Carbenium Ion Generation

1
Carbenium Ion Generation

• 1. Protonation of an unsaturated hydrocarbon
• If the olefin is not strongly basic, a strong
  acid is required to generate the carbocation
• 2. Protonation of a saturated hydrocarbon
• predominant mechanism in the cracking of alkanes
• carbonium ion
• 3. Hydride removal from a neutral molecule

2
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.

3
Carbenium Ion Reactions Hydride Abstraction

• 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

4
Carbenium Ion Reactions Alkyl Migration

• Skeletal isomerizations like the reactions below
• result 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 b position to the carbenium centre, the
  tendency to migrate relates to electron affinity
• CH3 gt C2H5 gt C3H7 gt t-C4H9

5
Carbenium Ion Reactions Alkyl Migration

• 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.

6
Carbenium Ion Reactions Alkyl Migration

• 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 hydride and alkyl 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.

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

8
Carbenium Ion Reactions b-Scission

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