- Email: [email protected]
Peculiarities of electrochemical reduction of aromatic nitro compounds on short-period dropping mercury electrodes
J. PlectroanaL Chem., 65 (1975) 635-649 @ Elsevier Sequoia S-A., Lgtisanne --Printed
in The Netherlands
PECULIA&ITIES OF ELECTROCHEMICAL NITRO COMPO-S ON SHORT-PERIOD ELECTRODES*
REDUCTION OF AROMATIC DROPPING MERCURY
J_ STRADINS and I. KRAVIS Insfitite of Organic Synthesis, Latvian
S;S.R.
P-cademy
of Sciences
of the Laivian S.S.R.,
Riga,
(US.S.R_)
(Received 1st August 19’75)
The electrochemical reduction of nitrobenzene and its derivatives on the dropping mercury electrode has been the subject of numerous studies dealing with various aspects of the process [I-14] _ This reduction is known to be strongly dependent on the nature of the medium and on conditions under which the process on the electrode is effected. The first stage of electroreduction of nitrobenzene
in aqueous
solution
consists
lh a four-election
reduction
to N-
phenylhydroxylamine [l--5], but in aprotic media, in t’ne transfer of one electron with formation of an anion-radical [X2,14] _ There exists, however, also an intermediate and up to now practically not studied region (cf. ref. 7), that of transition from the four-electron to a one-electron mechanism. Polarographic curves of aromatic nitro compounds obtained in this region permit us to gain an insight into the sequence and relative rate of separate stages of electrochemical and chemical reactions in the summary process of nitro gro=upelectroreduction, the latter being treated as a consecutive electron and proton transfer to the molecules and anion-radicals of nitro compounds adsorbed upon the electrode surface [ 3,4,9,10]. The kinetics of electrochemical reactions and protolytic surface processes can be particularly clearly observed under conditions of short duration of microelectrolysis, viz- on short-period dropping electrodes. Unlike capillaries usually employed in polarography and possessing a drop time t between 3 and 6 s, short-period electrodes (t = 0.2 to 0.5 s) possess considerably lower adsorbance of organic depolarizers and more pronounced effects of change in adsorbance with changes in medium properties and in structure of the derivative under study. This makes a short-period DME, first used for this purpose in ref. 15, a convenient means for studying the separate stages of electroreduction of aromatic nitro compounds_
* In_.honoukof Academician A.N. Frumkin’s -80th birthday_
636 EXPERTiMEMTAL
The compounds studied were nLtrobenzene &d 14 of its m- and p-substituted derivatives (formulae 1 and 11). The parameters df polarographic reduction waves obtained with these substances were studied under different conditions, the latter including varying composition of organo-aqueous mix&e, pH values, depolarizer concentration, buffer capacity, iotic strength, duration of microelectrolysis (drop time).
02NvXi
x; = H;
NH2;
X; =H;
CH3.
OH,
CHO;
COOH;
NH*; OH;
Cl,
NC&
Br, 1;
COOH;
(11
CHO;
CN
(II)
X,
The work was performed with a short-period DWX fitted with a special device for forcing the break-off of the mercury drop (by means of little blade fused to the end of the capillaryj. The drop-time was t = 0.25 S, with a massrate of m = 0.81 mg s- I_ The effect of changes in the dropping period on polarogram pammeters was studied on a special device (fitted with a micrometric screw), permitting a change of t from 0.2 s to 3.0 s, the mass rate of mercury flow remaining constant and equal to m = 1.4 mg s-l_ Variation of dropping period was effected by changing the distance between the end of the capillary and the movable glass fibre, the “blade”. A saturated calomel electrode was used as anode. The electrolyser consisted of a thermostatically controlled ceil, The electrode potential was measured by the compensation method using a three-elec%rode potentiometer circuit_ Measurements were at 25.0 t-‘015°C. Argon flow was used for deaeration of the solution. The heights of the_ polarographic waves were corrected by subtrdcting the current corresponding to the background electrolyte. Polarographic studies-were c-ied out in aqueous and alcohol + water uniVeYsal Britton~Ro‘biuson *buffer sol&ions (pH values 2-12) with ethanol content-from- 0 to 40 vol %.-II% case of fiecessity other buffer solutions with a narrower range of pH value were also used, as well as anhydrous dimethylform&de (DMF-)_ T&S a.koh~l~+~ water solutions were prepared ticcording to. requirement, with -variouS depoiarizer (r?)-and aloohol contents, with various pH values, ionic strengths
.-
637
&zmt values. Correlational statistical analysis methods, were used for procek-fg these data. . The po~arograms were recorded in integral and different% form on a LP-60 polaroglaph.
PoEarographic behaviuur of nitmbenrene akohol -i-water buffer sulattions
and its derivatives in aqueous and
The short-period DME (Fig. I) produces in aqueous medium only a fourelectron reduction of nitrobenzene and its derivatives to N-phenylhyclroxylamine (wave A). The Ella dependence (Ella values in the 43.2 to -0.8 V range) of the corresponding wave on pH value can be represented by an Sshaped Curve which we divide into three regions, or, /3, and y respectively, characterized each by different values of coefficients A_E,&ApW and semilogarithmical slopes (Fig, 1). In media with higher alcohol content a a-region also appears (Fig:, 1). Table 1 contains AE If2/ApH values for a, _Oand Cregions, as well as pH values corresponding to the break-points of slope change on the E,@--p H curve and showing the transition from the a-region to the P-region (p&3), from 0 to 6 (p&j), from /3 or S to y fpK,). The AElrz/ApH value in the y-region equals zero. Divergences of AZ I,z/ApH values from the theoretical value of -59 mV/pH unit in the case of organic partic?e reduction are characteristic for processes involving adsorbed particles cl6 j . Non-equal AE,,,/ApH values determine the S-shaped form of the E,,,-pH curve. In 10% afcohol + water solution, at pH 5 a two-electron wave is additionally
f
I
i
I
I
I
I
i
5.0 PH
I
I
r
,
10.0
PH
Fig, I. Dependence of. firiziting current txu~stant (r = &&%~~~f”~ f and of El12 on pH for p-nitroto&.xene cm&or&-period DME in ~.queous and akohul + water Britton-Robinson buffer sdlutipy& (I) 0, (2) 10, (3) 20, (4) 40% ah (X,2,3,4) -4’ wave, (3’, 4’) A” wave,
(2”,3”,4”)
B wavel
,’
.’
;’
:I
,’
,’
,,’
,,’
5,.
‘,
1,,
_,
‘I
,tj
8;
7’
,,
p.1 :’ ‘p:Br ‘,
,;@-
” ::POH
po- :
”
60 “o y
GO,
6O
48
ip:C,HO
; /; .,I14~ ‘.‘$CN’ ’ ,;,,,lb’: ;,,,1n-N92, ‘,’ ,,_~ ‘,,‘I ,,‘, ,’ I’ ,, /I ‘,, ,I,, ‘: I ,,’ ,,.“.
,’ ,:m’13,; I,,;
,“,I,,
: :: 12,, ‘, rwCHO ) ,’ ‘, ‘,
,,
:, ,tO.“” ” ?!,$6 +O.?L
.+0.3’(j
30 1 .;17
“43
po
“+0,03G ‘, ,‘I 11;‘,,,,, ,” tf~~cooI~I, I~CO’O” -0.io’ ,I,“0
+0.23, +0.‘26 0’
-0.37. -0.d2. +0.18 ; ,-co.23,,
0
.I
,,
5,O 4,3 Sal
72
-
4,7 -
8,7
0,4 $8
$6
60
46 30
50
(4G)
53
(3,O) 10,2
36 46
61
44
9.0
44 (60)
B,6 -
10.4
0,2
9,o
8,8 ‘(40)
67
67 67
110
GG
110 66 76
120 78 74
3,8 4,0,
6.8,
-
3.4
4,G
G,O
4.6 4.0
7-3
6.0
3.1
93
3,2 4,0 4,o
(0.050)
90
88 83
--
-
-
I
-
-
-
-
-
i
-
-
(8,0) -
9.7 9.8 8.4
10.4
--.C_r__-
---I
buffer fiolutions
---“_l__
6.6
8,l
9.8
,D.2
9.4
7.6
9.0
8.7
9.5
and nlcohol + water Rritton-Robinson -‘-----------7
!O% C2H50H --_‘-l.-,
10,2
4,l
6.7
4,o 7,7
74 67
40
74
(62)
70
‘73
120
7o
wo
(4.6)
70
62 12i 1iS
4.7 4,3
‘73 80
.a.-,
--_
on p~l,(mV/pH unit) Pm nitrobehacnc dcrivativos In n~cous
0% CjHgx
fill2 dC&denco
,G’: :m:OH
“,
‘. ‘,, ., ) : : 1 ;:j’ (1’&Cl :’ ,: ” : $,!a, ~p?CQOII ” ‘I’ “, ~J)‘~O~~,
/’
8’
,I
3,
:
,,‘,
&khf&nts,dk
:,l,,;H,‘:,
,‘,I,’ /,
,‘/,’ ,,
.’
