~roduet~ of reaction, i.t,. those s~t.mpled where the decomposition of the ariff~nal ,~zter w ~ fi~at complete, can be readily accounted far on the bas~ of the m.eehanism pre~att~ly pat forward far the flame react in,to of ethyl nitrate and the ~ropyl ~.itrate*, The essential feature of th'b~ mee,han.ism ia the ab~ctraetleu of ~ hydration atom from tl~ n,~trie eater by NO~ or I-[ONO. and s.~mpte rearrangement o] tit,: rem~ltin# tl,ydrott~.n. ,fetich'rot eater ntogeeale dlrect:it to the obsemed aldehydle ?~rodaets, The pr~rargrll prod ~cta o] six mononltrnt~ flames a ttd those of two dlnitrates are eompa.ed.
Iatroduetlon W~tER'~S certain aliphatic dinitrates appear ~ to degrade in their decomposition flames in the simple unimolccu!ar fashion exemplified by r~'actions 1 and 2 below, the reaction products of the mononitrates are not easiiy described by a similar mechanism. ONO 2 O-
I
I
CH~--CH--CH--CH 3 ~
~NO: ~
CH3--CH--CH ~CH~
t
f
ONO2 2CH.~CHO+2NO~
CNOz . . . . [1]
ONO2
O•
I
i
CH~--CHf-CHz--CH~ ~
CHf-CH2--CH~--Cii~
I
i
ONO2 ON02 + N() -.--> 2H...CO + C.I-I~:CH~+ 2N(.)= . . . [2] in ,,he above dinitrates high concentrations of ni:r~..,,n dioxide can be observed in a thin zone ~i )he flame front, and the finat products of ~'eaction at atmospheric pressure (not in therrno4yn.trnie equilibrium) are identical with those '~( ihc appropriate nitrogen dioxide-aldehyde(ok'fin) mixture. In an earlier report-" it was st~t:¢l that ~he prt~ducts of the ethyl nitrate
2ol
decomposition flame could be better accounted for if it were ~ s u m e d that nitrogen dioxide attack was not confined to the products of an initial fission of the nitric ester but occurred extensively on the nitric ester itself. It was sugges|ed that the greater part of the degradation of the nitric ester was brought about b y the removal of ~r hydrogen ~tom by NO:, HANG, or CH~-, and that unimolecular fission of the nitrate to a radical and NO.. wa~ only the initiating step. The stationary concentration of nitrogen dioxide in this fl.'mae was nowhere great enough to be observed. The products of th(- flames of normal and i s o p r o p y l nitrate have been dcsctbbcd ~ by a similar mechanism, but an alternative expkmafion, based on the unimolecular degradatiou o~ the esters to alkoxy radicals and NO._,, could not be confidently excluded in this ease. Again, in contrast to the dinitrates, the stationary coneentratioas of nitrogen dioxide were very small The reaction schemes postul:ttcd for these mononitrates m a y be found elsewhere "-'.,~, To examine thc possibilffics of a more extensive application of such a mechanism, the combustion of three further mononitrates of significantly different chemical structure (within the limited range of energy suitable for
202
Vol, 4
J. powling, w . A. W. Smith and J. Thynne
examination in the l a b o r a t o r y ) , h a s n o w been studied. These are 2 - m e t h o x y e t h y l nitrate. CH.~OCH~CI-L_ONO.; 2-ethoxyethyl nitrate, CH~CH,..OCH=CH~ONO~ a n d 2.hydroxy'ethyl nitrate. HOCH...CH=ONO=. Experimental Method
Preparation o] tke nitric esters 2-Hyd~oxyethyl nitrate~This compoond (ethylene glycol mononitmte) was prepared by adding 1.13 mole el ethylene bromohydrin to l mole of silver ~itrate dissolved in acetonitrite. Separatio~ of silver bromide began ahnost at once and the temperature rose in two hours to 4S"C. The mixture ~as cooled to roam temperature and showed no lurther tendency to heat up. Tt w ~ allowed to stand for six days in "the dzrk, .filtered. and the silver bromkle washed wit.h acetonitrile. The combined filtrate and ,washings were distilled at 20 mm of mercury to about half their volume and allowed to stand for three days. Silver tmmtide was again filtered off and wa.~hed: at thls stage the ~otal recovery of silver was 98,4 per cent. The comblamd filtrate and ~vashing~ were evaporated ~t reduced pressure and the residual 2-hydroxyethyl nitrate distilled irt a vacuum. The yield was 9S per cent; b.pt ST.S°C/1 mm Hg. 2-Ethoxycthyl nitrate~98 per cent nitric acid (t17 ml) was cooled below 5'C and 4.5 g urea slowly added. 2.Ethoxvel'har.)l 44S g wa:, added dropwisu over a period of" 20 mint,its with good stirring and cooling, so that the temperature remained below O*C. After-allowing to stand for 10 min at ,-20C, the ni"tratioa mixture was drowned in 1 kg of 501S0 ice/' water and then extracted three tlmes with 75 ml of methylene chloride. The combined extracts were washed twice with 50 ~1 of 3 per cent sodium car, bonate solution and twice with SO ml of w'Lter, The solutlotx was dried overnight with unhvdro~ sodium sulphate and the mettaylene ctfloride ~tistiIled off at atmospheric presstire. The residual 2-ethoxyethvl nitr:tte distilled vff at 63,,5°Ca.nd 1,5 mm I~g (Yield: 87 per cent). 2.Methoxyethyl nltrate~This was preparxl in a som(wh:.tt s~milar manner except that rt ~uiphurie acid-nitrlc acid nitrating mixture was ~sed. and the nitrate was separated, rather thm~ extract,.d with methylene chloride, from the drowned nitration mixture. B.pt S7°C/22 mm Hg.
Flame stabilizalion and probe sampling The flames were studied at a t m o s p h e r i c prcssure, using a flat flame b u r n e r and a simple probe sampling technique s,imilar to that used in the de.composition of the o t h e r nitrates~-,L Steady decomposition flames v e r y close to the (stationary) liquid surface were obtained with all the esters, Difficulties e n c o u n t e r e d were a t e n d e n c y of the 2-methoxyetbyl nitrate to boil at the surface, which m a d e sampting at close distances tedious, a n d the deposition of t a r r y
materials on the burner jacket, which obscured the flame.
Rates of burning The rate o f regression of b u r n i n g liquid surface was m e a s u r e d at elevated pressures in 4.5 m m d i a m e t e r glass tubes, a n d at a t m o s p h e r i c pressure in 24 m m d~ameter ttabcs. T h e rate of b u r n i n g / p r e s s u r e curves for the t h r e e esters are given in Figure !.
2-Hydroxyelhyl n trale 0"SF~ '2
/ n Z-Z- Methoxyethyl
f/ "6
0'3r °'
Smm 0 l"icurc I.
tubes 500
! _._ .... I , 1000 1500 Prossure
1
2000 lb/in z
I¢~d,.. of burnntg o/. Lhe nilmc ,'~l,'¢.~ In ,t'5 J~.:m D~t,v,s
At a t m o s p h e r i c pressure, the 2-ethoxyethyl nitrate required m be boated to a b o u t 60" to 80°C before it would burn steadily (0.007 to 0-009 c r n / s ) . It is interesting that 2-methoxyethyl nitrate b u r n e d almost twice as fast (0,013 cm]~) as ethyl nitrate (0.007 e r a / s ) .r 2-hy:lroxyethyl nitrate {O'O067cm/s), in spite . f a m u c h lower he,Ll of explosion a n d flarrw. front t e m p e r a t u r e . However, at pressures abo~e 200 lb/irf-' and 8¢10 Ill/in'-', respectively, the rates of burning of the httter esters are grealer.
Flame temp,eralure measureme~:~ Flame temperatures were mea.surcd with p l a t i n u m / p l a t l n u m - 1 0 per cent r h o d i u m thermo-
The flame decompositlort of some substituted ethyl nitrates
September 1960
couples, butt-welded from wires of 0.05rum diameter and covered with a thin layer of ~ilica to reduce catalytic action. The liquid sm'face was allowed to burn past the stationary the¢~aocouple, and the e.m.f, recorded on a potegtiometric recorder. Knowledge of the liquid consumpSon rate and of the recorder chart speed gave a reliable temperature distribution, with tlw. :xception that the surface tension of the Ikluid made it impossible to record the teal temperature within --,1 mm from the liquid surface. The temperature distributions are givee in Figure 2. ;?00
I000
2-Melhoxyethyt ,,... nilrat~.s.~ ~'" /
BOO
6012 -
LicNid r e
-
"
2.Ethoa~,ethyt
niira~e..R..._N.~
i
~eCt
Distance
I";::~, 2
moo
l',:~nperalupe ~,'c.vdin;;.~ lhcr~ut.h ~h¢ I~rla:d ntt.,'lc ester flalm'.~
,:lnuly,q,~ o~ combustion products "[t> pr.ducls from the flrunes were drawn (via a fin,:, silica probe situated at a rneasttred (ti.q;lliCt. from the liquid ~urface) through it cold isopc~tl;t[It' trap at -I2I)"C, and the n,,n. c.rl(l~..r~sable fraction c<)lleeted over rnerO~,ry [rtlr;i.rt:d analyses were then performed cm thc gas salnple, arid hydrogen and nitr, igen, which art. tr~lnsparent in tim hafrairt.'d, were estimated ':}lr~'Inat,,grapiiically using ;t 1211('rrl aetiv'ltt:,d ~:;l:tl'<',Jlile.lumn. The culd trap was rmn0Ved aml a~l i.r. a~alysis of the fraction vaporizing h,.twt,on --120"C and room ternpt'r.'tttu'e Was r,hlained. The eondens:ate was tl'lez~ weighed and ,.tivkled into parts as neo:ssary. T:¢p{cal trvatlmen? ¢,f a liquid would h w o l v e :
203
(i)
Infra-red anatygis at 1 2 0 ° C ~ b y this means less volatile products such as It:CO, HCO~H, CH~OH and nitro compounds cou!d be estimated. Water, in amounts encountered, can be estimated only approximately in this way. (it) Water content was estimated by the Karl Fischer method. (iii) The third fraction was titrated for acid, and the aldehyde content estimated by the sodium sulphite method. The products of the combustion were also examined using a Metropolitan Vickers 5IS 3 mass spectrometer, when certain specific components were being sought. llesu~ts The distributions of reaction products in the three nitric ester flames arc shown in l:igures 3 to 5. The number ol moles of reaction product derived from tO0 moles of the nitric ester is plotted against the distance of the sampling probe from the burning liquid ~urface. A mass balance between reactant and products bad to tie made because, with these esters, considerable amounts of unidentified material (of low volatility) were found in the liquid fraction. The atom balances were poor, ~herefore, They indicated the presence of unidentified nitrogout'ms material in the cases of 2-methoxyethyl nitrate and 2-ethoxyethyl nitrate, but not in thai of 2&ydroxyb.thyt nitrate. It is thought that tnost of the polymeric re.~idue is derived from the rather reactive aklehydes, me.thoxyac, qald,.,hyde, vlht,xyaet.ta[dehyde atld .,
204
J. Powllng, W, A. W, Smith and J..Thynne
Vo]. 4 ~"CO. . . . ~'HzO "
G~
/:
+
•7 H~~
~'
/
~
~
""~'._
~
o
.
