Investigation of the dimer structure of isohexenes using infrared spectroscopy and nuclear magnetic resonance

INVESTIGATION OF THE DIMER STRUCTURE OF ISOHEXENES USING INFRARED SPECTROSCOPY AND NUCLEAR MAGNETIC RESONANCE * Yu. G. OSOKIN, V. SH. FEL'DBLYUM, S. I. KRYUKOV, V. D. LOSHCHILOVA, O. P. YABLOI~SKII, N. ]~[. PASHCHENKO, A. F. MOSKVIN and A. G. PANKOV Scientific Research Institute of Monomers used for Synthetic Rubber Production, Yaroslavl' (Received 14 April 1967) ~-~IGHER olefins are valuable raw material for industrial organic syntheses [1]. One method of producing them is by dimerization of lower olefins [2, 3]. Thus C12 olefins are readily formed during the dimerization of methyl-2-pent-l-ene in the presence of sulphuric acid or solid acid catalysts [4, 5]. However, there is hardly any information in the literature concerning the structures of the hydrocarbons obtained. Only one paper [6] gives some indication of the possibility of 4,6,6-trimethylnon-3-ene formation. This paper deals with the study of the structure of the principal isohexene dimers (methyl-2-pent-l-ene and methyl-2-pent-2-ene). They are produced in high yield by dimerization of propylene [5] and could, in future, become available as industrial products. EXPERIMENTAL

Isohexene dimers were analysed by gas-liquid chromatography using a "Shandon" device, a heat-conductivity detector, a column 4 m in length and 4 m m in diameter, at a temperature of 70 ° and a carrier gas (hydrogen) velocity of 80 ml/min. Polyethyleneglyeol adipate, or polyethyleneglyeol 1000 (20 wt. % on diatomite) were used as stationary phases. The main components after separation in a chromatographic column were collected in traps, cooled with solid carbon dioxide and examined by I R spectroscopy and nuclear magnetic resonance. NMR spectra were observed with a JNM-3-60 high resolution device at a frequency o f 60 Mc/s with tetramethylsilane as internal standard. Samples were studied as 10% solutions in CCI4. I R absorption spectra were observed in the 700-3600 cm -1 range in a doublebeam UR-10 spectrophotometer (with a NaC1 prism). Vessels were used with an absorbing layer thickness of 0.004 cm. * :Neftekhimiya 7, No. 6, 871-877, 1967. 271

272

YtT. G. OSOKINet EFFECT

al.

OF C A T A L Y S T S A N D R E A C T I O N C O N D I T I O N S ON T H E COMP O S I T I O N OF I S O H E X E N E DIlYIERS

i

i Content of components in Temper- ~1 dimers wt.% Initial olefin : o ~...... ature, C , I ' il others, wt. '. 1 ] 2 i 3 ! O' . . . . . . . . . . . . . . . . . . . . . ........ ~' . . . . . (% ..... Catalyst, 70~o H,S04 Methyl-2-pent - l-ene Methyl-2-pent-2-ene Methyl-2-pent- 1-ene Methyl-2-pent-2-ene

~

30 30 50 50

39"91 68"31 12"8 37"1

32"61 18"4 14"01 9"6 39"0 25"9 30"4 19"0

9.1 8.1 23.3 13.5

Catalyst, KU- I sulpho-eation exchange resin Methyl-2-pent-l-ene

!

90

] 19.1 J 51"61 22.0]

7.3

Catalyst, AI,Os-- SiO2 Methyl-2-pent- 1-ene Methyl-2-pent-2-ene

~

22-0 53.0 14.7 39-4 35.4 12.3 I

120 120

10.3 12.9

Catalyst, CaA zeolite Methyl-2-pent-1-ene

'

150

i

5.3!47.5

21.9 I

25.3

I n addition, to determine the n u m b e r of structural isomers in the p r o d u c t investigated, h y d r o g e n a t i o n was carried o u t on palladium-coated asbestos with a nickel catalyst at 200 ° [7]. RESULTS AND DISCUSSION

Isohexene dimers (methyl-2-pent-l-ene a n d methyl-2-pent-2~ne) obtained b y m e t h o d s previously described [4, 5] were t r e a t e d w i t h various acid catalysts (Table}. Figure l a shows a c h r o m a t o g r a m of the p r o d u c t consisting of three m a i n components. P r e p a r a t o r y separation of the isohexene dimer in a chromatograph gave component 1 (97.3 wt.~/o), component 2 (89-4 wt.~/o) a n d component 3 (75.6 wt.% ). For comparison, Fig. lc shows a c h r o m a t o g r a m of a 190-200 ° narrow fraction of an industrial propylene t e t r a m e r sample obtained b y polymerization of propylene on a phosphoric acid catalyst, used for obtaining sod i u m dodecylbenzene-sulphonate, a synthetic detergent, a n d a c h r o m a t o g r a m of the p r o d u c t of the h y d r o g e n a t i o n of propylene t e t r a m e r is shown in Fig. ld. A t t e n t i o n is d r a w n to the considerably more complex composition of propylene tetramer, compared w i t h isohexene dimers. H y d r o g e n a t i o n of isohexene dlraers