,,
,‘,,’
.‘I
,’
‘I”
‘,,
‘,y.wmy+“--*;ll”.----~ ,, ” ,yl’i’I ’ ax I,,‘, ., ?i .‘,
/
-,,,:”
II.
TABLE’i.
,’ ,’ ;,,
‘I.,,,,
‘;‘,
,,
:,, / i
,g
’
‘/
,,
,”
_
,,’
,,
3,
'1, 2,
,-
.,,
:
H ‘-’ JkH3’ ttt-NH2
-a16
,‘O -0J-7
OX
;
.,
’
p42OOH
10
,:’
12
11
.I)-CHO
tn.COOH tt1.C‘oottbCH0
‘IJ$oo’
,’
-
,’
,PCN ,'.15' vi-NOi
‘13 ,,14,
‘, :
,/
I ','p-q,. 7,M ,' ,!f. e_Br,. ,' 9' pa
,p.OH’
ttr.o-
‘6,‘.
”
to,45 +0,66 to.71
+0.23 +0,26 0 MI.036 -0,lO to,36
-0,71 -0,37 -O,G2 +0,18 +0,23
93
28
(42)
G2
(G6)
20
46 48
31 08 GD
ll;O
lG0
90
-
GO
-
109 04 104 69 no 80
(38) -,
-
93
(53)
G4 63
88
102
-_
Gl
-
20% C$$OEI
of E 1’12dependence on pH (mV/pH unit) for nitrabenzcne
', rn-NFI; -to,63 ,' .', 4, P*NHz, --OS6 _' " 'b-NH3 ,," ; ,'I, 5 WOH +0.1’3
1,
,’
.
‘,,
TiBIhi.1 (continued)’
‘/ .;, ,,
:
,,
.,C?officihs “,,, ,I” Xi ,, .., : ‘,,
‘”
,,,
-
-
-
-
-
-
G.8 6.7
4.4
,,,
3.2 3.9
3.3
-
-
G,G
-
-
4.b ‘4,0 -
4.3
3.1 6.0
3,l
(3.6)
-
3,O 3,8
-
-
-
-
68
40
,.
8,G 884 36 a.9 34
9.2
8,G -
(58)
(30)
8-6 6.7
(43) (43)
3,9 9.1
9,G -
10.1
73
8.8
8,G
9.0
40% C2H50H
-
100
86 a4
62
170
160
70
3G
-
-
-
6,4
(46)
-
-
40
130
140 6B 32 90
76
68
50 46
130
3.03
100 106
-
388 4#1
-
4.9
5.7
-
7.0
-
5.0
4.4
6.0
6.9. G.3,
7.6 7.7
9.6
8.8
7,4
7.0
9.8
9.0 9,0
lo,2
10,l
7.4
,8.0
8.2 8.3
sdlutions
-
400
300
332
440
4,o 4,o
..”
380 3,6
b&x
__.--_.;_^
derivatives in aqueous and tic01101+ water Brittan-Robinson --
c (0
.,.,, ._
observed, ide.to
dorrespor,&2g- to reductiqn of the correspond~g.M-pheny~~_drox~~ obta.&d froti.nitrob&iene derivatives qapabl@-of losing -protons at high_pH v.alu&s-with&&f&mation of anions (X = COO& OH), is 5 splitt&g of the t&al four-eJec-i&n ‘zf7iGe.A into a one-electron and a ttiee&lectron wave in ILOT?alcohol -i; water solution _ In mixed media with higher alcohol content the sazlie wave is observed as in 10% alcohol -i-v!ater solution, except that the EI13 value at the c’orresponr ding pff values shifts towards higher cathode potentials, znd the limitmg current (ilim) diminishes-with increase in alcohol content. The product is qzi2 (q being +,heviscosity of the solution) diverges from the expected constant wzhxe, +hich may be due, presumably, to chang& in the salvation sh&ath of the depolarizer molecules. In case of nitrobenzene compounds which are not capable of losing protons (XL = H; p-CHs; p-NH~; m-NH2; p-CE\i) splitting of the four-&c&-on wave A intc; one- and- three+fectron waves Ar and Au occurs in media with higher alcohol content- The waves of nitrohenzene, p-nitroto_luene, p-nitroaniline and p-cyzmonitrobenzene split in 2070, the m-nitroaniline wave in 40%, and the p-nitrohalogenobenzene wave in 95% alcohol + water solution. Only in the case of G-Z-and ,p-nitrobenzaldehydes did none of the alcohol concentrations tested produce any splitting. An increase of &cohol content of the aqueous solution causes a shift of the value towards higher negative potentials, whilst the initia,l points of the y-regions of the E If2-pH curves shift towards higher positive potentials. At the same time we observe an increase in (A.E,,2/ApH) value, and the Sshape oftheElrz -pH curve becomes mere pronounced_ Ia the transition area between the four-electron and one-electron reduction region the _4_rwave is of kinetic nature which manifests itself in the dependence of fri, on buffer capacity (cb) {with increase in cb there is an increase in height of tbr A’ +zTe owing to increasing protonation rate of the z&on-radical) [11-J. The temperatyAre coefficient values are abnormally low for kinetic waves .(2.6-2.8% per “C) which indicates that we are dealing with kinetic “surface”waves. In anhydrous D&W all nitrobenzene derivatives studied exhibit a well defined one-ele&Qon wave corresponding to anion-radical formation, and a three-elect--on wave correqonding to further electroreduction of the anion-radical to N$henylhyc?.roxylamine_
anilibe (mm&B). A peculiari@ of~po’l~ograms,
$&i
Effect of adsorption of molecules pu&&-b~#pZri~ deduction
of aromatic
n&-o compoiuzds
on their
EJ&tr&e&&on of the-arornztid titio group proceeds ti v&.&y of electiocapillary zero-point (-3-4 to O-6 V) where m aximum adsorption- of organic moiecrrles occurs. kdsorpti&>. of nitrobenzene zm;dits &zrivtitivas on. the sur.face of $he_BME n&mfests~itseiL’ through a lowering df the ele&roca@iBey C&T& .-
.-
_
641
in the p&$&e of tfie nititibeniene derivative, as compared to base electrolyte solution (-Fig. -2). Depending on mole&.&r structure and medium conditions nitrobenzene tid ‘its derivatives are redu_ced at potentials corresponding to the positive branch of the electrocapill-&y curve, at zero charge potentials, or at potentials of the initial part of the negative branch. In case of absence of extraeous surface-a&iv& substances (s.a.s.) in aqueous or organo-aqueous media adsorption- of depolarizer molecules causes in all cases mentioned above a high surface concentration .of reducing particles, which is necessary for “surface protonation”, However, under certain conditions the-kinetics of adsorption and subsequent protonation starts limiting the rate of the electrode reaction as a whole POIInhibition ofpolarographicreduction
surfacereactions
by the aromatic
nitro group can be effected: -by increase of organic component content in the organo-aqueous solution; - by addition of extraneous s.a.s. [3,6] ; - by shortening the drop time of DME; - by changing the electronic structure of the molecu!e (through electrondonor substitution)_ Use of alcohol + watersolutionsinstead of aqueous onesreveakthe specific effects produced
by adsorption
of alcohol molecules
on the electrode,thus
displacing adsorbed nitro compound molecules: with increase of alcohol concentration Eli2 shifts towards higher cathode potentials; the semilogarithmic slope b of the polarographic wave and the coefficient AJ3112/ApH change in a
0e
10
5 PH
Fig.