,
v_ot~lile aldH'qcIe
.
m
,, ,
<4
+
NO/"
m o
AI~ del¢,¢|od(~ 3m<>les throughout) ~1~. HCN, CH~CNO, CH~OH ImL
0
2
6
4
I0
8
~i~r.cc. ,'rcm liquid surbce
rnrn
0+
~lt~ate f~ame
6°F
,++
0
2
4 6 8 ~0 Dir.+,ance from tiquid sutfac~
12 mm
b~+ di~,cu~,'.,~:.d r;v.re b;i~..fl.V hy ar+:d<~: T,
++
~+
I
~
~
~
.
,+~,+,~...... ~'+-----+----~. ", +"/; '+++
:L.Uc:hr, w+~thy[ ~utra~. fh~. ~tv.,j~r ]'~rr~l~(::~ ~,~ very rapid :~eacl;iorl m
I~+,'+'h,~n<,].
'Jh,' ,'d,,~'lc,~
+,rl]v rtnlzl~Jr ,tt,~+,titilh,~
~c
,j
2 II~,~th,+,x,+,',c'th:tn,,~l
,,t ~'tJ~pl~'[~l.:¢l etJlc+r, i~ <.i|
t~ <+ '[h~: ~h~ti;~Im~:~ ~,?~.,p m t h e de~,~r~,
lhc c,f
|hl!
<~ ~N .... b~,:Jd Ir,-dc~i,,~l 3) a n d could },~: .×[.'ch:'d ~ to I., f,,IIowed b y lurth~:r b r e a k ~ l u w n +,i :h, ;t.l~+,,xy radical at tI~c, h:~h It-ml~cr;+ttlre~, /"I;.'ii¢,
,~+
['rodu+~. +l ,t+'l NflOtl. "c.tl tl+:+, Z.,,l'It,.m;¢ll;.+¢.~
+:lrPll+' ~++it+Je
I +{J{|' [+* ~+JJ) ( i i
+rw+*lve+l, ~+cc<,n.lm~.t I+'j
rc'+Jt',t+,,a+
~eptembex 1960
205
The fl&me decomposition, of some substituted ethyl aJLrate.~
CH~0CH+.CH-+ONO= - + CH, OCH+aCH=O- + NO= ..... [:3] CH~0CH=CH=O' --:3" CH~OCH~- + H = C O .
CH~OCH.+' --~ CH~" + H = C O
.
.
.
[4]
.... [5]
The absence of 2-methoxycthanol, and t h e presence of only the small amounts of dimethyl ether, suggests either that the radicals produced in reactions ~ and 4 rapidly degrade before their expected hydrogen-capturlng reactions can take place, or that the main decomposition reaction involves a chain mechanism in which only a few ir, itiating steps are r e q u i r e d in o r d e r to bring about the subsequent breakdown of many nitric ester molecules. T h e r e l a t i v e l y small amOl.lllt ~f methane, an unknown a m o u n t of which is alway-.+ p r o d u c e d f r o m the thermal d,.,',.~tnl~sition of m e t h y l f o r m a t e , a d d s s u p p,,ri tr+ the view t h a t the number of initiating itot'+s ~s r<,Ialively small. Oi p;,rticular interest i.~ the p r o d u c t i o n of hrgt' a n m u n t s of metl,yi f o r m a t e , whictl c a n n o t :~,: ,atisf,tct,~rily aCC<+~llrlted fc+r on the imsis of ::rum,~h'cul;tr dccomI×~sitirm of the nitric ester. h.~ app,.ar~tnce c a l l tmwever, be d e s c r i b e d in ., <:q'& way by a rea<:ti, m ct'+inplt+te~y a n a l o g o u , ',, th.tt pr,>pos~,d :is an i r n [ ~ r t a n t s t e p in {he I,.;r,:l,h~bm *,f etilyl nitrate, |l wat-~ ~,ut;:ie,,ted :h.+: ,ff~vJ n i t r a t e was a ~ t a , : k d by nitrogtm :.!-'~A, + I.)r H O N O <,r C}t+.} with r'o~'~soi;~erl~ ",, ,,{ a |~,}'dr;,zer~ tt+~m, It a.r) >}lydr~r~.y.n *,'r.. r,'w,v,.d, the rm,,:+,um+ ;v,,qh.] b.e stmp+>'
If reactions analogous, to 6 and 7 occur in the decomposition of methoxyethyl nitrate, two different, but characteristic,products would be methoxyacetaldehyd¢ and methyl formate. respectively. NO.+ + CH~OCH=CH~ONO+. -+
[CH~OCH.,CHONO=]
+ HONO
$
CH:,OCH:CHO
.... [8] .... [8~]
+ NOr
N O = + CH.~OCH..CH=ONO= --+ [ C H ~ O C ~ H C H = O N O ~ ] +
HONO
... [9]
I[
l / H° CHa0C[" ":::::0
+, I-I;CO +o.N O ....
l: i, d',ffi<'v]1 ~o d, u e r m i r w t;mv n;uch o,' the r+qlctvm mb.:h~ pr,.>c,:cd I,v reac[iun 8. .~ince mc~h+,xy,.'~,~,d,i,,hy, k • c,,Ifid n,,t he p r e p a r e d h,r reler,:v.c+,. +pt,ctra. a,t+d £A[t!+r+ttit,tL The' ,t+k+,+,:;',.tc~,t,,i,.b by*,]+'. ;+r,~: k:v,wr+'+' t+~ +:. ~+L+'~t+.i!t t'' p[('+iLirt,' ,~Htt k¢N'[, l[L+rd +t'+l ~p,.,ct~';~ l'I.,Yc;ttci:l t]':r' I+rp<,'tlC<" <,t :l.+; tmkw+'+,,n :d{h'h;'d,' in the "+.tp.,1.h+ Ira+trim, .',ll,.J ,t ,tlhi!l ,tmourlt ,+f a ',lli+.L'~Jb~'r' J,~i%'iltU', ;:14",+ ['Jc,tk: +, A'[ 7++ a r i d
[' i ! ( tI~+N+):] .....~- t ' l t . , ( ' I t < + - N < > . . . . .
[6]
". i ,',v,,,+,.r , ~ +q hy, h',+<<'~t ;M,ml v,,-r,, },,,t, th<" +~ ,:::i;: in,heal ',,,++u!ll illlr.P:m~iati,lv ~",'a[Lt;Up' [ , lt,j++,.t' thu,.
t+CH.,ON
','<,~,./" o
'_,tl:t ',.,..X,, . [71
[ga]
7 3 wa.,+
[,r,',.~'zL+ N,¢ .+ cre.+r, dv,d ,+{ :p, cth~,xva fi,ttttd in ~ht' lh}I:id te.lcti,+'l..tlz, l it ix tilOtlKht {h;lt this "e¢,v4pI'OI+&'D1V a p~lt?,+;n,'r ,q' .'l'l~'likt,X~,TWvtd, b,lvhy+.le with SOtlTt! ~'~lI'g'rHIJlt:jC!l%'~lt'. "l'['l{" ~llott~+t4t.'r WaS riot rege[+10r .ttt'd wht'Ii l:lt',t~'{'~ to 120 't" ill the m f r a - r e d hot el:It. If t h e tttlktlow~'l ahl~'hydc a n d tile p o l y m e r ,ire assl:II~
tl~
nmfl~,+xyacetaldchyde.
their m o r e t h a n 40 mole per c~'nt o f the n h r a t e decomlx~ses to thi:, s u b s t a n c e . More tha~. :I0 mole lx~r cent of m e t h y l f o n n a t e w ~ f o u n d , and
206
J. PowEng, w. A. w. Smith and J. Thynne
probably some had already pyrolysed. Thermal decomposition of this material in a silica vessel at 600°C showed that it produced, very approximately, 90 per cent (CO+CHoOH) and 10 per cent ICO:+CH~). This reaction, and the pyrolysis of formaldehyde, could account for the observed hydrogen, methane, carbon monoxide ~md methanol. Water would result from reactions of HONO produced in reactions 8 and 9. For example, HONO+ CH~OCH:,CH:ONO: ~ + NO + [CH.~0d,.,HaON0:]
.... [10]
CH~OCH.CH,..0NO.., ---+ CH,0CH.~CI-I~O. NO: (initiation) . . . . [3] CH~OCH,CH,,O- --e" CI't.~+ 211.CO . . . . [4 and 5] (~H..,+ CH.~OCH..CH:ONO.. ~ . CH, ÷
+
[CH~0EJ.LON0=] ~
[~ I]
. . . .
HONO
-..[CH~O~.:HsONO:
]
.... IS]
[I0]
....
= [ C H . O C H dHON(~ 1 ._L>.'CH.OCH 2 CHO$NO' " ~='[8a 2 . . . .