Investigation of dlmer structure of isohexenes using infrared spectroscopy

273

revealed two peaks, 4 and 5 on the chromatogram, (Fig. lb), which proves that two types of structural isomer were formed. It is interesting that on palladium-coated asbestos component 1 is hydrogenated to a compound which corresponds to peak 5 and, on a nickel catalyst, to a mixture of compounds C~

2

L

l--~

~ ~ ,

L

~,

d

I

30

20

10

0

FIG. 1. Chromatograms of: a methyl-2-pent-l-ene dimer obtained on H~SO, (a); an industrial propylene tetramer fraction (c) and products obtained from hydrogenation (b) and (d), respectively.

producing peaks g and 5. This indicates that structural isomerization of olefins can be carried out on a nickel catalyst during hydrogenation. Re-groupings of a similar type were described in a recent review [8]. Components 2 and 3 on both catalysts hydrogenate to product 4, which proves that the carbon skeleton of component 1 differs from that of components 2 and 3. According to the caxbonium ion theory of Whitmore [9], methyl-2-pent-1-

Y~. G. OSOKL~ et al.

ene and methyl-2-pent-2-ene form a tertiary carbonium ion with an acid catalyst, which can combine with each olefin to form C12 cations. The decomposition of the latter would result in the following structures of isohexene dimers: CH3 CHsCH2CH2CCH2CCH2CH2CH3

i

I l)

H

CH3 CH2 CH3 CHsCH2CH2CCH~C = CHCH~CHs

f

(II)

I

CH~ CHs CH3

i

CHsCH~CH~CCH = CCH~CH2CH3

I

([II)

I

CH3

CH~

CH,

J

CH3CH2CH~C -- C H - - C = CH~ I I I CH3CH2 CH3

(IV)

i

CH3 CHa

t

CH3CH~CH~C-- C = CCH8

I

I I

CHaCH~

I

CHa

(v)

CH~

During dimerization of an isohexene mixture with a relatively higher methyl-2-pent-2-ene content it was natural to expect that structures IV and V would be chiefly formed. The Table indicates that component 1 predominates in isohexene dimers obtained from methyl-2-pent-2-ene with all the catalysts tested. Structures I - I I I would be obtained selectively from methyl-2-pent-1erie, if it were isomerized to methyl-2-pent-2-ene. However, as the dimerization of methyl-2-pent-l-ene is accompanied by isomerization to methyl-2-pent-2erie [5], the formation of structures of both types could be expected, which is also confirmed by hydrogenation. The NMR spectrum (Fig. 2) of component I isolated from isohexene dimers indicates that it consists of a compound of R~C=CH2 type. Resonance bands in the 4.65 and 4.80 p.p.m. * range correspond to two protons at the terminal ethylene carbon atom [10]. A considerable chemical shift between them is due to the difference of alkyl substituents, R, one of which is the CR s group giving a band at J = 1.65 p.p.m, with a constant of spin-spin interaction JH~, b-ca, z * p.p.m.=parts

per million.

Investigation of dimer structure of isohexenes using infrared spectroscopy

275

~1 c/s [11]. Signals in the range of 0.75 to 1.40 p.p.m, correspond to protons of methyl and methylene groups in another radical. The singlet at 6--0.80 p.p.m, represents protons of two Inethyl groups connected with a quaternary carbon atom, while signals at 6 ranging from 1.20 to 1.40 p.p.m, correspond to methylene groups in the fl-, 7- and 6-position to the double bond.

i cb g CH~ CH I I z ~ / H ~z CH,- CH,-CH,- C - CH-C =C-

g

CHa CH 3 e dc

f.f

ML I

I

2

CH a- CH[ CHz- C - CHi-C =CRCHf- cHs f I "

9

f

OH,.. c2; c e D b'

f

........... i

I

a

~.~v ,~g---,

I

I

I

I

I ~

i

T-

f~

,

i

3

p.p.s, s

c,:cb-

CHz-C - CH-C-C,~-CH~-CH~ cb ~H~

(z

b

.q

;

'

f

e

)

a

d c b

l

I

~

I

:

I

'

o

FIo. 2. l~-MR spectra of isohexene dimer components. Assignment of the separate absorption bands to structural elements is denoted by letters.