2_ Electrocapitfary
curves,obtainedin
solution at pH 5.4; (I) background in presence of m-nitroaniline1
curve;
40%
(2)
alcohol+ water Brition-Robinson curve in presence of nitrobenzene;
Fig. 3. E~~~.depetidence on p&I for nitrobenzene in tqueous ,md alcohol + water Robinsoti soiutiotis at varions DME dropping.periodL: (l,l’.‘l”) f = 4.2 s, (2,2’,2”) (3,3’,3”) f = 3 s, -( 1-3) H,O; (I’--3”) 40% ale.
buffer
(3)
curve
Brittos0.8 s,
642
rather complex manner with incr&se of a.&ohol. content, the S-shape of the _E 112-pH curve being enhanded.~ A specific effect of nitro compound molecule desorp&n from the electrode surface with l?ncrease of aJcoho1 content is $he splitting of the four-electron wave in media possessing low protogenic activity. This splitting is due to lowering of proton activity of the solution, as well as to desorption of the anionradicals formed from the electrode surface, the latter process becoming more marked with increasing distance of electroreduction potentials from electro-capillary zero-point towards higher cathode potentials_ yn- andp-nitrobenzaldehydes which are reduced at potentials lying on the positive part of the e.c.c., do not produce a split four-electron wave. Addition of high alcohol content to aqueous solutions produces an effect on nitrobenzene electroreduction similar t.o that of such surfactants as camphor and tylose, as earlier described by Holleck and Kastening f4]. The $3 dependence of E,/, is S-shaped even in complete absence of surfactants, provided the following conditions are observed: - organo-aqueous solutions have to be used instead of aqueous ones with sufficiently-high non-aqueous component content. Increase of alcohol content raises the value of the AEI12/ApH coefficient and, consequently, enhances the S-shape of the _&,s-pH .curve; - the molecule is substituted with an electron donating group; - the duration of microelectrolysis has to be reduced by shortening the drop time of DME. An essential point is “straightening out” of the S-shaped (EII, vs. pH) curve with lengthening of microelectrolysis t.ime (Fig. 3). The effect of dropping period change upon the (E,,, vs. pH) curve is more pronounced at high alcohol content and in case of such substituents of nitrobenzene-which are capable of electrolytical dissociation (-COOH; -OH). The S-shape of the (E1,2 vs. pH) curve at small values of t and the “straightening out” of this curve with increase in t [17] can be explained by the fact that in the course of polarographic reduction of nitrobenzene and its derivatives only particles adsorbed at the surface of the electrode are involved in the electrochemical process and protonated before electron transfer_ On a short-period electrode (I: < 1 s) surface adsorption equilibrium cannot get established in -. time< hence the Erinetics of adsorptidn and subsequent protonation sets certain limits to the rate of the electrode process as a whole, thus causing the jump on -the (E,,, vs_ pH) curve. ~Effect of buffer cbpucity onpofarographic
reduction
ofaromatk
nitro com-
pOi..L?tdS
Ir_the transition part from the four-electron to the one-electron kegion of reduction ihe A’ wave assumes kinetic nature which affects also the dependence of iErnon buffer capacity cc& -Eli] _ The dependence -of im %nd of E1/2 on cb ,c&n tie observed in weak alkaline-and alcohol + Water soluti_ons (Fig; _4)_
.643
0 -2.0
““““‘1
051 -1.0
0
19 Cb
I
-2.0
I
I
I
I
-1.0
I
I
I
I
I
0
‘9 Cb
Fig-Q_ El, and ilim valuesofthe Awave(A'and A”) fornitrobenzene,as plotted against buffer capacity ofalcohoS+ water solutions atpH7.45: (1)veronaI buffer,1070 ale.,(2,s) Verona1 buffer , 40% alc_,(4,5)borate buffer,40% ale.
With increase of buffer capacity the Q2 value ofthe four-electron wave shifts towards positive potentials (mllZ/Alog cb = SO-100 mV). In the region
of splitting
of the four-electron
A
wave A into a one-electron
and a three-electron wave A’ and A”, the Elj2 value of the A’ wave does not depend on c,, whilst that of the A” wave shifts towards positive potentials with increase in cb_ Decrease in cb causes splitting of the -4 wave into A’ and A”, similarly to the effect produced by increase in pH value. The semilogarith- i)--E: curves corroborate the reversibility of mic slopes b on the log(i/i,, one-electron reduction corresponding to the-A’ wave (b = 60 mV) and irreversibility of the A" wave (b # 60 mV). The E1,2 and ili, dependence on c, confirms the fact that in acid medium the protonated nitro group is reduced, whereas in neutral medium the reversible transfer of the first electron is followed by reduction of the protonated anion-radical. Effectofnitro
compound
concentration on itselectroreductionpotentials
On the surface of the DME the 4e-reduction product - N-phenylhydroxylamine - accumulates as well. Accordingly, the effect of increased depolarizer content, on the one hand, and of addition of extraneous N-phenylhydroxylamine, on the other, upon the A wave parameters (or, correspondingly, upon those or”&he A’ and A” waves) of nitrobenzene has been studied within the concentration range between 5 X 10e5 and 2 X 10B2 mol l-’ and under various experimental conditions. In both cases simile effects were observed. With increase in depolarizer content
-644
0.70, 0.65 I
L
d
2-o-o-~Q
2
O_
n 50
Q50
/* ,
10-0-o I I I
1 1 f
/ 1 1 I
-4
I
-3
I
I
!
1 -2
Fig_ 5_ E1,2 values of uitrobenzene A wave, as plotted against depolarizer content in aqueous and alcohol + aqueous Britton-Robinson buffer solutions, on short-period D-ME: (1) acetate buffer, pi!! 4-8, 0% ale_; (2) Britton-Robinson buffer, pH 12. 0% ale.; (3) B&ton-Robinson buffer, pH 12, 40% ak. {A’ wave); (4) Britton-Robinson buffer, p&I 12, 40% a.k. (A” wave); (5) Sritton--Robinson buffer, pH 6.4, 0% ale. (nitrobellzene mol 1-l ; M-phenylhydroxylmine added)_ content I X 10” _Pecutiurities DME
of
p-nitruphenol
and p-nitroaniline
reduction
on short-period
Polarographic reduction of nitrobenzene and its derivatives in aqueous solutions is usually represented by a four-electron process which in acidic media is followed by an‘additional two-electron process (in form of a separate B-wave). Only a few o- and p-substituted nitrobenzene derivatives fnitrophenols, nitroanilines and structurally related compounds) are reduced by means of a specific six-electron mechanism. The latter may be due to the possibility of dehydration.of the corresponding aromatic N-phenylhydroxylanxine with fonnat.ion
of e.&ly
reduceable
qukoneimine
structure:
645
The intermediate cherrtical reaction of change from p-hydroxy- or p-aminoN-phenylhydroxylarnine into p-quinoneimine (or into p-quinonediimine) is catalysed by acids and bases and has a rate of the order of I-10 s-l [ 18]. Hence, this-reaction can be successfully effected within the life-time of the mercury drop with ordinary polarographic capillaries (t = 3-6 s). We could show [I93 that in case of a short-period DME (t = 0.20 s) no such continuous six-electron wave is observed in the electroreduction of p-nitrophenol and p-nitroanihne: we have here a separate four-electron wave corresponding to reduction to the respective N-phenylhydroxylamine and a separate wave representing furtiier reduction to an aromatic amine. Hence, the intermediate chemical reaction of quinoneimine formation cannot take place if the duration of microelectrolysis does not exceed 0.2 s_ Changing the drop period of the electrode within the O-I-1.0 s range it was found that, beginning with a drop time of 0.3-0.5 s, the first wave reaches the six-electron level, i.e. the intermediate chemical reaction becomes noticeable. It thus became possible to determine the corresponding intermediate reaction rate constants which were found to be O-35 s-l for p-nitrophenol and 0.40 s-l for p-nitroaniline_ This kind of change of a six-electron wave into a four-electron one with shortening of the drop time of DME has already been observed previously in the course of studying 5-nitrofuryhdenehydrazone [20] _ A similar phenomenon appears also during electrochemical reduction of p-nitroanihne on a rotating disc-ring electrode at higher rotation velocities [ZI] . Influence reduction
of electronic effects on substituents of nitrobenzene derivatives
on parameters
of polarographic
The specific electronic effects of substituents in the nitrobenzene
series
PH
Fig.
6. p,-constant
alcohol 0.25
$ water s),
dependence solutions
for
on pH arxd on alcohol nitrobenzene
(5) DME yit’n t = 3.5 s, (1,5)
HzO,
-series:
(2).10%
content
(I-4)
de.,
using
(3)
in Brittoa-Robinson short-period
20% de.,
(4)
DIME
40%
ak.
(t =
are due to the fact the corresponding EIi3_ v alues of electroredu&on that the electrochemical process takes place at potentials at which considerable adsqrption is observed. A substituerit pgssessing electron donating or acceptor properties, when introduced into a nitrobenzene mofecule, does not simply-shift the Eli2 value towards higher or lower cathode potentials, but is capable of transferring the whole process of electroreductioti into a different region of e.c.c., diff&g substantially in adsorption of the components of the electrode reaction 1223. In order to estimate the E,,, dependence on the structure of the n-electron
upon
system in nifxobenzene derivatives we considered it necessary to test the validity of the _&Iaccoll-Hoijtink equation for aqueous and alcohol + water solutions ofthisreaction series. We have
wherep= %l+1 c
bond
titegral (resonance
= energy coefficients orbS&; = empirical constant.
integral};
of electron
bonding
on the first vacant
molecular
wave A and the corIt was found that the Exj2 values of the four-electron DlMF responding m, +1 values are linearly correlated not only in anhydrous (r = O-99), but also in mixed alcohol + water solutions (0.90 < r < 0.99). The values of the coefficient /3 vary, however, with the nature of t’he medium, being particularly high ‘in the P-region on the (F -1,2 vs. pH) curve and showing mirrimum value in the a-region of these curves and in anhydrous DME’. The points corresponding to m- and p-nitrobenzaldehydes in alcohol + water solutions fall oat. of linear correlation, owing to hydration and semiacetalation of I the aldehyde group. For more detailed study of the effect of various factors on E1,2 dependence on electronic action of substituents we have used the Hammett correlation between E1lz and ox-constants:
where E&
= half-wave
potential
of substituted
nitrobenzene;
E’?,z = half-wave potential of nitrobenzene; PX = susceptibility constant of electroreduction
Elj2 towards polar effect of substituent Xi; ox- = Har&mett constant characterizing polar effect-of subsfZtx.ient. The fi@ear~corret_atiotibetween the E1,2 va.lues~ancJ ox-constants in the
I
hofds wittin &he &hole $3 range studied (2-~12)~and for Cdmpos?tion$ of _the aqueous -&ohol. ml%xn& The Corr&tion coefficients._ -. : differ, however, for &rious conditions :[0.90 < % i 0_99)-.