[CH~Od,H.,ONO.,]
[CH~OC.H~ONO.,] = [ C H ~ O C H C H , , O N O -->- L # i , , o c F I o ,
H:co
+
NO
"
....
No adequate check is possible however, because, even in the earliest samples taken, some oxidation and pyrolysis of aldehydes had occurred and part of the nitric oxide had been reduced.
This ester hz: a less favourable oxygen balance than the 2-methoxyethyl nitrate, but a stable flame can be obtained at atmospheric pressure by preheating. The flame temperature at the point of complete disappearance of the nitrate is not m~ch different from that of the 2-methoxyethyl nitrate if the minimum amount of preheat for stable burning is given. The products of greatest interest from this ester are formaldehyde, ethyl formate and acetaldehyde. The amount of carbon monoxide is much smaller than usual and indicates that not much pyrolysis of aldehydes has occurred. No ethoxyethanol, methylethyl ether or nitrogen dioxide could be detected, and less than 0','; mole of ethane per 100 moles of nitric ester wa, present Sigmifieant amounts (~-~20 mote per cent) of a high-boiling nitro compound were found. Its mass spectrum w ~ consistent with the fraknnents expected from CH:,CH~OCH.,NO.,. but this couk! not b,:, prepared for comparison. InJtJatlon is again t'lken to be fission of the weakest bond Cl taCH,:OC H ..C [ l..ONO.. --)- CH:,CI'I._.Of~H...CI:I:O..+NO..,
. . . . [l:q
"flit: ab~erice of 2-ethoxycthanol suggests that
HON'O+CH:,OCH:CH~ON0: ---~ H=(} + N O + [CH~0d:H.0NO...]
CH~OCH,.,CH=ONO= - - ~ 0' ICH~ + 0"gH~CO + 0"6CH.~OCHO + 0'gCH~OCH.~CHO + 0-4H..O + 1.0NO . . . . [12]
2-EJhoxyetkyl nitrate
H..O
A small amount of a nitro compoui/ of low volatility was estimated by assuming the same extinction coefficient at 6.80 microns as fcr nitromethane, its infra-red spectrum and its mass spectrum were compatible with the structure CHaOCH:NO., but this material was not successfully prepared, so no positive identification could be made. The postulated reaction scheme for 2-methoxyethyl nitrate, which wilt describe the appearance of all the observed reaction products, and which is completely analogous to that suggested for ethyl nitrate, is summarized below'.
NO: + CHo0CH.CI~:0NO~
Vol. 4
l ~"[~a]
An overall equation for the initial stages of the reaction according to the above scheme, with a chain length ~f ten, would be
the atk~xy radical breaks down rapidly C H.,C I f/K~} I..Ctl .O"'-" CIlaCI-/:O(~,H= + I-I~CO . . . . [1.1] The absence of rnethylefl~yl ether and the tow concentrations of etliane and ethylene would be evidence to suggest that, in fact, no large proportion of the nitric ester decomposed according to r,aetion 13 because none of the expected products of the resulting alkoxy radical would have been prcdnced:
~eptembea"1960
The t'Lamc decomposition o.f some substituted ethyl nitrat.es
CH,~CH=OCH.,+ R H - + CH.~CHzOCH~ + R" . . . .
Os]
'~ll~7
The radical CH~f~HOCH=CH,ONO.,, producecl in reaction 19, could rearrange, by"way of either of the two forms below, to give acetaldehyde, formaldehyde and nitric oxide:
CH~CH,.OCH= -'->" CH~CHz"+ H:CO ....[16] However, if the nitro compound is indeed [CH,,dHOCH.:CH~ONOa] -->- CH:,CHO + tCH:--CH2 CHaCH.,OCH=NO=, then it is possible that it w.~sformed by combination of CH~CH..OCH=. 0.. x 0 and NO,, although this reaction would have to be very much faster than the hydrogenI ii .~bstracting reaction 16 since the concentration H-,C--->C 0 0 of both CH..CH.0CH.," and NO,, must be vcry sm,~ll, whcreas ' R H ' is always large. The O CH2 amount of stable hydrocarbon from the alkoxy ~/\ / .NO+ 2 H..,CO radical (reaction 16) cannot in this case bc taken o~N'-.....Of~ CH2 . , , [1 ,ga] as some guide to the reaction chain length. On the other hand, the nitro compound may well have t,e.en produced by a blmoleeular reaction 17, lwl,.,w, which is not a chain-terminating CHaCHO + 2H=CO + NO . . [19b] process. The other hydrogen-stripping reaction (20 N(L:.!.C }| ~C 1-|:f.}CH ..C H :O NO: betow) would produce ethoxyacetaldehyde. --->- CI I~,CI.L.,OCH..NO..,+ H~CO + NO,., . . . [17] The ]~trge amount of polymeric suhstance in the The clhy] fommtc is produced by rc,'mtkms liquid fractkm might well be dt~e to this unstable analogous to 9 and 7: aldehyde, but no positive infra-red evidence could be obtained for this suggestion, because NO..,+ C I I.,C H...OC H C H _ONO.z of the interfenmce of acetaldehyde, formaldehyde, ethyl f,,rmate and water, tlONO+ [CIrI:,CH=O(~I[ICH'~ONO:,] . . [18] NO,+ CH:jCH._,OCH~CH..ONO., ~ HON0
1
/
/
\
\
I
+ [CI-(,CI-I=OCH:CHONO:] [20]
-q
C'H.,C H=OCH..CHO + NC).
0=~N
I °
I C l INC..H,.OCHO + t I.,C.O + NO . . [ 18a] 'l'w,~ r~ther ilydrog('.'n-strippilag reactions would
appear t, be eqttally pr(~bal)le in this ester. Ore, ,f them could account for the acetakh,hyde obsor'v~.~d in large amotmts NO,, + CH.~CH..OCH.,CH.X)NO= ~> [CH~(;HOCH.~CH.,ONO=] + H O N O
. . [19]
[20a]
A reaction scheme closely similar to that for the 2-mcthoxyethyl 16irate, bu~ with the additi,~al reaction,~ ~o :mcotmt for the acetaldehwtc (reaction t9) and the nitro compound (e.g. reaction 17), will :.tccmnlt fl,r the main Menl.iticd pr.,:iucts. The large :unoup.ts of tlnkaowtt snb,~t0.r:tcc [rottl this more complex estt, r (see Figm.~'~ 4) npsc.t attempts t . describe the reacti,n rnorc fully. [k'vund the flame front, it would s,.,cm that, in t'bis cool-burning ester, polymerization of aldehydic products is faster than their pyrolysk~,. In Figure 4 it may I,e seen that the cn~'es of the Mdehydes and of the unknown polymeric matcriM are inversdy feinted to each other. The nitro compound deeomp~ses steadily in die p.rcduet gases.
2o8
J, Fowling. W. A. W. Smith and J. Thynne
2-Hydroxyethyl ni$rate The decomposition products of this ester are all simple molecules and do not provide much evidence about the course of the flame degradation unless examined in relation to the other nitrates, when abnormally large amounts of formic acid, hydrogen, carbon monoxide, water and carbon dioxide are seen to be produced {Figure 5). No methanol or glycol is observed, and, although the amount of hydrogen is larger than commonly observed in the flame front of similar nitric esters, the trend of the hydrogen curve in Figure $ indicates that its initial concentration (when the decomposition of the nitric ester is just complete) is much smaller. The amount of 2-hydroxyethyl nitrate consumed according to reaction 21 (plus reactions 22 and 23) is again probably not large: HOCH~CH--ONO~ ~
ttOCH_~CH:O.+ NO. ....
~m]
HOCH~CH--O" --->. HOCH--'+ H--CO . . . . [.02] HOCH.. ~
H- + H,CO . . . .
[~]
It is suggested that the observations arc better explained by a mechanism similar to that of the other menonitrates, with reaction 21 being the iai;:'.'ating step in a chain process, the main reactions being 24 and probably 25 below: NO.. + HOCH~CI-I~ONO~ --9- HONO + [HOCHCH,,ONO:] .../I H OjCH - - C H2 , \ C..
0
k_
~ - N O . . . [~-4al I-l~O+ CO or H~ + C()--
NO~ + HOCH--CH.,ONO~ ~
HONe
+ [HOCH.~C2HONO--] HOCH~,CHO+ NO~
the trap, and of the rapidity of the pyrolysis of the aldehydes in this flame. The 'unknown' in Figure 5 has been given the molecular weight of glycollie aldehyde. Synthetic samples of glycollie aldehyde, both as the anhydrous solid and as the hydrated form, were examined spectroscopically. It was considered that the latter was the form in which it was most likely to occur in the liquid products, but this, when heated to 120°C, gave a negligible vapour-phase i.r. spectrum. Its spectrum as a thin film was compared with that of the liquid products, but the latter consisted of broad featureless bands due to the mixture of water, formaldehyde and formic acid, which were also present in the samples in which the unknown occurred. Thus, no positive inference as to the presence or absence of the aldehyde in the products could be made. HNCO is an interesting and unusual minor product of this particular flame, but there are no clues regarding its mode of formation. The reaction scheme for this ester would be summarized as follows: HOCH--CH..,ONO.~ - - + H, + 2H.,CO + NO~ . . . . [2 t, 22 and 23] H.+ HOCH:CH~ONO 2 - . + l'l~ + [HO(~..,H~ONO=]
....
[261
NO= + HOCH--CH--ONO,_, .-->. HONe + [HOC--H~ONO..] [24 or 26] H O N e + I'IOCH~:CH~ONO~ --->. ['[--O + N O 4- [HOC:rLONO~]... ["7]
[-l(:(')~')f-l-t I..,CO
"~,'N. / II 0
{24]
Vet. 4
....