276

Y~T. G. OSOK[N et al.

Absorption bands with centres at 6----2.]0 p.p.m, should be a t t r i b u t e d to t h e only proton of the t e r t i a r y carbon a t o m [611. I n the I R spectl"um of component 1 (Fig. 3) an absorption band was ol>~ H ~erved at 890 cm -1, which is due to the deformation vibrations of :--:( ~" \

bonds

H and a b a n d at 1640 cm -1, which is assigned to bond-stretching vibrations of the C : C bond [12]. The b a n d at 3085 cm -1 is assigned to bond-stretching H / vibrations of C - - H bonds in the = c group. Absorption bands at 1380 and

\

15 1465 cm -1 are t y p i c a l of C H a a n d C H 2 groups, respectively. Thus, I1%- a n d NMI% spectroscopic d a t a for c o m p o n e n t 1 are consistent w i t h a s t r u c t u r e o f 2 , 4 , 4 - t r i m e t h y l - 3 - e t h y l h e p t - 1-ene (IV).

80

/

60'

I

i ZO

2 o~ 80 .~ 80

zo ,..j

3

. . . .

3,?0(7 3ggg 2800

'

,

1600

I

i

I

•

I

.

1;oo Izoo 1~oo' e'oocm-~

FIG. 3. I R absorption spectra of isohexene dimer components.

Investigation of dimer structure of isohexenesusing infrared spectroscopy

277

Component 2 is 4,6,6-trimethylnon-3-ene (II). In the NMR spectrum (Fig. 2) there is a characteristic triplet with a centre at ~ 5 . 0 5 p.p.m. (proton at an unsaturated carbon atom in RCH~CH=CR~), a band at ~ 1.60 p.p.m. (methyl group in RCHg.CH----C(CH3)R) and singlets at ~=1.82 p.p.m. (methylene group in the fragment =C(CH3)CH2CR3) and ~-=0.80 p.p.m. (methyl groups at the quaternary carbon atom). In addition, a resonance signal was observed in the I~MR spectrum in the region of 4.83 p.p.m, which points to the apparent presence of 1 as an olefin impurity of the R~C=CH~ structure. The IR spectrum also confirms the structure of component 2. Absorption bands at 850, 1660 cm -1 and 890, 1640 cm -1 indicate the presence of olefin structures, R2C=CHR and R~C=CH2, respectively. Separation of the band at 1380 c~n-1 into a doublet with absorption maxima at 1370 and 1390 cm -~ and the presence of bands at 1190 and 1210 cm -1 are characteristic of methyl groups combined with the quaternary carbon atom [13]. The structure of component 2 is also confirmed by the fact that the relative retention time during chromatographic analysis appeared to be identical with that of the standard synthesized b y t h e Grignard-Wiirtz reaction from 2-chloro2-methylpentane and 3-chloro-2-methylpent-l-ene. (The latter, by interaction with tert-hexylmagnesium chloride, via allyl re-grouping, was converted to primary chloride [15], which further reacted with the formation of C12 olefin). The structure of standard 4,6,6-trimethylnon-3-ene (II)(b.p. 73° (10 mmttg), n~ = 1.4410) was confirmed by NMR spectra. Products obtained from the hydrogenation of the standard compound and the compound studied gave a joint peak 4 on the chromatogram (Fig. lb). IR and NMR spectra of component 3 indicated an olefin mixture. In the NMR spectrum signals were observed of olefin protons of the R3C--CH=CRg. (5.10 p.p.m.) and R~C----CH~ (4'65 and 4.80 p.p.m.) types, of the methyl group in the RsC--CH=C(CH3)I~ 1.65 p.p.m.) structure, methylene groups in the a-position (1.85 p.p.m.), fl-position (1.40 p.p.m.) and 7-position (1.15 p.p.m.) to the double bond and methyl groups remote from the double bond (0.70+0.9 p.p.m.), among others at the quaternary carbon atom (0.90 p.p.m.). Deformation vibrations were observed in the IR spectrum of C--H bonds in R~C=CHR (850 cm -1) and R~C~CH 2 (890 cm -1) structures, bond-stretching vibrations of the C=C bonds in the same structures (1660 and 1640 cm -~, respectively), a doublet at 1370 and 1390 cm-~ and bands at 1190 and 1210 cm -1, representing two methyl groups at the quaternary carbon atom. The product obtained from the hydrogenation of component 3 gave the main peak 4 on the chromatogram (Fig. 2b) identical with that of 4,6,6-trimethylnonane. These data indicated the presence in component 3 of 4,6,6-trimethylnon-4-ene (III)and 'olefin impurities of R(CH3)C=CH ~ type, the structure of which has not been elucidated in detail. It is possible that, during dimerization of methyl-2-pent1-ene, a certain amount of 2,5-dimethyl-3-ethyloct-l-ene (VI)'is obtained according to a mechanism [14], involving the formation of an intermediate