: nitrdbefizem-sties; afl
-
.. .--
:
_
..
. -.
647
Pig_ 7. p,-constant dependence of one-electron reduction wave on dielectric constant of the medium for nitrobenzene derivatives: (1) Britton--Robinson buffer, pH 12, 0% ale_; (2) acetonitrile; (3) DMF; (Cr-11) Britton--Robinson alcohol + water solutions (10,20, -?10,50,6O,T70,80,96.5% ale_).
The p,-constant for the nitrobenzene series has a sufficientl$ high positive value in all cases which speaks in favour of nucleophilic addition of electrons as potential-determining stage of the electrochemical process. Similarly to the coefficients of the (EI12 vs. m,,,) dependence the values of the p,-constant are in rather complex relation to pH-value and alcohol content (Fig. 6). It appears that electronic effects of the substituent have an influence not only on the electroreduction pot%tials, but also on the constants of preceding protonation, i.e. on the AE,,,/ApH coefficients. Indeed, we have a linear correlation between the values of (EIIP/pH) and Hammett ox-constants in the P-region. With increase in alcohol content we have a narrowing of the acid medium region on the (p, vs. pH) graph where pn does not depend on pH, and the peak of the prr vs. pH curve shifts towards lower pH values. The following interpretation of the pT vs. pH curves is suggested: - in strong acidic medium in which the protonation rate of particles entering into electrochemical reaction is sufficiently high for all members of the series, the pX value depends but little on pH, since the protonation rate does not affect the prr value,_ and electron transfer forms the potential-determining stage; - a change from aqueous to alcohol t- water solutions leads to decrease of protonation rate, the region of constancy shifts towards lower pH values, and vanishes at ail pH values tested in ‘&se of 40% alcohol; - on the rising branch of the (pn vs. pHj curve the value of Ellz and, accordirigly, that of -px is determined by the protonation rate of the nitro group. -With decrease of electron-acceptor properties of the substituent Xi the protonatiofi rate of the n&o group ought to increase, but the electron transfer r&e to diminish. Thus we-g& an increase in the value of the pl,-constant, owin.$to the increased contribution of the protonation kinetics of the oxidized form;
043
.- the reverse holds for the falling branch of the (p,,-vs. pH) curve: the value of px is determined by the protonation rate- of the anion-radical the rmmber of which diminishes with decrease of protonation raf~ of the reduced form; -the shift of the (ok vs. pH) curve peak towards lower pH v&h& with increase in alcohol content in the solution may be due to a decrease in absorption of depolarizer particles and a fall in their protonation rate; - in the region of pH values where we have addition of an electron to the unprotonated particle, the. pX value becomes constant snd independent of pH which lies approximately within the range of [email protected] _.. +0.30 V. On a short-period DIME desorption of molecules takes pf&e, and protonation is considerably slowed down. This leads to more pronounced changes of p,-value in media with high alcohol content, than in aqueous solutions. With usualpolarographic capillaries we have less pronounced p,-dependence on pH, 02 else this depenpence is linear. Similar phenomena of electronic effects have been observe&in our previous work on the 5-nitrofuran series [23,24]. It may therefore be assumed that they are sufficiently general for electroreduction of organic nitro compounds. The above mentioned character&tic features of electronic effects of substituents on potentials and mechanism of electroreduction of nitrobenzene and 5-nitrofuran derivatives are caused by electrochemical processes at adsorption poterdids of depolarizer molecules_ Adsorbance decreases as EXi2 values shift away from maximal adsorption potentials towards the positive or negative branch of the electrocapillary curve, as well as with displacement of adsorbed depolarizer molecules from the electrode surface by alcohol molecuies. En the nitro compound series under conside&ion substitution by electron donor or electron acceptor groups does not simply shift Elia values towards higher or lower cathode potentials, but transfers the whole process of electroreduction to a-different region of the electrocapillary curve, differing considerably in it& adsorba.&e of the components of the electrode tea&ion. This inevitably affects the parameters of electron addition and of protonation reactions on the surface, In our c&se desorption of depolarizer molecules lowers the protonation rtite,_which leads to more pronounced pH effect-upon prconstant in media with-high alcohol content, as compared to aqueous soluticns- ft may be noted that the p,-cons+;arrt of the nitrobenzene series increases linearly with increase in the dielectric.constant. of the medium [254. The latter circumstance is due to the fact that a high dielectric constant of the. medium favours -formation of nitrobenzene or primary Bnion-radical solvates w&h solvent molecules. This leads to lowering of “ionic” properties of ..the~electrochemlical reaction-product with respect to the initial molecule. --.Withincrease in dielectric-con&& of _the medium we get weakening of electrostatic interaction between substituent Xi and the nitro group. In mixed alcohol + water solutions this dependence of px on dielectric con-. &ant has: been observed to hoki tie witI& ~c_ertainlimits, titil hypersolvation effects-set in [Fig. 7). :
649 REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
J. Pearson, Trans. Faraday Sot., 44 (1948) 683. D. StoEesova, Coil., 14 (1949) 615. B. Kastening and L. Holleck, 2. Elektrochem., 64x1960) 823. L. Holleck and B_ Kastening, Rev_ Polarogr. (Japan), 11 (1963) 129. M. Heyrovsky, S. VavriiSka, L_ Holleck and B. Kastening, J_ Electroanal. Chem., 26 (3.970) 399. M. Person, Bull. Sot_ Chim. France, (1966) 1832. ES. Levin, Dokl. Akad. Nauk SSSR, 151 (1963) 1375. J.P. Stradins, Polarography of Organic Nitrocompounds (in Russ.), Izd. Akad. Nauk Latv. SSR, Riga, 1961. J.P. Stradins and 1-J. Kravis in -4-N. Frumkin (Ed.), Electrochemical Processes with Participation of Organic Compounds fin Russ.), Nauka, Moskow, 1970, pp. 81-89. J.P. Stradins and I.J. Kravis_ Latv. PSR Zinat. Akad. Vestis, Kim. Ser., (1971) 572. S.G. Mairanovskii, J.P. Stradins azxd I.J. Kravis, Elektrokhimiya, 8 (1972) 784. L.H. Piette, P. Ludwig and R.N. Adams, J. Amer. Chem. Sot., 84 (1962) 4212. W. Kemula and Z. Kublik, Bull_ Acad. Polon. Sci., Ser. Sci. Chim. Geol., Geogr., 6 (1958) 653. L. Hslleck and D. Becher, J. Electroanal. Chem., 4 (1962) 321. S.G. Mairanovskii and J-P. Stradins, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, (1961) 2239. S.G. Mairanovskii, J. Electroanal. Chem., 4 (1962) 166. J_P_ Stradins and I.J. Kravis, Lafx. PSR Zinat. Akad. Vestis, Kim. Ser., (1971) 5@0. G-S. Alberts and I_ Shain, Anal. Chem., 35 (1963) 1859_ J.P. Stradins and 1-J. Kravis. Latv. PSR Zinat. Akad. Vestis, Kim_ Ser.. (1974) 374_ J.P. Stradins and G-0. Reihmanis, Latv. PSR Zinat. Akad. Vestis, Kim. Ser., (1969) 377;
21 22 23 24 25
(1972)
180.
B.G. Podlibner and L.N. Nekrasov, Elektrokhimiya, 7 (1971) 1191. J.P. Stradins and 1-J. Kravis in A.N. Frumkin (Ed.), Electrochemical Processes with Participitation of Organic Compounds, Nauka, Moskow, 1970, pp_ 110-116. J. Stradins and S. Iiillers, Tetrahedron, 20, Suppl. 1 (Nitro compounds) (1964) 409. J.P. Straditis and G;O_ Reihmanis, Elektrohimiya, 3 (1967) 178. J.P. Stradins, Latv_ PSR Zinat. Akad. Vestis, Kim_ Ser., (1968) 633.
in The Netherlands
PECULIA&ITIES OF ELECTROCHEMICAL NITRO COMPO-S ON SHORT-PERIOD ELECTRODES*
REDUCTION OF AROMATIC DROPPING MERCURY
J_ STRADINS and I. KRAVIS Insfitite of Organic Synthesis, Latvian
S;S.R.