[2S]
. . , . [2Sa]
Agai~L the relative exteut of reaction 25 could not be estimated because of the difficulty of analysing for polymerized glycollic aldehyde in
[HO(:,:I-~oONL)...] ~ HC()OH + H...CO+ N() "~ . . . . [24a] IIOCII~CI-IO+ N(.I~ . . [25a]
Reactions brhin,.! .q'e tt',,n.e ]rents In all the tlames exaa~ined, relatively slow reactions fol!ow in the wake of the very fast reactions occurring in the flame front. The former are h;rgely aldehyde pyrolyses, and do not result ir~ any further l~c;'~t rclcase, un]e.~, nitric oxide becomes involved in the reaction and is reduced. This reduction of nitric oxide is an important feature in relation to the rate of energy release in nitric ester combustion, and has been discussed eIsewhere~.*. I:a a!l nitric
~lztember 1960
The flltme decomposition of some substituted ethyl nitrates
ester flames in which formaldehyde is observed to be decomposing, nitric oxide is reduced at a rate which seems (from the concentration carves) to be related to the rate of aldehyde pyrolysis. In the 2-hydroxyethyl nitrate the decomposition of formaldehyde is rapid, and the asmciatcd reduction of nitric oxide stops at abou~ the point when the aldehyde has disappeared. The continued temperature rise resulting from this nitric oxide redaction is seen as at " :,;vd!qg c;ff' of the temperature profile io Figure 2. In the 2-merhoxyethyl nitrate flame, the temperature is lower and the aldehyde pyrolyses arc sMwer. Nitric oxide is reduced continuously. ttxenitri,:: oxide curves and those of the formal&hydv and methyl formate being roughly parallel over the range covered (Figure 3). The flame temperature continues to rise fairly steepfy beyond the flame front (Figure 2). The ~itt,.~tiorlwith regard to nitric oxide reduction in ue 2-ethoxyethyl nitrate is complicated by the ,~arly production and subsequent slow decoml~Jsiti,.m of significant amounts of a nitro c0mpm,nd: nitric oxide is produced more rapidly tha~l it is decomposed to nitrogen. Aldehyde pymlyses, as judged by the production of carbon :lom~xide, are not rapid in the cool-burning e,ter, but the increase in the amount of unknown eaterraI (tarry residue) with distance from the iI',une fr-at (Figure 4} indicates extensive oulymerizati.r~ of the "aldehydes No temper}.ture rise ,,tours beyond tile ttame from in this ester. "/he l,r,,pyl nitrates, burning with simil~Lr flame l"mp,,r,,ture, produce I;t:ger amounts . f more
<1 cemparison o[ the initial ,~roducts [ ~ m u ~l~*mt.:.,. el *dtric ester flames ifieee ,:<(t'.nsion of: this analytical :4udy el the
209
effect o£ chemical structure on degradation mechanisms in nitric ester flames is not coxxtemplated at the present time, it was considered useful to summarize and compare briefly the results for the eight esters examined. The primary reaction products (i.e. the products at the point in the flame at which virtually all the original nitric ester has disappeared) from these esters arc listed in Table I. Some of the values for the le~,s volatile com. ponents should be regarded as senfiquantitative because the complexity of the mixtures in wb.ieh they were encountered would have made a better estimate unprolitably time-consuming. It was hoped that at least a semiquantitative answer for all the significant stable reaction products of every ester coukl be obtained, and it is considered that this has been achieved in ,all cases except tile three more difficult esters reported in tile present paper. The aldehyde mlxtur~s produced ill these alkuxy nitrates gave polymeric n'laterhds not readily anMysed and sometimes representing a large proportkm of the products. It is considered that the divers products from all these mononltrates, in contrast to the two dinitrates studied, can bc more reasonably and rn,.,re cornpletely described by a degradation mechanism which does not rerll~ire extensive nnlrnMecular breakdown of the nitric ester i n t o alkoxy radical and nitrogen dioxide, but which depends upon hydrogen abstraction from tile eslers by radicals and reactive species, particularly NO. and FIt)NO. Such a mecltanism prvsePts n,~ ditlictlhy in dv~cribitLg th,.' appeari,m'e ,,[ tile matj,.~r pr'tuhlcls fr~.n 'dl the :n,~;a,,:. him.des ,'xamint'd. [t i-~ r,,m, ditfic)ll~ 1¢) lliider. >land why ~'lil[rogtHi ,li,,xidv ;rod radicM at{ael: ,,a huta.,.,.Z,zi-,lit,1 clhli{I,atv ~ is n~q as rapid rt:~ ill tllt' lu¢m,mitrat,:.s. hi flit. lll,lnt,ll[l?-o.[u IIdltl(.s ill a.tt'aospheric [ID'N:-;IID.'. II,, t'VJ~II'P.C~ hits t~t,q.,tl f,:,lltlet l,.r the pr.dttctioxl el alkoxy radicals, ials,,far ',is the
aplw*,pri;~te :.flc-h,d has not h,'l, ql ob~,crvvd, [t ~. p r , hA~Io th;tt sm:h atk.xy radicals :is ;ire pr,.hmt,d i. th,' itlilial.ing steps ,m, l,r~fl.:en down t,~ Mky] radical ~md :,ldehs'de ,it the high t.vlnper;ttttrv involve, P, Wtle l.w pressure ethyl nitrate ll0me (h'mpvrattu'e ab-ut 860°C) has, tlowevcr, receiil[y been sl'u)wn ~ to yield lip to
~J
I~ " ~ =
~
I I~'~V~==
I I
~
I~ A
- -
"~
b
I
~
B~
September 1960
The
flame decomlx~sition
o! some
IS mole per cent ethanol. Alkyl radicals and hydrogen atoms, produced either from the alko~.y mdicaI from the original ester or b y radical attack of early reaction products s,aeh as aldehydes, are also expected to abstract a hydrogen atom from the nitric ester~ but, according to the reaction scheme suggested here, none of these are regenerated when the hydrogen-deficient nitric ester radical decomposes, the only reactive species produced being ~0.. It is not considered that reactions of the type R- + NO., - - ~ RO. + NO
"x
RNO:
take place to any major extent for the following reasons. ROH, expected to be prodnced by hydrogen abstraction by R()., is rarely a significant product. The concentration~ of Ra~d NO., arc very smaU, and although the activation energy for reaction may be sufficiently low to m.'.tke this a ~ssible competitor wb.h R' + RONO_, -->- RH, etc., and NO: + R'ONO~ -->. HONO, etc., for which the activation energy may be 7 to 10kcal and 15 to 20kcal rcs~,ctively', the products of other equ,-dly probable reactions (e.g. 2R.-->- products, R. + NO ---+ products} are not generally observed. Where the NO,.. concentration is known Io be large, and CH:,CHO is pyrolysing tO C]'t+ a r i d
CO
(the
butane-2,g-diol
dinitrate
.~yst~'ml, no Ct't~N(),, or C14~,()I-[ (from CHaO. reacfi,r;s) is observed. In fact, it has not been l~ssi~,le to detect any significant amount.¢ of ,ihro ceni~pound, rlitriie.~ or hydrogen cyanide hi ~[a.m.:sof nitrogen dioxide ar,d numerous fuels whh:l; ;,~ight be expcctcd lo produce rnctl'tyl ra¢tic,ils iii a similar l:enll×,xature range:'. There Js much evidci~ce for the oecurrcm'c of these reacti,m~ at lower temperatures~, Significant qtm.i~:ilh,s o[ nitro compounds arc produced only
substituted
ethyl
2.11
nitrates
in the two cool-burning nitric esters. It is of interest to take nitro compound formation as an example, and c~mpare the behaviour o! 2-methoxyethyl nitrate and isopropyl nitrate. If it were considered that unimoleeular breakdown of the nitric esters was ~. major reaction in flames, and that the alkoxy radicals degraded to aldehyde and simple atkyl radical (see P. GRAYand A. WILLIa~S '~ a ad A, F, TROT,~ta~Dicst~tcsor~", p 806), the products of these two esters might be expee+ed to be similar, apart from the aldehvde produced: CHaOCH,_,CH,,ONO_~ ~ CH:,. + 2H~CO + NO,_,
....
[26]
(CHa),CHONO., ~ : - CI'[.¢ + CH,,~CHO + NO,,
....
[27]
No nitromcthane is found in the products of the 2-methoxyethyl nitrate flame, and o~:ly minor quantities of other nitro compounds and ifitritcs, whereas the yields in isopropyl nitrate are very high.
The authors are grateful ~o Mr ]. V. (~riffi.ths [or .preparing the 2-hydroxyett~yl nitrate and the 2-¢thoxyethyl nitrate+ ~,e.~ct'el'Ice,5 i P o w r , h~C;, J. ".tnd S',.~rrr[. \V, A. VV. Combrtst~on ~,, Fl,.o~h", 1988, ;2, 187 = N~.zr~vl.ca~r. D. P, a n d P o w t , i,~c;, J . Prcc. Roy, Sac, ,'1, 1985, 232, 337
Flare,.', 1987, 1, 3{)8 '~ GRA',', [L 7 rtot,~, Faraday Noc, I955, 5.1, 13H7 :' l~p~ab:l,:, S . [,., I)tti:tatJ,, t[. M., JAct'mg, T. L.. '1 Iill)lU',~'ll.~, []. "l'. Oth;[ ,'~l~NNIt lINES, t:~'. ~]..]', A " , ' r . ch,'m. S - c . l,q:¢,q, 60. 73 +~ AI,~I;.N, ."~. I~, :tll~]. ["OWI.ING, J. C~,mb;t::tion ¢:.+ F.tamc+ 193~, Z, 55 ~ l l i c ~ ; s . . ] . A. E . N . L L h : , , \V<,hham ..\l,b.,.y, l,L,,s~,:,: 4't¢i hi' puldi~hl'l.l) rt "I'It(I.I..,II.,IN. IIIi:K1-NS(~.~, ~. F. (;,':S /+.'¢llr.',tf+".,. :[JtLt|.t'r" x~¢,r( h'.~: Ltm~ l
l)~tailed reaction product concentration ~rofilem have bee)~ obta{ned far the d,eeompoMt~on flames of 2.h!tdro~e?leth!fl nitrate, 2-methox*dethyl nitrate and ~-ethoxyeHi,yl Mtrate nt~)ilized in a simple fiat flame bp~rner. The reactions irt the flames at atrno~pheri~ preasu.re do not proceed to eqttil~brtrtm, and the temperatures (meamtred in 34 mm d~ameter tubes) ju,,t behind the flame frottt~ are 910", ~,900 and 450°G, re~peet~mbd, for the abram esters, the 2-ethoxyethfl ),.¢trate req.uiring about 60"U preheat for stable burnl)~3. ~he rates of bara~nff of the l~quids are .q.~venfor pressare~ u?~ to 1300 to 2000 lb/~n ~. Up to about 800 lblin~ o.methoxyethyl nitrate b~rns faster titan 2.hydroxyethyl ~dtrate, 'in. ~pite oJ a coru~ide~ably I~we~ heat of ez'plonlon and flame front tempera.lure. The primary ~roduet~ of reaction, i.t,. those s~t.mpled where the decomposition of the ariff~nal ,~zter w ~ fi~at complete, can be readily accounted far on the bas~ of the m.eehanism pre~att~ly pat forward far the flame react in,to of ethyl nitrate and the ~ropyl ~.itrate*, The essential feature of th'b~ mee,han.ism ia the ab~ctraetleu of ~ hydration atom from tl~ n,~trie eater by NO~ or I-[ONO. and s.~mpte rearrangement o] tit,: rem~ltin# tl,ydrott~.n. ,fetich'rot eater ntogeeale dlrect:it to the obsemed aldehydle ?~rodaets, The pr~rargrll prod ~cta o] six mononltrnt~ flames a ttd those of two dlnitrates are eompa.ed.