278

¥~7. G. 0SOKIN et al.

allyl cation: CH2 : C-- CH ÷ CH~= C-- CH2CH-I2CH3->CH2 = C-- CHCH2-- C-- CH~CH2CH3 ~> HsCH2 CH3

CH3CH2

(VI)

CH~

CH

In addition, a somewhat lower relative intensity of absorption bands corresponding to structures of R2C:-CHR and R2C----CH2 in the IR spectrum of component 3 may point to the presenee of a small amount of tetra-alkylethylenes R2C-----CR2, for example V, which do not give characteristic absorption bands in the IR range of the spectrum [Ill. The above investigation confirmed the formation of olefins of structures I I - I V as the main products of acid catalytic dimerization of methyl-2-pent1-ene and methyl-2-pent-2-ene, which is in agreement with Whitmore's carbonium ion theory. Isohexene dimers formed on various catalysts do not qualitatively differ in composition, but are characterized by various quantitative component ratios which is, apparently, due to the different rates of izomerization and dimerization of methyl-2-pent-l-ene and methyl-2pent-2-ene. The fact that the isohexene dimer consists of olefins with a lateral alkyl group at the unsaturated carbon atom and a very limited number of structural isomers should demonstrate its successful application in synthesis of tertiary alkylbenzenes, mercaptans and other products of industrial organic synthesis. SUMMARY

1. A study was made of the dimer structure of 2-methylpent-l-ene and 2-methylpent-2-ene prepared in the presence of various acid catalysts. 2. 2,4,4-trimethyl-3-ethylhept-l-ene; 4,6,6-trimethylnon-3-ene and 4,6,6trimethylnon-4-ene were the main products of dimerizati on. Translated by E. S E ~ R E

REFERENCES 1. V. T. SKLYAR, Ye. V. LEBEDEV and V. A. Z A K U P R A , Vysshie monoolefiny (High er Mono-Olefms). ToldmltrR,, Kiev, 1964 2. F. ASINGER, K h i m i y a i teldmologiya monoolefinov (Chemistry and Technology of Mono-Olefins). Gostoptekhizdat, Moscow, 1960 3. L. SHM]F~LING, Sb. Kataliz v organicheskoi khimii (Catalysis in Organic Chemistry). Izd-vo inostr, lit., Moscow, 1952 4. S. I. KRYUKOV a n d M. I. FAR~EROV, Zh. prikl, khimii 35, No. 10, 2319, 1962 5. V. Sh. FEL'DBLYUM, S. I. ILRYUKOV, M. I. F A P ~ E R O V , L. V. GOLOVKO, I. Ya. TYRYAYEV and A. G. PANKOV, Neftekhimiya 3, No. 1, 20, 1963 6. F. C. STEHI.ING and K. W. BARTZ, Analyt. Chem. 38, No. 11, 1467, 1966 7. Yu. G. OSOKIN, V. Sh. FEL'DBLYUM and S. I. KRYUKOV, Neftekhimiya 6, No. 2, 333, 1966

Investigation of dlmer structure of isohexenes using infrared spectroscopy

279

8. F. ASINGER and B. FELL, Erd~l und Kohle 19, No. 6, 406, 1966 9. F. C. WHITMORE, Industr. and Engng. Chem. 26, 94, 1934 10. Dzh. POPL, V. SHNEIDER and G. BI~RI~TEIN, Spektry yadernogo magaitnogo rezonansa vysokogo razresheniya (High-Resolution Nuclear Magnetic Resonance Spectra) Izd-vo inostr, lit., Moscow, 1962 11. S. A. FRANCIS and E. D. ARCHER, Analyt. Chem. 35, No. 10, 1363, 1963 12. L. BELAMI, Infrakrasnye spektry slozhnykh molekul (IR Spectra of Complex Molecules), Izd-vo inostr, lit., Moscow, 1963 13. K. NAKANISI, Infrakrasnye spektry i stroenie organicheskikh soedinenii (IR Spectra and Structure of Organic Compounds). Mir, Moscow, 1965 14. N. P. LEFTIN and E. HERMANA, Prec. Third Intern. Congr. of Catalysis, Amsterdam, 2, 1965 15. T. I. TEMNIKOVA, Kurs teoreticheskikh osnov khimii (Course on the Theoretical Principles of Chemistry). Goskhimizdat, Leningrad 1962