P-cademy
of Sciences
of the Laivian S.S.R.,
Riga,
(US.S.R_)
(Received 1st August 19’75)
The electrochemical reduction of nitrobenzene and its derivatives on the dropping mercury electrode has been the subject of numerous studies dealing with various aspects of the process [I-14] _ This reduction is known to be strongly dependent on the nature of the medium and on conditions under which the process on the electrode is effected. The first stage of electroreduction of nitrobenzene
in aqueous
solution
consists
lh a four-election
reduction
to N-
phenylhydroxylamine [l--5], but in aprotic media, in t’ne transfer of one electron with formation of an anion-radical [X2,14] _ There exists, however, also an intermediate and up to now practically not studied region (cf. ref. 7), that of transition from the four-electron to a one-electron mechanism. Polarographic curves of aromatic nitro compounds obtained in this region permit us to gain an insight into the sequence and relative rate of separate stages of electrochemical and chemical reactions in the summary process of nitro gro=upelectroreduction, the latter being treated as a consecutive electron and proton transfer to the molecules and anion-radicals of nitro compounds adsorbed upon the electrode surface [ 3,4,9,10]. The kinetics of electrochemical reactions and protolytic surface processes can be particularly clearly observed under conditions of short duration of microelectrolysis, viz- on short-period dropping electrodes. Unlike capillaries usually employed in polarography and possessing a drop time t between 3 and 6 s, short-period electrodes (t = 0.2 to 0.5 s) possess considerably lower adsorbance of organic depolarizers and more pronounced effects of change in adsorbance with changes in medium properties and in structure of the derivative under study. This makes a short-period DME, first used for this purpose in ref. 15, a convenient means for studying the separate stages of electroreduction of aromatic nitro compounds_
* In_.honoukof Academician A.N. Frumkin’s -80th birthday_
636 EXPERTiMEMTAL
The compounds studied were nLtrobenzene &d 14 of its m- and p-substituted derivatives (formulae 1 and 11). The parameters df polarographic reduction waves obtained with these substances were studied under different conditions, the latter including varying composition of organo-aqueous mix&e, pH values, depolarizer concentration, buffer capacity, iotic strength, duration of microelectrolysis (drop time).
02NvXi
x; = H;
NH2;
X; =H;
CH3.
OH,
CHO;
COOH;
NH*; OH;
Cl,
NC&
Br, 1;
COOH;
(11
CHO;
CN
(II)
X,
The work was performed with a short-period DWX fitted with a special device for forcing the break-off of the mercury drop (by means of little blade fused to the end of the capillaryj. The drop-time was t = 0.25 S, with a massrate of m = 0.81 mg s- I_ The effect of changes in the dropping period on polarogram pammeters was studied on a special device (fitted with a micrometric screw), permitting a change of t from 0.2 s to 3.0 s, the mass rate of mercury flow remaining constant and equal to m = 1.4 mg s-l_ Variation of dropping period was effected by changing the distance between the end of the capillary and the movable glass fibre, the “blade”. A saturated calomel electrode was used as anode. The electrolyser consisted of a thermostatically controlled ceil, The electrode potential was measured by the compensation method using a three-elec%rode potentiometer circuit_ Measurements were at 25.0 t-‘015°C. Argon flow was used for deaeration of the solution. The heights of the_ polarographic waves were corrected by subtrdcting the current corresponding to the background electrolyte. Polarographic studies-were c-ied out in aqueous and alcohol + water uniVeYsal Britton~Ro‘biuson *buffer sol&ions (pH values 2-12) with ethanol content-from- 0 to 40 vol %.-II% case of fiecessity other buffer solutions with a narrower range of pH value were also used, as well as anhydrous dimethylform&de (DMF-)_ T&S a.koh~l~+~ water solutions were prepared ticcording to. requirement, with -variouS depoiarizer (r?)-and aloohol contents, with various pH values, ionic strengths
.-
637
&zmt values. Correlational statistical analysis methods, were used for procek-fg these data. . The po~arograms were recorded in integral and different% form on a LP-60 polaroglaph.
PoEarographic behaviuur of nitmbenrene akohol -i-water buffer sulattions
and its derivatives in aqueous and
The short-period DME (Fig. I) produces in aqueous medium only a fourelectron reduction of nitrobenzene and its derivatives to N-phenylhyclroxylamine (wave A). The Ella dependence (Ella values in the 43.2 to -0.8 V range) of the corresponding wave on pH value can be represented by an Sshaped Curve which we divide into three regions, or, /3, and y respectively, characterized each by different values of coefficients A_E,&ApW and semilogarithmical slopes (Fig, 1). In media with higher alcohol content a a-region also appears (Fig:, 1). Table 1 contains AE If2/ApH values for a, _Oand Cregions, as well as pH values corresponding to the break-points of slope change on the E,@--p H curve and showing the transition from the a-region to the P-region (p&3), from 0 to 6 (p&j), from /3 or S to y fpK,). The AElrz/ApH value in the y-region equals zero. Divergences of AZ I,z/ApH values from the theoretical value of -59 mV/pH unit in the case of organic partic?e reduction are characteristic for processes involving adsorbed particles cl6 j . Non-equal AE,,,/ApH values determine the S-shaped form of the E,,,-pH curve. In 10% afcohol + water solution, at pH 5 a two-electron wave is additionally
f
I
i
I
I
I
I
i
5.0 PH
I
I
r
,
10.0
PH
Fig, I. Dependence of. firiziting current txu~stant (r = &&%~~~f”~ f and of El12 on pH for p-nitroto&.xene cm&or&-period DME in ~.queous and akohul + water Britton-Robinson buffer sdlutipy& (I) 0, (2) 10, (3) 20, (4) 40% ah (X,2,3,4) -4’ wave, (3’, 4’) A” wave,
(2”,3”,4”)
B wavel
,’
.’
;’
:I
,’
,’
,,’
,,’
5,.
‘,
1,,
_,
‘I
,tj
8;
7’
,,
p.1 :’ ‘p:Br ‘,
,;@-
” ::POH
po- :
”
60 “o y
GO,
6O
48
ip:C,HO
; /; .,I14~ ‘.‘$CN’ ’ ,;,,,lb’: ;,,,1n-N92, ‘,’ ,,_~ ‘,,‘I ,,‘, ,’ I’ ,, /I ‘,, ,I,, ‘: I ,,’ ,,.“.
,’ ,:m’13,; I,,;
,“,I,,
: :: 12,, ‘, rwCHO ) ,’ ‘, ‘,
,,
:, ,tO.“” ” ?!,$6 +O.?L
.+0.3’(j
30 1 .;17
“43
po
“+0,03G ‘, ,‘I 11;‘,,,,, ,” tf~~cooI~I, I~CO’O” -0.io’ ,I,“0
+0.23, +0.‘26 0’
-0.37. -0.d2. +0.18 ; ,-co.23,,
0
.I
,,
5,O 4,3 Sal
72
-
4,7 -
8,7
0,4 $8
$6
60
46 30
50
(4G)
53
(3,O) 10,2
36 46
61
44
9.0
44 (60)
B,6 -
10.4
0,2
9,o
8,8 ‘(40)
67
67 67
110
GG
110 66 76
120 78 74
3,8 4,0,
6.8,
-
3.4
4,G
G,O
4.6 4.0
7-3
6.0
3.1
93
3,2 4,0 4,o
(0.050)
90
88 83
--
-
-
I
-
-
-
-
-
i
-
-
(8,0) -
9.7 9.8 8.4
10.4
--.C_r__-
---I
buffer fiolutions
---“_l__
6.6
8,l
9.8
,D.2
9.4
7.6
9.0
8.7
9.5
and nlcohol + water Rritton-Robinson -‘-----------7
!O% C2H50H --_‘-l.-,
10,2
4,l
6.7
4,o 7,7
74 67
40
74
(62)
70
‘73
120
7o
wo
(4.6)
70
62 12i 1iS
4.7 4,3
‘73 80
.a.-,
--_
on p~l,(mV/pH unit) Pm nitrobehacnc dcrivativos In n~cous
0% CjHgx
fill2 dC&denco
,G’: :m:OH
“,
‘. ‘,, ., ) : : 1 ;:j’ (1’&Cl :’ ,: ” : $,!a, ~p?CQOII ” ‘I’ “, ~J)‘~O~~,
/’
8’
,I
3,
:
,,‘,
&khf&nts,dk
:,l,,;H,‘:,
,‘,I,’ /,
,‘/,’ ,,
.’
