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GEORGE WlGER Calitornla state Unlversify Carson. CA 90747
Stability and Formation of lsobutylene Dimers Robert H. Goldsmith St. Mary's College of Maryland, St. Mary's City, MD 20686 Isohutvlene is an important bulk chemical for the petroleum industry. Dimerization and hydrogenation reactions produce the standard for fuel octane rating comparison. This classic chemistry is often misrepresented in modern texts, however, and this paper attempts to correlate the physical organic principlesthat apply. The dimerization process was first described by V. N. Ipatieff and B. B. Corson ( 1 ) . In this work isohutylene was subjected to gentle acid-catalyzed polymerization a t 30°C and 1atm pressure to give a diisohutylene fraction. Conversion of isobutvlene to isooctane was described shortly thereafter (2). The identification and relative percentage of the two isomers formed during this isohutylene dimerization was established ilhout 1950 and the maior ~ r o d u c was t found to be 2,4,4-tri-
- .
.
A proton is ejected in the last step and the two different 2,4,4-trimethylpentenes are formed.
The Saytzeff rule, which is often used to explain El elimination reactions. states that eiven a choice of ~ r o d u c t sthe more substituted alkene is preierred (4). consideration of the transition state reveals that, as the solvent removes the proton in the last step, one carbon changes hybridization, and electron flow towards the empty orbital on the adjacent carbon leads to a new pi hond characteristic. it is correct that most simnle carbonium ions derived from a ~El r situation do producr the IIIMP hiyhl? s ~ ~ l i ~ t :ulkt:ne i r ~ ~ as ~ tthe ~ l major pr n d u c t - l i k e features. A relevant example would he the El reactions of several related alkyl bromides with the general structure of I which can give rise to two different alkenes represented by I1 and 111. Journal of Chemical Education
I I ~r
R-CH,-C-CH,
I
-
R,
RCH,
,CH.,
dC=\ CHI 11
-
/H >M
CH, TI1
H'
The effect of several different R groups upon alkene product formation is illustrated in the table ( 5 ) . Composition of Alkene ( % ) c4
I,
111
.
A crnsensus exists about the first two steps in the overall reaction. Isobutvlene is attacked bv proton to form the t .a . hntyl carhonium ion which in turn attacks another molecule of isohutylene to form a new tertiary carhoninm ion.
596
CII,
This table reveals that while the most substituted alkene is formed in most situations, the less substituted alkene increases in proportion as the size of the R group increases. In the last case, the less substituted alkene becomes the major product, which is essentially the same as in our dimerization situation. What principles apply to the formation of the less suhstituted alkene as the major product? One approach to this question might he to focus upon the relative transition states leading to the isomeric products formed. Two different perspectives of the reacting molecule can be formulated; each includes the carhonium ion as a focal point. One perspective is obtained by examining the conformational situation about the C1-C2hond as in IV. A different view of this same transitional state is obtained by rotating the structure 120" and thereby focusing on the spatial relationships about the C& hond as in V.
Loss of a proton from view V would give rise to a steric crowding around the pi hond since the t-hutyl group would be cis to l-methyl. We would, therefore, not expect to see formed here, much of the product, 2,4,4-trimethyl-2-pentene and indeed, this is the minor product. It is more likely that the proton will be lost from carhon atom numher one, sterically which analogous to IV, giving rise to 2,4,4-trimethyl-l-pentene is the major product. An alternative method of considering this problem would be to consider the conformational space about the C& hond in a manner represented by V(a). The t-butyl group is attached to carhon numher 3
and is parallel with the vacant p orbital on carbon number two. Also, the t-hutyl group is now distant from the two methyl groups on this carbon. This situation gives the lowest conformer energy about this C 2 - C 3 bond and could be considered to be the favored situation. However, we now note that neither proton on Cn is in the same plane as the vacant p orbital on carbon number 2. Since a proton should be aligned with an openp orbital so that the pi bond can be formed most easilv. - . it becomes obvious that pi bond formation here is not favorable. Up to this point, we have been focusing upon the relative transition state energies as our key to determining which of our products is the major one and which is the minor one. A different approach to this problem might he to focus upon the thermodynamic stability of the products. The heat of hydrogenation gives data indicating that the more substituted
alkenes are more stable. This is certainly true for the vast majority of alkenes. Steric factors may also play a role. It is observed, for instance, that trans-2-butene has a lower heat of hydrogenation than cis-2-butene. However, in our situation has a heat of hydrowe find that 2,4,4-trimethyl-l-pentene genation of AH = 27.2 kcallmole (gas phase at 8Z°C) which is lower than the value of AH = 28.4 kcallmole (pas phase at 82°C) determined for 2,4,4-trimethyl-2-penteie(6). This means that the maior oroduct formed in dimerization is in. . deed the more stable isomer, and product stability is as imnortant as the difference in transition states examined pre;iously. Our two exceptional isomers probably reflect a situation in which the steric factors outweight the normal alkyl substitution effect noted in most alkenes. Literature Cited (1) I m i e f f , V. N., and Corson, B. B., lnduatriol and Enginreriw Chemi8Lry, 2'7, 1070 (1935). (2) 1pafieff.v. N., andKomarewsky.V.1.. J. Amrr. Chem.Soc.,59,720 (1937). (3) MescheryaLou. M. I.. Batueu, M. I., and P e ~ o v , AD.,lzuest. . Akad. NcukSSSR., Otdd Khim. Nouk., 1950.282 11950). (4) Saytzeu,A. M.. Ann., 179,296 (1875). (5) Liberks, A., "Theoretics1 Organic Chemistry," Macmillsn, New York,1968. (6) DoUiuer. M. A.,Gresham.TL., Kiitiik~mky,G. B.,and Vaughan, W.EE,J.AAAA.Chem. Sor., 59,831 l18113.
Volume GO
Number 7
Julv 1983
597