Iatroduetlon W~tER'~S certain aliphatic dinitrates appear ~ to degrade in their decomposition flames in the simple unimolccu!ar fashion exemplified by r~'actions 1 and 2 below, the reaction products of the mononitrates are not easiiy described by a similar mechanism. ONO 2 O-
I
I
CH~--CH--CH--CH 3 ~
~NO: ~
CH3--CH--CH ~CH~
t
f
ONO2 2CH.~CHO+2NO~
CNOz . . . . [1]
ONO2
O•
I
i
CH~--CHf-CHz--CH~ ~
CHf-CH2--CH~--Cii~
I
i
ONO2 ON02 + N() -.--> 2H...CO + C.I-I~:CH~+ 2N(.)= . . . [2] in ,,he above dinitrates high concentrations of ni:r~..,,n dioxide can be observed in a thin zone ~i )he flame front, and the finat products of ~'eaction at atmospheric pressure (not in therrno4yn.trnie equilibrium) are identical with those '~( ihc appropriate nitrogen dioxide-aldehyde(ok'fin) mixture. In an earlier report-" it was st~t:¢l that ~he prt~ducts of the ethyl nitrate
2ol
decomposition flame could be better accounted for if it were ~ s u m e d that nitrogen dioxide attack was not confined to the products of an initial fission of the nitric ester but occurred extensively on the nitric ester itself. It was sugges|ed that the greater part of the degradation of the nitric ester was brought about b y the removal of ~r hydrogen ~tom by NO:, HANG, or CH~-, and that unimolecular fission of the nitrate to a radical and NO.. wa~ only the initiating step. The stationary concentration of nitrogen dioxide in this fl.'mae was nowhere great enough to be observed. The products of th(- flames of normal and i s o p r o p y l nitrate have been dcsctbbcd ~ by a similar mechanism, but an alternative expkmafion, based on the unimolecular degradatiou o~ the esters to alkoxy radicals and NO._,, could not be confidently excluded in this ease. Again, in contrast to the dinitrates, the stationary coneentratioas of nitrogen dioxide were very small The reaction schemes postul:ttcd for these mononitrates m a y be found elsewhere "-'.,~, To examine thc possibilffics of a more extensive application of such a mechanism, the combustion of three further mononitrates of significantly different chemical structure (within the limited range of energy suitable for
202
Vol, 4
J. powling, w . A. W. Smith and J. Thynne
examination in the l a b o r a t o r y ) , h a s n o w been studied. These are 2 - m e t h o x y e t h y l nitrate. CH.~OCH~CI-L_ONO.; 2-ethoxyethyl nitrate, CH~CH,..OCH=CH~ONO~ a n d 2.hydroxy'ethyl nitrate. HOCH...CH=ONO=. Experimental Method
Preparation o] tke nitric esters 2-Hyd~oxyethyl nitrate~This compoond (ethylene glycol mononitmte) was prepared by adding 1.13 mole el ethylene bromohydrin to l mole of silver ~itrate dissolved in acetonitrite. Separatio~ of silver bromide began ahnost at once and the temperature rose in two hours to 4S"C. The mixture ~as cooled to roam temperature and showed no lurther tendency to heat up. Tt w ~ allowed to stand for six days in "the dzrk, .filtered. and the silver bromkle washed wit.h acetonitrile. The combined filtrate and ,washings were distilled at 20 mm of mercury to about half their volume and allowed to stand for three days. Silver tmmtide was again filtered off and wa.~hed: at thls stage the ~otal recovery of silver was 98,4 per cent. The comblamd filtrate and ~vashing~ were evaporated ~t reduced pressure and the residual 2-hydroxyethyl nitrate distilled irt a vacuum. The yield was 9S per cent; b.pt ST.S°C/1 mm Hg. 2-Ethoxycthyl nitrate~98 per cent nitric acid (t17 ml) was cooled below 5'C and 4.5 g urea slowly added. 2.Ethoxvel'har.)l 44S g wa:, added dropwisu over a period of" 20 mint,its with good stirring and cooling, so that the temperature remained below O*C. After-allowing to stand for 10 min at ,-20C, the ni"tratioa mixture was drowned in 1 kg of 501S0 ice/' water and then extracted three tlmes with 75 ml of methylene chloride. The combined extracts were washed twice with 50 ~1 of 3 per cent sodium car, bonate solution and twice with SO ml of w'Lter, The solutlotx was dried overnight with unhvdro~ sodium sulphate and the mettaylene ctfloride ~tistiIled off at atmospheric presstire. The residual 2-ethoxyethvl nitr:tte distilled vff at 63,,5°Ca.nd 1,5 mm I~g (Yield: 87 per cent). 2.Methoxyethyl nltrate~This was preparxl in a som(wh:.tt s~milar manner except that rt ~uiphurie acid-nitrlc acid nitrating mixture was ~sed. and the nitrate was separated, rather thm~ extract,.d with methylene chloride, from the drowned nitration mixture. B.pt S7°C/22 mm Hg.
Flame stabilizalion and probe sampling The flames were studied at a t m o s p h e r i c prcssure, using a flat flame b u r n e r and a simple probe sampling technique s,imilar to that used in the de.composition of the o t h e r nitrates~-,L Steady decomposition flames v e r y close to the (stationary) liquid surface were obtained with all the esters, Difficulties e n c o u n t e r e d were a t e n d e n c y of the 2-methoxyetbyl nitrate to boil at the surface, which m a d e sampting at close distances tedious, a n d the deposition of t a r r y
materials on the burner jacket, which obscured the flame.
Rates of burning The rate o f regression of b u r n i n g liquid surface was m e a s u r e d at elevated pressures in 4.5 m m d i a m e t e r glass tubes, a n d at a t m o s p h e r i c pressure in 24 m m d~ameter ttabcs. T h e rate of b u r n i n g / p r e s s u r e curves for the t h r e e esters are given in Figure !.
2-Hydroxyelhyl n trale 0"SF~ '2
/ n Z-Z- Methoxyethyl
f/ "6
0'3r °'
Smm 0 l"icurc I.
tubes 500
! _._ .... I , 1000 1500 Prossure
1
2000 lb/in z
I¢~d,.. of burnntg o/. Lhe nilmc ,'~l,'¢.~ In ,t'5 J~.:m D~t,v,s
At a t m o s p h e r i c pressure, the 2-ethoxyethyl nitrate required m be boated to a b o u t 60" to 80°C before it would burn steadily (0.007 to 0-009 c r n / s ) . It is interesting that 2-methoxyethyl nitrate b u r n e d almost twice as fast (0,013 cm]~) as ethyl nitrate (0.007 e r a / s ) .r 2-hy:lroxyethyl nitrate {O'O067cm/s), in spite . f a m u c h lower he,Ll of explosion a n d flarrw. front t e m p e r a t u r e . However, at pressures abo~e 200 lb/irf-' and 8¢10 Ill/in'-', respectively, the rates of burning of the httter esters are grealer.