,,
,‘,,’
.‘I
,’
‘I”
‘,,
‘,y.wmy+“--*;ll”.----~ ,, ” ,yl’i’I ’ ax I,,‘, ., ?i .‘,
/
-,,,:”
II.
TABLE’i.
,’ ,’ ;,,
‘I.,,,,
‘;‘,
,,
:,, / i
,g
’
‘/
,,
,”
_
,,’
,,
3,
'1, 2,
,-
.,,
:
H ‘-’ JkH3’ ttt-NH2
-a16
,‘O -0J-7
OX
;
.,
’
p42OOH
10
,:’
12
11
.I)-CHO
tn.COOH tt1.C‘oottbCH0
‘IJ$oo’
,’
-
,’
,PCN ,'.15' vi-NOi
‘13 ,,14,
‘, :
,/
I ','p-q,. 7,M ,' ,!f. e_Br,. ,' 9' pa
,p.OH’
ttr.o-
‘6,‘.
”
to,45 +0,66 to.71
+0.23 +0,26 0 MI.036 -0,lO to,36
-0,71 -0,37 -O,G2 +0,18 +0,23
93
28
(42)
G2
(G6)
20
46 48
31 08 GD
ll;O
lG0
90
-
GO
-
109 04 104 69 no 80
(38) -,
-
93
(53)
G4 63
88
102
-_
Gl
-
20% C$$OEI
of E 1’12dependence on pH (mV/pH unit) for nitrabenzcne
', rn-NFI; -to,63 ,' .', 4, P*NHz, --OS6 _' " 'b-NH3 ,," ; ,'I, 5 WOH +0.1’3
1,
,’
.
‘,,
TiBIhi.1 (continued)’
‘/ .;, ,,
:
,,
.,C?officihs “,,, ,I” Xi ,, .., : ‘,,
‘”
,,,
-
-
-
-
-
-
G.8 6.7
4.4
,,,
3.2 3.9
3.3
-
-
G,G
-
-
4.b ‘4,0 -
4.3
3.1 6.0
3,l
(3.6)
-
3,O 3,8
-
-
-
-
68
40
,.
8,G 884 36 a.9 34
9.2
8,G -
(58)
(30)
8-6 6.7
(43) (43)
3,9 9.1
9,G -
10.1
73
8.8
8,G
9.0
40% C2H50H
-
100
86 a4
62
170
160
70
3G
-
-
-
6,4
(46)
-
-
40
130
140 6B 32 90
76
68
50 46
130
3.03
100 106
-
388 4#1
-
4.9
5.7
-
7.0
-
5.0
4.4
6.0
6.9. G.3,
7.6 7.7
9.6
8.8
7,4
7.0
9.8
9.0 9,0
lo,2
10,l
7.4
,8.0
8.2 8.3
sdlutions
-
400
300
332
440
4,o 4,o
..”
380 3,6
b&x
__.--_.;_^
derivatives in aqueous and tic01101+ water Brittan-Robinson --
c (0
.,.,, ._
observed, ide.to
dorrespor,&2g- to reductiqn of the correspond~g.M-pheny~~_drox~~ obta.&d froti.nitrob&iene derivatives qapabl@-of losing -protons at high_pH v.alu&s-with&&f&mation of anions (X = COO& OH), is 5 splitt&g of the t&al four-eJec-i&n ‘zf7iGe.A into a one-electron and a ttiee&lectron wave in ILOT?alcohol -i; water solution _ In mixed media with higher alcohol content the sazlie wave is observed as in 10% alcohol -i-v!ater solution, except that the EI13 value at the c’orresponr ding pff values shifts towards higher cathode potentials, znd the limitmg current (ilim) diminishes-with increase in alcohol content. The product is qzi2 (q being +,heviscosity of the solution) diverges from the expected constant wzhxe, +hich may be due, presumably, to chang& in the salvation sh&ath of the depolarizer molecules. In case of nitrobenzene compounds which are not capable of losing protons (XL = H; p-CHs; p-NH~; m-NH2; p-CE\i) splitting of the four-&c&-on wave A intc; one- and- three+fectron waves Ar and Au occurs in media with higher alcohol content- The waves of nitrohenzene, p-nitroto_luene, p-nitroaniline and p-cyzmonitrobenzene split in 2070, the m-nitroaniline wave in 40%, and the p-nitrohalogenobenzene wave in 95% alcohol + water solution. Only in the case of G-Z-and ,p-nitrobenzaldehydes did none of the alcohol concentrations tested produce any splitting. An increase of &cohol content of the aqueous solution causes a shift of the value towards higher negative potentials, whilst the initia,l points of the y-regions of the E If2-pH curves shift towards higher positive potentials. At the same time we observe an increase in (A.E,,2/ApH) value, and the Sshape oftheElrz -pH curve becomes mere pronounced_ Ia the transition area between the four-electron and one-electron reduction region the _4_rwave is of kinetic nature which manifests itself in the dependence of fri, on buffer capacity (cb) {with increase in cb there is an increase in height of tbr A’ +zTe owing to increasing protonation rate of the z&on-radical) [11-J. The temperatyAre coefficient values are abnormally low for kinetic waves .(2.6-2.8% per “C) which indicates that we are dealing with kinetic “surface”waves. In anhydrous D&W all nitrobenzene derivatives studied exhibit a well defined one-ele&Qon wave corresponding to anion-radical formation, and a three-elect--on wave correqonding to further electroreduction of the anion-radical to N$henylhyc?.roxylamine_
anilibe (mm&B). A peculiari@ of~po’l~ograms,
$&i
Effect of adsorption of molecules pu&&-b~#pZri~ deduction
of aromatic
n&-o compoiuzds
on their
EJ&tr&e&&on of the-arornztid titio group proceeds ti v&.&y of electiocapillary zero-point (-3-4 to O-6 V) where m aximum adsorption- of organic moiecrrles occurs. kdsorpti&>. of nitrobenzene zm;dits &zrivtitivas on. the sur.face of $he_BME n&mfests~itseiL’ through a lowering df the ele&roca@iBey C&T& .-
.-
_
641
in the p&$&e of tfie nititibeniene derivative, as compared to base electrolyte solution (-Fig. -2). Depending on mole&.&r structure and medium conditions nitrobenzene tid ‘its derivatives are redu_ced at potentials corresponding to the positive branch of the electrocapill-&y curve, at zero charge potentials, or at potentials of the initial part of the negative branch. In case of absence of extraeous surface-a&iv& substances (s.a.s.) in aqueous or organo-aqueous media adsorption- of depolarizer molecules causes in all cases mentioned above a high surface concentration .of reducing particles, which is necessary for “surface protonation”, However, under certain conditions the-kinetics of adsorption and subsequent protonation starts limiting the rate of the electrode reaction as a whole POIInhibition ofpolarographicreduction
surfacereactions
by the aromatic
nitro group can be effected: -by increase of organic component content in the organo-aqueous solution; - by addition of extraneous s.a.s. [3,6] ; - by shortening the drop time of DME; - by changing the electronic structure of the molecu!e (through electrondonor substitution)_ Use of alcohol + watersolutionsinstead of aqueous onesreveakthe specific effects produced
by adsorption
of alcohol molecules
on the electrode,thus
displacing adsorbed nitro compound molecules: with increase of alcohol concentration Eli2 shifts towards higher cathode potentials; the semilogarithmic slope b of the polarographic wave and the coefficient AJ3112/ApH change in a
0e
10
5 PH
Fig.
