CHEE 323 J.S. Parent 1

Carbenium Ion Reactions: Hydride Abstraction

Hydride abstraction is a key intermolecular H (hydride) transfer reaction. It is important in alkylation processes and oligomerization of olefins.

Hydride abstractions are extremely rapid when both the reactant and product are tertiary. The overall rate is strongly dependent upon the stability of the product cation.

Abstractions that yield secondary or primary carbenium ions are much slower.

The activation energy the above reaction is 3.5 kcal/mol, while the formation of a secondary carbenium ion from H abstraction from a paraffin has an activation energy of 14 kcal/mol.

CHEE 323 J.S. Parent 2

Carbenium Ion Reactions: Isomerization

One of the means of carbenium ion isomerization is H transfer along the hydrocarbon chain (intramolecular abstraction)

1,2 hydride shift:

1,3 hydride shift:

C+

CH3 CH3

H

CCH3 CH2+

H

H

CCH3H

CH3CH2

C+ CH3

CH3

C+

CH3

CH3CH2

CCH3

CH3H

CHEE 323 J.S. Parent 3

Carbenium Ion Reactions: Skeletal Isomerization

Skeletal isomerizations like the reactions below:

results from alkyl migration reactions of carbenium ions:

The alkyl group, like a hydride, migrates as a negative ion under heteropolar rupture of a C-C bond. When several alkyl groups are available in the position to the carbenium centre, the tendency to migrate relates to electron affinity:

CH3 > C2H5 > C3H7 > t-C4H9

CHEE 323 J.S. Parent 4

Carbenium Ion Reactions: Skeletal Isomerization

The isomerization of paraffins under the action of acid catalysts can be rationalized on the basis of a series of hydride and methyl shift reactions, as shown below for the rearrangement of 3-methylhexene:

CHEE 323 J.S. Parent 5

Isomerization of 3-methylpentane with H2SO4

Concentrated sulfuric acid catalyzes the isomerization of methylalkanes, while normal alkanes and paraffins containing quaternary carbons are not isomerized.

The rate of 3-methylpentane loss is pseudo-first order, yielding a constant that varies linearly with H0.

CHEE 323 J.S. Parent 6

Carbenium Reactions: Branching Rearrangements

Reactions leading to changes in the degree of branching are known to have activation energies that are much lower than expected on the basis of sequences of H and CH3 shifts.

A protonated cyclopropyl intermediate is believed to be formed, from which any carbon-carbon bond can be broken.

By this mechanism, an energetically unfavourable primary carbenium ion is avoided.

CHEE 323 J.S. Parent 7

Carbenium Ion Reactions: Alkylation

Alkylation is a synthetic reaction replacing a proton by an alkyl group. Although the alkyl group can be provided by an alcohol or alkyl halide, refinery operations employ olefins.

Relatively poor selectivity for a given product is expected, given the influence of side-reactions (isomerization, hydrogen transfer, cracking)

The alkylate is usually a mixture of paraffins with a wide range of carbon numbers.

The first reported reaction of this kind was isobutane with ethylene in the presence of BF3 or AlCl3 at 0oC to give a 45% yield of C6 paraffins:

CHEE 323 J.S. Parent 8

Carbenium Ion Reactions: -Scission

The reaction leading to hydrocarbon cracking is -scission of the carbenium ion to generate an alpha-olefin and a second cation.

-scission occurs most rapidly when a tertiary carbenium ion is generated, and can be very slow when the only product is a primary or secondary cation.

In the following example, the top pathway is slow due to the production of a secondary carbenium ion, while the bottom reaction is hindered by an unfavourable isomerization.

CHEE 323 J.S. Parent 9

Carbenium Ion Reactions: -Scission