Flame temp,eralure measureme~:~ Flame temperatures were mea.surcd with p l a t i n u m / p l a t l n u m - 1 0 per cent r h o d i u m thermo-
The flame decompositlort of some substituted ethyl nitrates
September 1960
couples, butt-welded from wires of 0.05rum diameter and covered with a thin layer of ~ilica to reduce catalytic action. The liquid sm'face was allowed to burn past the stationary the¢~aocouple, and the e.m.f, recorded on a potegtiometric recorder. Knowledge of the liquid consumpSon rate and of the recorder chart speed gave a reliable temperature distribution, with tlw. :xception that the surface tension of the Ikluid made it impossible to record the teal temperature within --,1 mm from the liquid surface. The temperature distributions are givee in Figure 2. ;?00
I000
2-Melhoxyethyt ,,... nilrat~.s.~ ~'" /
BOO
6012 -
LicNid r e
-
"
2.Ethoa~,ethyt
niira~e..R..._N.~
i
~eCt
Distance
I";::~, 2
moo
l',:~nperalupe ~,'c.vdin;;.~ lhcr~ut.h ~h¢ I~rla:d ntt.,'lc ester flalm'.~
,:lnuly,q,~ o~ combustion products "[t> pr.ducls from the flrunes were drawn (via a fin,:, silica probe situated at a rneasttred (ti.q;lliCt. from the liquid ~urface) through it cold isopc~tl;t[It' trap at -I2I)"C, and the n,,n. c.rl(l~..r~sable fraction c<)lleeted over rnerO~,ry [rtlr;i.rt:d analyses were then performed cm thc gas salnple, arid hydrogen and nitr, igen, which art. tr~lnsparent in tim hafrairt.'d, were estimated ':}lr~'Inat,,grapiiically using ;t 1211('rrl aetiv'ltt:,d ~:;l:tl'<',Jlile.lumn. The culd trap was rmn0Ved aml a~l i.r. a~alysis of the fraction vaporizing h,.twt,on --120"C and room ternpt'r.'tttu'e Was r,hlained. The eondens:ate was tl'lez~ weighed and ,.tivkled into parts as neo:ssary. T:¢p{cal trvatlmen? ¢,f a liquid would h w o l v e :
203
(i)
Infra-red anatygis at 1 2 0 ° C ~ b y this means less volatile products such as It:CO, HCO~H, CH~OH and nitro compounds cou!d be estimated. Water, in amounts encountered, can be estimated only approximately in this way. (it) Water content was estimated by the Karl Fischer method. (iii) The third fraction was titrated for acid, and the aldehyde content estimated by the sodium sulphite method. The products of the combustion were also examined using a Metropolitan Vickers 5IS 3 mass spectrometer, when certain specific components were being sought. llesu~ts The distributions of reaction products in the three nitric ester flames arc shown in l:igures 3 to 5. The number ol moles of reaction product derived from tO0 moles of the nitric ester is plotted against the distance of the sampling probe from the burning liquid ~urface. A mass balance between reactant and products bad to tie made because, with these esters, considerable amounts of unidentified material (of low volatility) were found in the liquid fraction. The atom balances were poor, ~herefore, They indicated the presence of unidentified nitrogout'ms material in the cases of 2-methoxyethyl nitrate and 2-ethoxyethyl nitrate, but not in thai of 2&ydroxyb.thyt nitrate. It is thought that tnost of the polymeric re.~idue is derived from the rather reactive aklehydes, me.thoxyac, qald,.,hyde, vlht,xyaet.ta[dehyde atld .,
204
J. Powllng, W, A. W, Smith and J..Thynne
Vo]. 4 ~"CO. . . . ~'HzO "
G~
/:
+
•7 H~~
~'
/
~
~
""~'._
~
o
.
,
v_ot~lile aldH'qcIe
.
m
,, ,
<4
+
NO/"
m o
AI~ del¢,¢|od(~ 3m<>les throughout) ~1~. HCN, CH~CNO, CH~OH ImL
0
2
6
4
I0
8
~i~r.cc. ,'rcm liquid surbce
rnrn
0+
~lt~ate f~ame
6°F
,++
0
2
4 6 8 ~0 Dir.+,ance from tiquid sutfac~
12 mm
b~+ di~,cu~,'.,~:.d r;v.re b;i~..fl.V hy ar+:d<~: T,
++
~+
I
~
~
~
.
,+~,+,~...... ~'+-----+----~. ", +"/; '+++
:L.Uc:hr, w+~thy[ ~utra~. fh~. ~tv.,j~r ]'~rr~l~(::~ ~,~ very rapid :~eacl;iorl m
I~+,'+'h,~n<,].
'Jh,' ,'d,,~'lc,~
+,rl]v rtnlzl~Jr ,tt,~+,titilh,~
~c
,j
2 II~,~th,+,x,+,',c'th:tn,,~l
,,t ~'tJ~pl~'[~l.:¢l etJlc+r, i~ <.i|
t~ <+ '[h~: ~h~ti;~Im~:~ ~,?~.,p m t h e de~,~r~,
lhc c,f
|hl!
<~ ~N .... b~,:Jd Ir,-dc~i,,~l 3) a n d could },~: .×[.'ch:'d ~ to I., f,,IIowed b y lurth~:r b r e a k ~ l u w n +,i :h, ;t.l~+,,xy radical at tI~c, h:~h It-ml~cr;+ttlre~, /"I;.'ii¢,
,~+
['rodu+~. +l ,t+'l NflOtl. "c.tl tl+:+, Z.,,l'It,.m;¢ll;.+¢.~
+:lrPll+' ~++it+Je
I +{J{|' [+* ~+JJ) ( i i
+rw+*lve+l, ~+cc<,n.lm~.t I+'j
rc'+Jt',t+,,a+
~eptembex 1960
205
The fl&me decomposition, of some substituted ethyl aJLrate.~
CH~0CH+.CH-+ONO= - + CH, OCH+aCH=O- + NO= ..... [:3] CH~0CH=CH=O' --:3" CH~OCH~- + H = C O .
CH~OCH.+' --~ CH~" + H = C O
.
.
.
[4]
.... [5]
The absence of 2-methoxycthanol, and t h e presence of only the small amounts of dimethyl ether, suggests either that the radicals produced in reactions ~ and 4 rapidly degrade before their expected hydrogen-capturlng reactions can take place, or that the main decomposition reaction involves a chain mechanism in which only a few ir, itiating steps are r e q u i r e d in o r d e r to bring about the subsequent breakdown of many nitric ester molecules. T h e r e l a t i v e l y small amOl.lllt ~f methane, an unknown a m o u n t of which is alway-.+ p r o d u c e d f r o m the thermal d,.,',.~tnl~sition of m e t h y l f o r m a t e , a d d s s u p p,,ri tr+ the view t h a t the number of initiating itot'+s ~s r<,Ialively small. Oi p;,rticular interest i.~ the p r o d u c t i o n of hrgt' a n m u n t s of metl,yi f o r m a t e , whictl c a n n o t :~,: ,atisf,tct,~rily aCC<+~llrlted fc+r on the imsis of ::rum,~h'cul;tr dccomI×~sitirm of the nitric ester. h.~ app,.ar~tnce c a l l tmwever, be d e s c r i b e d in ., <:q'& way by a rea<:ti, m ct'+inplt+te~y a n a l o g o u , ',, th.tt pr,>pos~,d :is an i r n [ ~ r t a n t s t e p in {he I,.;r,:l,h~bm *,f etilyl nitrate, |l wat-~ ~,ut;:ie,,ted :h.+: ,ff~vJ n i t r a t e was a ~ t a , : k d by nitrogtm :.!-'~A, + I.)r H O N O <,r C}t+.} with r'o~'~soi;~erl~ ",, ,,{ a |~,}'dr;,zer~ tt+~m, It a.r) >}lydr~r~.y.n *,'r.. r,'w,v,.d, the rm,,:+,um+ ;v,,qh.] b.e stmp+>'
If reactions analogous, to 6 and 7 occur in the decomposition of methoxyethyl nitrate, two different, but characteristic,products would be methoxyacetaldehyd¢ and methyl formate. respectively. NO.+ + CH~OCH=CH~ONO+. -+
[CH~OCH.,CHONO=]
+ HONO
$
CH:,OCH:CHO
.... [8] .... [8~]
+ NOr
N O = + CH.~OCH..CH=ONO= --+ [ C H ~ O C ~ H C H = O N O ~ ] +
HONO
... [9]
I[
l / H° CHa0C[" ":::::0
+, I-I;CO +o.N O ....
l: i, d',ffi<'v]1 ~o d, u e r m i r w t;mv n;uch o,' the r+qlctvm mb.:h~ pr,.>c,:cd I,v reac[iun 8. .~ince mc~h+,xy,.'~,~,d,i,,hy, k • c,,Ifid n,,t he p r e p a r e d h,r reler,:v.c+,. +pt,ctra. a,t+d £A[t!+r+ttit,tL The' ,t+k+,+,:;',.tc~,t,,i,.b by*,]+'. ;+r,~: k:v,wr+'+' t+~ +:. ~+L+'~t+.i!t t'' p[('+iLirt,' ,~Htt k¢N'[, l[L+rd +t'+l ~p,.,ct~';~ l'I.,Yc;ttci:l t]':r' I+rp<,'tlC<" <,t :l.+; tmkw+'+,,n :d{h'h;'d,' in the "+.tp.,1.h+ Ira+trim, .',ll,.J ,t ,tlhi!l ,tmourlt ,+f a ',lli+.L'~Jb~'r' J,~i%'iltU', ;:14",+ ['Jc,tk: +, A'[ 7++ a r i d
[' i ! ( tI~+N+):] .....~- t ' l t . , ( ' I t < + - N < > . . . . .
[6]
". i ,',v,,,+,.r , ~ +q hy, h',+<<'~t ;M,ml v,,-r,, },,,t, th<" +~ ,:::i;: in,heal ',,,++u!ll illlr.P:m~iati,lv ~",'a[Lt;Up' [ , lt,j++,.t' thu,.
t+CH.,ON
','<,~,./" o
'_,tl:t ',.,..X,, . [71
[ga]
7 3 wa.,+
[,r,',.~'zL+ N,¢ .+ cre.+r, dv,d ,+{ :p, cth~,xva
tl~
nmfl~,+xyacetaldchyde.
their m o r e t h a n 40 mole per c~'nt o f the n h r a t e decomlx~ses to thi:, s u b s t a n c e . More tha~. :I0 mole lx~r cent of m e t h y l f o n n a t e w ~ f o u n d , and
206
J. PowEng, w. A. w. Smith and J. Thynne
probably some had already pyrolysed. Thermal decomposition of this material in a silica vessel at 600°C showed that it produced, very approximately, 90 per cent (CO+CHoOH) and 10 per cent ICO:+CH~). This reaction, and the pyrolysis of formaldehyde, could account for the observed hydrogen, methane, carbon monoxide ~md methanol. Water would result from reactions of HONO produced in reactions 8 and 9. For example, HONO+ CH~OCH:,CH:ONO: ~ + NO + [CH.~0d,.,HaON0:]
.... [10]
CH~OCH.CH,..0NO.., ---+ CH,0CH.~CI-I~O. NO: (initiation) . . . . [3] CH~OCH,CH,,O- --e" CI't.~+ 211.CO . . . . [4 and 5] (~H..,+ CH.~OCH..CH:ONO.. ~ . CH, ÷
+
[CH~0EJ.LON0=] ~
[~ I]
. . . .