2_ Electrocapitfary
curves,obtainedin
solution at pH 5.4; (I) background in presence of m-nitroaniline1
curve;
40%
(2)
alcohol+ water Brition-Robinson curve in presence of nitrobenzene;
Fig. 3. E~~~.depetidence on p&I for nitrobenzene in tqueous ,md alcohol + water Robinsoti soiutiotis at varions DME dropping.periodL: (l,l’.‘l”) f = 4.2 s, (2,2’,2”) (3,3’,3”) f = 3 s, -( 1-3) H,O; (I’--3”) 40% ale.
buffer
(3)
curve
Brittos0.8 s,
642
rather complex manner with incr&se of a.&ohol. content, the S-shape of the _E 112-pH curve being enhanded.~ A specific effect of nitro compound molecule desorp&n from the electrode surface with l?ncrease of aJcoho1 content is $he splitting of the four-electron wave in media possessing low protogenic activity. This splitting is due to lowering of proton activity of the solution, as well as to desorption of the anionradicals formed from the electrode surface, the latter process becoming more marked with increasing distance of electroreduction potentials from electro-capillary zero-point towards higher cathode potentials_ yn- andp-nitrobenzaldehydes which are reduced at potentials lying on the positive part of the e.c.c., do not produce a split four-electron wave. Addition of high alcohol content to aqueous solutions produces an effect on nitrobenzene electroreduction similar t.o that of such surfactants as camphor and tylose, as earlier described by Holleck and Kastening f4]. The $3 dependence of E,/, is S-shaped even in complete absence of surfactants, provided the following conditions are observed: - organo-aqueous solutions have to be used instead of aqueous ones with sufficiently-high non-aqueous component content. Increase of alcohol content raises the value of the AEI12/ApH coefficient and, consequently, enhances the S-shape of the _&,s-pH .curve; - the molecule is substituted with an electron donating group; - the duration of microelectrolysis has to be reduced by shortening the drop time of DME. An essential point is “straightening out” of the S-shaped (EII, vs. pH) curve with lengthening of microelectrolysis t.ime (Fig. 3). The effect of dropping period change upon the (E,,, vs. pH) curve is more pronounced at high alcohol content and in case of such substituents of nitrobenzene-which are capable of electrolytical dissociation (-COOH; -OH). The S-shape of the (E1,2 vs. pH) curve at small values of t and the “straightening out” of this curve with increase in t [17] can be explained by the fact that in the course of polarographic reduction of nitrobenzene and its derivatives only particles adsorbed at the surface of the electrode are involved in the electrochemical process and protonated before electron transfer_ On a short-period electrode (I: < 1 s) surface adsorption equilibrium cannot get established in -. time< hence the Erinetics of adsorptidn and subsequent protonation sets certain limits to the rate of the electrode process as a whole, thus causing the jump on -the (E,,, vs_ pH) curve. ~Effect of buffer cbpucity onpofarographic
reduction
ofaromatk
nitro com-
pOi..L?tdS
Ir_the transition part from the four-electron to the one-electron kegion of reduction ihe A’ wave assumes kinetic nature which affects also the dependence of iErnon buffer capacity cc& -Eli] _ The dependence -of im %nd of E1/2 on cb ,c&n tie observed in weak alkaline-and alcohol + Water soluti_ons (Fig; _4)_
.643
0 -2.0
““““‘1
051 -1.0
0
19 Cb
I
-2.0
I
I
I
I
-1.0
I
I
I
I
I
0
‘9 Cb
Fig-Q_ El, and ilim valuesofthe Awave(A'and A”) fornitrobenzene,as plotted against buffer capacity ofalcohoS+ water solutions atpH7.45: (1)veronaI buffer,1070 ale.,(2,s) Verona1 buffer , 40% alc_,(4,5)borate buffer,40% ale.
With increase of buffer capacity the Q2 value ofthe four-electron wave shifts towards positive potentials (mllZ/Alog cb = SO-100 mV). In the region
of splitting
of the four-electron
A
wave A into a one-electron
and a three-electron wave A’ and A”, the Elj2 value of the A’ wave does not depend on c,, whilst that of the A” wave shifts towards positive potentials with increase in cb_ Decrease in cb causes splitting of the -4 wave into A’ and A”, similarly to the effect produced by increase in pH value. The semilogarith- i)--E: curves corroborate the reversibility of mic slopes b on the log(i/i,, one-electron reduction corresponding to the-A’ wave (b = 60 mV) and irreversibility of the A" wave (b # 60 mV). The E1,2 and ili, dependence on c, confirms the fact that in acid medium the protonated nitro group is reduced, whereas in neutral medium the reversible transfer of the first electron is followed by reduction of the protonated anion-radical. Effectofnitro
compound
concentration on itselectroreductionpotentials
On the surface of the DME the 4e-reduction product - N-phenylhydroxylamine - accumulates as well. Accordingly, the effect of increased depolarizer content, on the one hand, and of addition of extraneous N-phenylhydroxylamine, on the other, upon the A wave parameters (or, correspondingly, upon those or”&he A’ and A” waves) of nitrobenzene has been studied within the concentration range between 5 X 10e5 and 2 X 10B2 mol l-’ and under various experimental conditions. In both cases simile effects were observed. With increase in depolarizer content
-644
0.70, 0.65 I
L
d
2-o-o-~Q
2
O_
n 50
Q50
/* ,
10-0-o I I I
1 1 f
/ 1 1 I
-4
I
-3
I
I
!
1 -2
Fig_ 5_ E1,2 values of uitrobenzene A wave, as plotted against depolarizer content in aqueous and alcohol + aqueous Britton-Robinson buffer solutions, on short-period D-ME: (1) acetate buffer, pi!! 4-8, 0% ale_; (2) Britton-Robinson buffer, pH 12. 0% ale.; (3) B&ton-Robinson buffer, pH 12, 40% ak. {A’ wave); (4) Britton-Robinson buffer, p&I 12, 40% a.k. (A” wave); (5) Sritton--Robinson buffer, pH 6.4, 0% ale. (nitrobellzene mol 1-l ; M-phenylhydroxylmine added)_ content I X 10” _Pecutiurities DME
of
p-nitruphenol
and p-nitroaniline
reduction
on short-period
Polarographic reduction of nitrobenzene and its derivatives in aqueous solutions is usually represented by a four-electron process which in acidic media is followed by an‘additional two-electron process (in form of a separate B-wave). Only a few o- and p-substituted nitrobenzene derivatives fnitrophenols, nitroanilines and structurally related compounds) are reduced by means of a specific six-electron mechanism. The latter may be due to the possibility of dehydration.of the corresponding aromatic N-phenylhydroxylanxine with fonnat.ion
of e.&ly
reduceable
qukoneimine
structure:
645
The intermediate cherrtical reaction of change from p-hydroxy- or p-aminoN-phenylhydroxylarnine into p-quinoneimine (or into p-quinonediimine) is catalysed by acids and bases and has a rate of the order of I-10 s-l [ 18]. Hence, this-reaction can be successfully effected within the life-time of the mercury drop with ordinary polarographic capillaries (t = 3-6 s). We could show [I93 that in case of a short-period DME (t = 0.20 s) no such continuous six-electron wave is observed in the electroreduction of p-nitrophenol and p-nitroanihne: we have here a separate four-electron wave corresponding to reduction to the respective N-phenylhydroxylamine and a separate wave representing furtiier reduction to an aromatic amine. Hence, the intermediate chemical reaction of quinoneimine formation cannot take place if the duration of microelectrolysis does not exceed 0.2 s_ Changing the drop period of the electrode within the O-I-1.0 s range it was found that, beginning with a drop time of 0.3-0.5 s, the first wave reaches the six-electron level, i.e. the intermediate chemical reaction becomes noticeable. It thus became possible to determine the corresponding intermediate reaction rate constants which were found to be O-35 s-l for p-nitrophenol and 0.40 s-l for p-nitroaniline_ This kind of change of a six-electron wave into a four-electron one with shortening of the drop time of DME has already been observed previously in the course of studying 5-nitrofuryhdenehydrazone [20] _ A similar phenomenon appears also during electrochemical reduction of p-nitroanihne on a rotating disc-ring electrode at higher rotation velocities [ZI] . Influence reduction
of electronic effects on substituents of nitrobenzene derivatives
on parameters
of polarographic
The specific electronic effects of substituents in the nitrobenzene
series
PH
Fig.
6. p,-constant
alcohol 0.25
$ water s),
dependence solutions
for
on pH arxd on alcohol nitrobenzene
(5) DME yit’n t = 3.5 s, (1,5)
HzO,
-series:
(2).10%
content
(I-4)
de.,
using
(3)
in Brittoa-Robinson short-period
20% de.,
(4)
DIME
40%
ak.
(t =
are due to the fact the corresponding EIi3_ v alues of electroredu&on that the electrochemical process takes place at potentials at which considerable adsqrption is observed. A substituerit pgssessing electron donating or acceptor properties, when introduced into a nitrobenzene mofecule, does not simply-shift the Eli2 value towards higher or lower cathode potentials, but is capable of transferring the whole process of electroreductioti into a different region of e.c.c., diff&g substantially in adsorption of the components of the electrode reaction 1223. In order to estimate the E,,, dependence on the structure of the n-electron
upon
system in nifxobenzene derivatives we considered it necessary to test the validity of the _&Iaccoll-Hoijtink equation for aqueous and alcohol + water solutions ofthisreaction series. We have
wherep= %l+1 c
bond
titegral (resonance
= energy coefficients orbS&; = empirical constant.
integral};
of electron
bonding
on the first vacant
molecular
wave A and the corIt was found that the Exj2 values of the four-electron DlMF responding m, +1 values are linearly correlated not only in anhydrous (r = O-99), but also in mixed alcohol + water solutions (0.90 < r < 0.99). The values of the coefficient /3 vary, however, with the nature of t’he medium, being particularly high ‘in the P-region on the (F -1,2 vs. pH) curve and showing mirrimum value in the a-region of these curves and in anhydrous DME’. The points corresponding to m- and p-nitrobenzaldehydes in alcohol + water solutions fall oat. of linear correlation, owing to hydration and semiacetalation of I the aldehyde group. For more detailed study of the effect of various factors on E1,2 dependence on electronic action of substituents we have used the Hammett correlation between E1lz and ox-constants:
where E&
= half-wave
potential
of substituted
nitrobenzene;
E’?,z = half-wave potential of nitrobenzene; PX = susceptibility constant of electroreduction
Elj2 towards polar effect of substituent Xi; ox- = Har&mett constant characterizing polar effect-of subsfZtx.ient. The fi@ear~corret_atiotibetween the E1,2 va.lues~ancJ ox-constants in the
I
hofds wittin &he &hole $3 range studied (2-~12)~and for Cdmpos?tion$ of _the aqueous -&ohol. ml%xn& The Corr&tion coefficients._ -. : differ, however, for &rious conditions :[0.90 < % i 0_99)-.
