10.10: Alkylation of Alkenes

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Addition of a saturated hydrocarbon \(\left( \ce{R-H} \right)\) to an alkene to yield a saturated hydrocarbon of higher molecular weight is known as alkylation:

Roberts and Caserio Screenshot 10-9-1.png

Such reactions are used by the petroleum industry to produce medium-molecular-weight hydrocarbons from smaller molecules. A particularly important example is afforded by the addition of 2-methylpropane to 2-methylpropene in the presence of sulfuric acid or anhydrous hydrogen fluoride to yield 2,2,4-trimethylpentane:

Roberts and Caserio Screenshot 10-9-2.png

The overall reaction appears to be different from any so far discussed, because it involves addition of a nonpolar reagent \(\left( \ce{RH} \right)\) to an alkene bond.

The key to the mechanism of hydrocarbon alkylation was provided by the discovery by P. D. Bartlett, in 1940, that a carbocation can react rapidly with a hydrocarbon having a tertiary hydrogen to yield a new carbocation and a new hydrocarbon. Some of these "hydrogen-transfer" reactions are extraordinarily fast and may be complete in seconds or less. The hydrogen is transferred with both bonding electrons \(\left( \ce{H}^\ominus \right)\). For example,

Roberts and Caserio Screenshot 10-9-3.png

With the knowledge that the hydrogen transfer is fast, the alkylation of 2-methylpropene with 2-methylpropane can be formulated as involving first polymerization of two 2-methylpropene molecules under the influence of the sulfuric acid catalyst to give the same octyl cation as was postulated for the dimerization of 2-methylpropene:

Roberts and Caserio Screenshot 10-9-4.png

The octyl cation then can undergo a hydrogen-transfer reaction with 2-methylpropane to furnish 2,2,4-trimethylpentane and a tert-butyl cation:

Roberts and Caserio Screenshot 10-9-5.png

Attack by the tert-butyl cation on another molecule of 2-methylpropene produces an eight-carbon tertiary cation, which them proceeds to another molecule of "alkylate":

Roberts and Caserio Screenshot 10-9-6.png

This is an important example of a cationic chain reaction.

References

John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."