HONO
-..[CH~O~.:HsONO:
]
.... IS]
[I0]
....
= [ C H . O C H dHON(~ 1 ._L>.'CH.OCH 2 CHO$NO' " ~='[8a 2 . . . .
[CH~Od,H.,ONO.,]
[CH~OC.H~ONO.,] = [ C H ~ O C H C H , , O N O -->- L # i , , o c F I o ,
H:co
+
NO
"
....
No adequate check is possible however, because, even in the earliest samples taken, some oxidation and pyrolysis of aldehydes had occurred and part of the nitric oxide had been reduced.
This ester hz: a less favourable oxygen balance than the 2-methoxyethyl nitrate, but a stable flame can be obtained at atmospheric pressure by preheating. The flame temperature at the point of complete disappearance of the nitrate is not m~ch different from that of the 2-methoxyethyl nitrate if the minimum amount of preheat for stable burning is given. The products of greatest interest from this ester are formaldehyde, ethyl formate and acetaldehyde. The amount of carbon monoxide is much smaller than usual and indicates that not much pyrolysis of aldehydes has occurred. No ethoxyethanol, methylethyl ether or nitrogen dioxide could be detected, and less than 0','; mole of ethane per 100 moles of nitric ester wa, present Sigmifieant amounts (~-~20 mote per cent) of a high-boiling nitro compound were found. Its mass spectrum w ~ consistent with the fraknnents expected from CH:,CH~OCH.,NO.,. but this couk! not b,:, prepared for comparison. InJtJatlon is again t'lken to be fission of the weakest bond Cl taCH,:OC H ..C [ l..ONO.. --)- CH:,CI'I._.Of~H...CI:I:O..+NO..,
. . . . [l:q
"flit: ab~erice of 2-ethoxycthanol suggests that
HON'O+CH:,OCH:CH~ON0: ---~ H=(} + N O + [CH~0d:H.0NO...]
CH~OCH,.,CH=ONO= - - ~ 0' ICH~ + 0"gH~CO + 0"6CH.~OCHO + 0'gCH~OCH.~CHO + 0-4H..O + 1.0NO . . . . [12]
2-EJhoxyetkyl nitrate
H..O
A small amount of a nitro compoui/ of low volatility was estimated by assuming the same extinction coefficient at 6.80 microns as fcr nitromethane, its infra-red spectrum and its mass spectrum were compatible with the structure CHaOCH:NO., but this material was not successfully prepared, so no positive identification could be made. The postulated reaction scheme for 2-methoxyethyl nitrate, which wilt describe the appearance of all the observed reaction products, and which is completely analogous to that suggested for ethyl nitrate, is summarized below'.
NO: + CHo0CH.CI~:0NO~
Vol. 4
l ~"[~a]
An overall equation for the initial stages of the reaction according to the above scheme, with a chain length ~f ten, would be
the atk~xy radical breaks down rapidly C H.,C I f/K~} I..Ctl .O"'-" CIlaCI-/:O(~,H= + I-I~CO . . . . [1.1] The absence of rnethylefl~yl ether and the tow concentrations of etliane and ethylene would be evidence to suggest that, in fact, no large proportion of the nitric ester decomposed according to r,aetion 13 because none of the expected products of the resulting alkoxy radical would have been prcdnced:
~eptembea"1960
The t'Lamc decomposition o.f some substituted ethyl nitrat.es
CH,~CH=OCH.,+ R H - + CH.~CHzOCH~ + R" . . . .
Os]
'~ll~7
The radical CH~f~HOCH=CH,ONO.,, producecl in reaction 19, could rearrange, by"way of either of the two forms below, to give acetaldehyde, formaldehyde and nitric oxide:
CH~CH,.OCH= -'->" CH~CHz"+ H:CO ....[16] However, if the nitro compound is indeed [CH,,dHOCH.:CH~ONOa] -->- CH:,CHO + tCH:--CH2 CHaCH.,OCH=NO=, then it is possible that it w.~sformed by combination of CH~CH..OCH=. 0.. x 0 and NO,, although this reaction would have to be very much faster than the hydrogenI ii .~bstracting reaction 16 since the concentration H-,C--->C 0 0 of both CH..CH.0CH.," and NO,, must be vcry sm,~ll, whcreas ' R H ' is always large. The O CH2 amount of stable hydrocarbon from the alkoxy ~/\ / .NO+ 2 H..,CO radical (reaction 16) cannot in this case bc taken o~N'-.....Of~ CH2 . , , [1 ,ga] as some guide to the reaction chain length. On the other hand, the nitro compound may well have t,e.en produced by a blmoleeular reaction 17, lwl,.,w, which is not a chain-terminating CHaCHO + 2H=CO + NO . . [19b] process. The other hydrogen-stripping reaction (20 N(L:.!.C }| ~C 1-|:f.}CH ..C H :O NO: betow) would produce ethoxyacetaldehyde. --->- CI I~,CI.L.,OCH..NO..,+ H~CO + NO,., . . . [17] The ]~trge amount of polymeric suhstance in the The clhy] fommtc is produced by rc,'mtkms liquid fractkm might well be dt~e to this unstable analogous to 9 and 7: aldehyde, but no positive infra-red evidence could be obtained for this suggestion, because NO..,+ C I I.,C H...OC H C H _ONO.z of the interfenmce of acetaldehyde, formaldehyde, ethyl f,,rmate and water, tlONO+ [CIrI:,CH=O(~I[ICH'~ONO:,] . . [18] NO,+ CH:jCH._,OCH~CH..ONO., ~ HON0
1
/
/
\
\
I
+ [CI-(,CI-I=OCH:CHONO:] [20]
-q
C'H.,C H=OCH..CHO + NC).
0=~N
I °
I C l INC..H,.OCHO + t I.,C.O + NO . . [ 18a] 'l'w,~ r~ther ilydrog('.'n-strippilag reactions would
appear t, be eqttally pr(~bal)le in this ester. Ore, ,f them could account for the acetakh,hyde obsor'v~.~d in large amotmts NO,, + CH.~CH..OCH.,CH.X)NO= ~> [CH~(;HOCH.~CH.,ONO=] + H O N O
. . [19]
[20a]
A reaction scheme closely similar to that for the 2-mcthoxyethyl 16irate, bu~ with the additi,~al reaction,~ ~o :mcotmt for the acetaldehwtc (reaction t9) and the nitro compound (e.g. reaction 17), will :.tccmnlt fl,r the main Menl.iticd pr.,:iucts. The large :unoup.ts of tlnkaowtt snb,~t0.r:tcc [rottl this more complex estt, r (see Figm.~'~ 4) npsc.t attempts t . describe the reacti,n rnorc fully. [k'vund the flame front, it would s,.,cm that, in t'bis cool-burning ester, polymerization of aldehydic products is faster than their pyrolysk~,. In Figure 4 it may I,e seen that the cn~'es of the Mdehydes and of the unknown polymeric matcriM are inversdy feinted to each other. The nitro compound deeomp~ses steadily in die p.rcduet gases.
2o8
J, Fowling. W. A. W. Smith and J. Thynne
2-Hydroxyethyl ni$rate The decomposition products of this ester are all simple molecules and do not provide much evidence about the course of the flame degradation unless examined in relation to the other nitrates, when abnormally large amounts of formic acid, hydrogen, carbon monoxide, water and carbon dioxide are seen to be produced {Figure 5). No methanol or glycol is observed, and, although the amount of hydrogen is larger than commonly observed in the flame front of similar nitric esters, the trend of the hydrogen curve in Figure $ indicates that its initial concentration (when the decomposition of the nitric ester is just complete) is much smaller. The amount of 2-hydroxyethyl nitrate consumed according to reaction 21 (plus reactions 22 and 23) is again probably not large: HOCH~CH--ONO~ ~
ttOCH_~CH:O.+ NO. ....
~m]
HOCH~CH--O" --->. HOCH--'+ H--CO . . . . [.02] HOCH.. ~
H- + H,CO . . . .
[~]
It is suggested that the observations arc better explained by a mechanism similar to that of the other menonitrates, with reaction 21 being the iai;:'.'ating step in a chain process, the main reactions being 24 and probably 25 below: NO.. + HOCH~CI-I~ONO~ --9- HONO + [HOCHCH,,ONO:] .../I H OjCH - - C H2 , \ C..
0
k_
~ - N O . . . [~-4al I-l~O+ CO or H~ + C()--
NO~ + HOCH--CH.,ONO~ ~
HONe
+ [HOCH.~C2HONO--] HOCH~,CHO+ NO~
the trap, and of the rapidity of the pyrolysis of the aldehydes in this flame. The 'unknown' in Figure 5 has been given the molecular weight of glycollie aldehyde. Synthetic samples of glycollie aldehyde, both as the anhydrous solid and as the hydrated form, were examined spectroscopically. It was considered that the latter was the form in which it was most likely to occur in the liquid products, but this, when heated to 120°C, gave a negligible vapour-phase i.r. spectrum. Its spectrum as a thin film was compared with that of the liquid products, but the latter consisted of broad featureless bands due to the mixture of water, formaldehyde and formic acid, which were also present in the samples in which the unknown occurred. Thus, no positive inference as to the presence or absence of the aldehyde in the products could be made. HNCO is an interesting and unusual minor product of this particular flame, but there are no clues regarding its mode of formation. The reaction scheme for this ester would be summarized as follows: HOCH--CH..,ONO.~ - - + H, + 2H.,CO + NO~ . . . . [2 t, 22 and 23] H.+ HOCH:CH~ONO 2 - . + l'l~ + [HO(~..,H~ONO=]
....
[261
NO= + HOCH--CH--ONO,_, .-->. HONe + [HOC--H~ONO..] [24 or 26] H O N e + I'IOCH~:CH~ONO~ --->. ['[--O + N O 4- [HOC:rLONO~]... ["7]
[-l(:(')~')f-l-t I..,CO
"~,'N. / II 0
{24]
Vet. 4
....