: nitrdbefizem-sties; afl
-
.. .--
:
_
..
. -.
647
Pig_ 7. p,-constant dependence of one-electron reduction wave on dielectric constant of the medium for nitrobenzene derivatives: (1) Britton--Robinson buffer, pH 12, 0% ale_; (2) acetonitrile; (3) DMF; (Cr-11) Britton--Robinson alcohol + water solutions (10,20, -?10,50,6O,T70,80,96.5% ale_).
The p,-constant for the nitrobenzene series has a sufficientl$ high positive value in all cases which speaks in favour of nucleophilic addition of electrons as potential-determining stage of the electrochemical process. Similarly to the coefficients of the (EI12 vs. m,,,) dependence the values of the p,-constant are in rather complex relation to pH-value and alcohol content (Fig. 6). It appears that electronic effects of the substituent have an influence not only on the electroreduction pot%tials, but also on the constants of preceding protonation, i.e. on the AE,,,/ApH coefficients. Indeed, we have a linear correlation between the values of (EIIP/pH) and Hammett ox-constants in the P-region. With increase in alcohol content we have a narrowing of the acid medium region on the (p, vs. pH) graph where pn does not depend on pH, and the peak of the prr vs. pH curve shifts towards lower pH values. The following interpretation of the pT vs. pH curves is suggested: - in strong acidic medium in which the protonation rate of particles entering into electrochemical reaction is sufficiently high for all members of the series, the pX value depends but little on pH, since the protonation rate does not affect the prr value,_ and electron transfer forms the potential-determining stage; - a change from aqueous to alcohol t- water solutions leads to decrease of protonation rate, the region of constancy shifts towards lower pH values, and vanishes at ail pH values tested in ‘&se of 40% alcohol; - on the rising branch of the (pn vs. pHj curve the value of Ellz and, accordirigly, that of -px is determined by the protonation rate of the nitro group. -With decrease of electron-acceptor properties of the substituent Xi the protonatiofi rate of the n&o group ought to increase, but the electron transfer r&e to diminish. Thus we-g& an increase in the value of the pl,-constant, owin.$to the increased contribution of the protonation kinetics of the oxidized form;
043
.- the reverse holds for the falling branch of the (p,,-vs. pH) curve: the value of px is determined by the protonation rate- of the anion-radical the rmmber of which diminishes with decrease of protonation raf~ of the reduced form; -the shift of the (ok vs. pH) curve peak towards lower pH v&h& with increase in alcohol content in the solution may be due to a decrease in absorption of depolarizer particles and a fall in their protonation rate; - in the region of pH values where we have addition of an electron to the unprotonated particle, the. pX value becomes constant snd independent of pH which lies approximately within the range of [email protected] _.. +0.30 V. On a short-period DIME desorption of molecules takes pf&e, and protonation is considerably slowed down. This leads to more pronounced changes of p,-value in media with high alcohol content, than in aqueous solutions. With usualpolarographic capillaries we have less pronounced p,-dependence on pH, 02 else this depenpence is linear. Similar phenomena of electronic effects have been observe&in our previous work on the 5-nitrofuran series [23,24]. It may therefore be assumed that they are sufficiently general for electroreduction of organic nitro compounds. The above mentioned character&tic features of electronic effects of substituents on potentials and mechanism of electroreduction of nitrobenzene and 5-nitrofuran derivatives are caused by electrochemical processes at adsorption poterdids of depolarizer molecules_ Adsorbance decreases as EXi2 values shift away from maximal adsorption potentials towards the positive or negative branch of the electrocapillary curve, as well as with displacement of adsorbed depolarizer molecules from the electrode surface by alcohol molecuies. En the nitro compound series under conside&ion substitution by electron donor or electron acceptor groups does not simply shift Elia values towards higher or lower cathode potentials, but transfers the whole process of electroreduction to a-different region of the electrocapillary curve, differing considerably in it& adsorba.&e of the components of the electrode tea&ion. This inevitably affects the parameters of electron addition and of protonation reactions on the surface, In our c&se desorption of depolarizer molecules lowers the protonation rtite,_which leads to more pronounced pH effect-upon prconstant in media with-high alcohol content, as compared to aqueous soluticns- ft may be noted that the p,-cons+;arrt of the nitrobenzene series increases linearly with increase in the dielectric.constant. of the medium [254. The latter circumstance is due to the fact that a high dielectric constant of the. medium favours -formation of nitrobenzene or primary Bnion-radical solvates w&h solvent molecules. This leads to lowering of “ionic” properties of ..the~electrochemlical reaction-product with respect to the initial molecule. --.Withincrease in dielectric-con&& of _the medium we get weakening of electrostatic interaction between substituent Xi and the nitro group. In mixed alcohol + water solutions this dependence of px on dielectric con-. &ant has: been observed to hoki tie witI& ~c_ertainlimits, titil hypersolvation effects-set in [Fig. 7). :
649 REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
J. Pearson, Trans. Faraday Sot., 44 (1948) 683. D. StoEesova, Coil., 14 (1949) 615. B. Kastening and L. Holleck, 2. Elektrochem., 64x1960) 823. L. Holleck and B_ Kastening, Rev_ Polarogr. (Japan), 11 (1963) 129. M. Heyrovsky, S. VavriiSka, L_ Holleck and B. Kastening, J_ Electroanal. Chem., 26 (3.970) 399. M. Person, Bull. Sot_ Chim. France, (1966) 1832. ES. Levin, Dokl. Akad. Nauk SSSR, 151 (1963) 1375. J.P. Stradins, Polarography of Organic Nitrocompounds (in Russ.), Izd. Akad. Nauk Latv. SSR, Riga, 1961. J.P. Stradins and 1-J. Kravis in -4-N. Frumkin (Ed.), Electrochemical Processes with Participation of Organic Compounds fin Russ.), Nauka, Moskow, 1970, pp. 81-89. J.P. Stradins and I.J. Kravis_ Latv. PSR Zinat. Akad. Vestis, Kim. Ser., (1971) 572. S.G. Mairanovskii, J.P. Stradins azxd I.J. Kravis, Elektrokhimiya, 8 (1972) 784. L.H. Piette, P. Ludwig and R.N. Adams, J. Amer. Chem. Sot., 84 (1962) 4212. W. Kemula and Z. Kublik, Bull_ Acad. Polon. Sci., Ser. Sci. Chim. Geol., Geogr., 6 (1958) 653. L. Hslleck and D. Becher, J. Electroanal. Chem., 4 (1962) 321. S.G. Mairanovskii and J-P. Stradins, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, (1961) 2239. S.G. Mairanovskii, J. Electroanal. Chem., 4 (1962) 166. J_P_ Stradins and I.J. Kravis, Lafx. PSR Zinat. Akad. Vestis, Kim. Ser., (1971) 5@0. G-S. Alberts and I_ Shain, Anal. Chem., 35 (1963) 1859_ J.P. Stradins and 1-J. Kravis. Latv. PSR Zinat. Akad. Vestis, Kim_ Ser.. (1974) 374_ J.P. Stradins and G-0. Reihmanis, Latv. PSR Zinat. Akad. Vestis, Kim. Ser., (1969) 377;
21 22 23 24 25
(1972)
180.
B.G. Podlibner and L.N. Nekrasov, Elektrokhimiya, 7 (1971) 1191. J.P. Stradins and 1-J. Kravis in A.N. Frumkin (Ed.), Electrochemical Processes with Participitation of Organic Compounds, Nauka, Moskow, 1970, pp_ 110-116. J. Stradins and S. Iiillers, Tetrahedron, 20, Suppl. 1 (Nitro compounds) (1964) 409. J.P. Straditis and G;O_ Reihmanis, Elektrohimiya, 3 (1967) 178. J.P. Stradins, Latv_ PSR Zinat. Akad. Vestis, Kim_ Ser., (1968) 633.