Page 474 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

to reduce pollution from partial combustion. In the 1980s methyl t-butyl ether (MTBE) 455
was introduced as a major antiknock component, but it was soon discovered that
MTBE contaminated groundwater as a result of leakage and spillage. 157 It is being TOPIC 4.1
phased out as a gasoline performance enhancer. 158 As a result, new emphasis has been The Role Carbocations
and Carbonium Ions in
placed on catalytic reforming as a means of meeting engine performance requirements. Petroleum Processing
A possible replacement for MTBE is the mixture of branched C hydrocarbons
8
prepared by dimerization of C compounds. 159 This is economically attractive since
4
the C compounds are by-products of other stages of petroleum refining. Isobutane
4
and isobutene react with strong acid to give C products. 160 The reaction involves
8
intermolecular hydride transfers.
CH 3 H +
C CH 2 (CH ) C +
3 3
CH 3
CH 3
+
(CH ) C + CH 2 C (CH 3 ) 3 CCH 2 C (CH 3 ) 2
3 3
CH 3
+
(CH ) CCH C (CH ) + H C(CH3)3 (CH ) CCH CH(CH ) + + C(CH )
3 2
3 3
2
3 3
2
3 2
3 3
The same kind of product can be obtained by acid-catalyzed dimerization of isobutene,
followed by hydrogenation.
CH 3 H +
C CH 2 (CH ) C +
3 3
CH 3
CH 3
+
(CH ) C + H C C (CH ) CCH C (CH ) (CH ) CCH C(CH ) +
3 2
3 3
3 3
2
2
3 2
3 3
CH 3
H 2 (CH ) CCH C CH 2
2
3 2
(CH ) CCH CH(CH ) CH 3
2
3 2
3 3
The hydrocarbon mixtures formed by these processes have octane numbers ranging
from 90 to 95.

Cracking, which is done at high temperatures 480–550 C in flow reactors
with short contact times (seconds), converts high-boiling components of petroleum
to hydrocarbons in the gasoline-boiling range. The catalysts are rapidly degraded
and are regenerated by high-temperature 700 C exposure to air. 161 The product

mixture is complex but is enriched in hydrocarbons in the gasoline range. Low-boiling
hydrocarbons such as methane and ethane are produced as by-products. Reactivity
toward cracking increases with molecular weight and branching. Carbocations are
intermediates in the cracking process, which leads to isomerizations.
157 S. Fiorenza, M. P. Suarez, and H. S. Rifai, J. Envir. Eng., 128, 773 (2002); S. Erdal and B. D. Goldstein,
Ann. Rev. Energy Environ., 25, 765 (2000).
158
A. K. Kolah, Q. Zhiwen, and S. M. Mahajani, Chem. Innovation, 31, 15 (2001).
159 J. M. Meister, S. M. B. Black, B. S. Muldoon, D. H. Wei, and C. M. Roesseler, Hydrocarbon Process.,
79, 63 (2000).
160 G. A. Olah, P. Batamack, D. Deffieux, B. Toeroek, Q. Wang, A. Molnar, and G. K. S. Prakash, Appl.
Catal. A, 146, 107 (1996).
161
Y. V. Kissin, J. Catal., 126, 600 (1990); Y. V. Kissin, Catal. Rev., 43, 85 (2001).