[2S]
. . , . [2Sa]
Agai~L the relative exteut of reaction 25 could not be estimated because of the difficulty of analysing for polymerized glycollic aldehyde in
[HO(:,:I-~oONL)...] ~ HC()OH + H...CO+ N() "~ . . . . [24a] IIOCII~CI-IO+ N(.I~ . . [25a]
Reactions brhin,.! .q'e tt',,n.e ]rents In all the tlames exaa~ined, relatively slow reactions fol!ow in the wake of the very fast reactions occurring in the flame front. The former are h;rgely aldehyde pyrolyses, and do not result ir~ any further l~c;'~t rclcase, un]e.~, nitric oxide becomes involved in the reaction and is reduced. This reduction of nitric oxide is an important feature in relation to the rate of energy release in nitric ester combustion, and has been discussed eIsewhere~.*. I:a a!l nitric
~lztember 1960
The flltme decomposition of some substituted ethyl nitrates
ester flames in which formaldehyde is observed to be decomposing, nitric oxide is reduced at a rate which seems (from the concentration carves) to be related to the rate of aldehyde pyrolysis. In the 2-hydroxyethyl nitrate the decomposition of formaldehyde is rapid, and the asmciatcd reduction of nitric oxide stops at abou~ the point when the aldehyde has disappeared. The continued temperature rise resulting from this nitric oxide redaction is seen as at " :,;vd!qg c;ff' of the temperature profile io Figure 2. In the 2-merhoxyethyl nitrate flame, the temperature is lower and the aldehyde pyrolyses arc sMwer. Nitric oxide is reduced continuously. ttxenitri,:: oxide curves and those of the formal&hydv and methyl formate being roughly parallel over the range covered (Figure 3). The flame temperature continues to rise fairly steepfy beyond the flame front (Figure 2). The ~itt,.~tiorlwith regard to nitric oxide reduction in ue 2-ethoxyethyl nitrate is complicated by the ,~arly production and subsequent slow decoml~Jsiti,.m of significant amounts of a nitro c0mpm,nd: nitric oxide is produced more rapidly tha~l it is decomposed to nitrogen. Aldehyde pymlyses, as judged by the production of carbon :lom~xide, are not rapid in the cool-burning e,ter, but the increase in the amount of unknown eaterraI (tarry residue) with distance from the iI',une fr-at (Figure 4} indicates extensive oulymerizati.r~ of the "aldehydes No temper}.ture rise ,,tours beyond tile ttame from in this ester. "/he l,r,,pyl nitrates, burning with simil~Lr flame l"mp,,r,,ture, produce I;t:ger amounts . f more
<1 cemparison o[ the initial ,~roducts [ ~ m u ~l~*mt.:.,. el *dtric ester flames ifieee ,:<(t'.nsion of: this analytical :4udy el the
209
effect o£ chemical structure on degradation mechanisms in nitric ester flames is not coxxtemplated at the present time, it was considered useful to summarize and compare briefly the results for the eight esters examined. The primary reaction products (i.e. the products at the point in the flame at which virtually all the original nitric ester has disappeared) from these esters arc listed in Table I. Some of the values for the le~,s volatile com. ponents should be regarded as senfiquantitative because the complexity of the mixtures in wb.ieh they were encountered would have made a better estimate unprolitably time-consuming. It was hoped that at least a semiquantitative answer for all the significant stable reaction products of every ester coukl be obtained, and it is considered that this has been achieved in ,all cases except tile three more difficult esters reported in tile present paper. The aldehyde mlxtur~s produced ill these alkuxy nitrates gave polymeric n'laterhds not readily anMysed and sometimes representing a large proportkm of the products. It is considered that the divers products from all these mononltrates, in contrast to the two dinitrates studied, can bc more reasonably and rn,.,re cornpletely described by a degradation mechanism which does not rerll~ire extensive nnlrnMecular breakdown of the nitric ester i n t o alkoxy radical and nitrogen dioxide, but which depends upon hydrogen abstraction from tile eslers by radicals and reactive species, particularly NO. and FIt)NO. Such a mecltanism prvsePts n,~ ditlictlhy in dv~cribitLg th,.' appeari,m'e ,,[ tile matj,.~r pr'tuhlcls fr~.n 'dl the :n,~;a,,:. him.des ,'xamint'd. [t i-~ r,,m, ditfic)ll~ 1¢) lliider. >land why ~'lil[rogtHi ,li,,xidv ;rod radicM at{ael: ,,a huta.,.,.Z,zi-,lit,1 clhli{I,atv ~ is n~q as rapid rt:~ ill tllt' lu¢m,mitrat,:.s. hi flit. lll,lnt,ll[l?-o.[u IIdltl(.s ill a.tt'aospheric [ID'N:-;IID.'. II,, t'VJ~II'P.C~ hits t~t,q.,tl f,:,lltlet l,.r the pr.dttctioxl el alkoxy radicals, ials,,far ',is the
aplw*,pri;~te :.flc-h,d has not h,'l, ql ob~,crvvd, [t ~. p r , hA~Io th;tt sm:h atk.xy radicals :is ;ire pr,.hmt,d i. th,' itlilial.ing steps ,m, l,r~fl.:en down t,~ Mky] radical ~md :,ldehs'de ,it the high t.vlnper;ttttrv involve, P, Wtle l.w pressure ethyl nitrate ll0me (h'mpvrattu'e ab-ut 860°C) has, tlowevcr, receiil[y been sl'u)wn ~ to yield lip to
~J
I~ " ~ =
~
I I~'~V~==
I I
~
I~ A
- -
"~
b
I
~
B~
September 1960
The
flame decomlx~sition
o! some
IS mole per cent ethanol. Alkyl radicals and hydrogen atoms, produced either from the alko~.y mdicaI from the original ester or b y radical attack of early reaction products s,aeh as aldehydes, are also expected to abstract a hydrogen atom from the nitric ester~ but, according to the reaction scheme suggested here, none of these are regenerated when the hydrogen-deficient nitric ester radical decomposes, the only reactive species produced being ~0.. It is not considered that reactions of the type R- + NO., - - ~ RO. + NO
"x
RNO:
take place to any major extent for the following reasons. ROH, expected to be prodnced by hydrogen abstraction by R()., is rarely a significant product. The concentration~ of Ra~d NO., arc very smaU, and although the activation energy for reaction may be sufficiently low to m.'.tke this a ~ssible competitor wb.h R' + RONO_, -->- RH, etc., and NO: + R'ONO~ -->. HONO, etc., for which the activation energy may be 7 to 10kcal and 15 to 20kcal rcs~,ctively', the products of other equ,-dly probable reactions (e.g. 2R.-->- products, R. + NO ---+ products} are not generally observed. Where the NO,.. concentration is known Io be large, and CH:,CHO is pyrolysing tO C]'t+ a r i d
CO
(the
butane-2,g-diol
dinitrate
.~yst~'ml, no Ct't~N(),, or C14~,()I-[ (from CHaO. reacfi,r;s) is observed. In fact, it has not been l~ssi~,le to detect any significant amount.¢ of ,ihro ceni~pound, rlitriie.~ or hydrogen cyanide hi ~[a.m.:sof nitrogen dioxide ar,d numerous fuels whh:l; ;,~ight be expcctcd lo produce rnctl'tyl ra¢tic,ils iii a similar l:enll×,xature range:'. There Js much evidci~ce for the oecurrcm'c of these reacti,m~ at lower temperatures~, Significant qtm.i~:ilh,s o[ nitro compounds arc produced only
substituted
ethyl
2.11
nitrates
in the two cool-burning nitric esters. It is of interest to take nitro compound formation as an example, and c~mpare the behaviour o! 2-methoxyethyl nitrate and isopropyl nitrate. If it were considered that unimoleeular breakdown of the nitric esters was ~. major reaction in flames, and that the alkoxy radicals degraded to aldehyde and simple atkyl radical (see P. GRAYand A. WILLIa~S '~ a ad A, F, TROT,~ta~Dicst~tcsor~", p 806), the products of these two esters might be expee+ed to be similar, apart from the aldehvde produced: CHaOCH,_,CH,,ONO_~ ~ CH:,. + 2H~CO + NO,_,
....
[26]
(CHa),CHONO., ~ : - CI'[.¢ + CH,,~CHO + NO,,
....
[27]
No nitromcthane is found in the products of the 2-methoxyethyl nitrate flame, and o~:ly minor quantities of other nitro compounds and ifitritcs, whereas the yields in isopropyl nitrate are very high.
The authors are grateful ~o Mr ]. V. (~riffi.ths [or .preparing the 2-hydroxyett~yl nitrate and the 2-¢thoxyethyl nitrate+ ~,e.~ct'el'Ice,5 i P o w r , h~C;, J. ".tnd S',.~rrr[. \V, A. VV. Combrtst~on ~,, Fl,.o~h", 1988, ;2, 187 = N~.zr~vl.ca~r. D. P, a n d P o w t , i,~c;, J . Prcc. Roy, Sac, ,'1, 1985, 232, 337
Flare,.', 1987, 1, 3{)8 '~ GRA',', [L 7 rtot,~, Faraday Noc, I955, 5.1, 13H7 :' l~p~ab:l,:, S . [,., I)tti:tatJ,, t[. M., JAct'mg, T. L.. '1 Iill)lU',~'ll.~, []. "l'. Oth;[ ,'~l~NNIt lINES, t:~'. ~]..]', A " , ' r . ch,'m. S - c . l,q:¢,q, 60. 73 +~ AI,~I;.N, ."~. I~, :tll~]. ["OWI.ING, J. C~,mb;t::tion ¢:.+ F.tamc+ 193~, Z, 55 ~ l l i c ~ ; s . . ] . A. E . N . L L h : , , \V<,hham ..\l,b.,.y, l,L,,s~,:,: 4't¢i hi' puldi~hl'l.l) rt "I'It(I.I..,II.,IN. IIIi:K1-NS(~.~, ~. F. (;,':S /+.'¢llr.',tf+".,. :[JtLt|.t'r" x~¢,r( h'.~: Ltm~ l