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Chapter 5:
Structure and Preparation of Alkenes.
Elimination Reactions
Ch 5 contents

The general stability order of simple alkyl carbocations is: (most stable) 3o > 2o > 1o > methyl (least stable)

[carbocation stability order]

This is because alkyl groups are weakly electron donating due to hyperconjugation and inductive effects. Resonance effects can further stabilise carbocations when a adjacent π system is present (delocalisation of charge is a stabilising effect).
Note that reactions that occur via 3o and 2o are known. 1o cations have been observed under very special conditions, but methyl cations have never been observed. This means methyl systems don't undergo reactions via carbocations.

A simple representation of a carbocation
Alkyl carbocations are sp2 hybridised, planar systems at the cationic C centre. 
The p-orbital that is not utilised in the hybrid set is empty and is often shown bearing the positive charge since it represents the orbital available to accept electrons.
Computer model of CH3+


electrostatic potential of CH3+ (side view) As they have an incomplete octet, carbocations are excellent electrophiles and react readily with nucleophiles (i.e. substitution via SN1 pathway).
Alternatively, loss of H+ can generate a
p bond (elimination).

The electrostatic potential diagrams clearly show the cationic center in blue, this is where the nucleophile will attack.

electrostatic potential of CH3+ (top view)

Carbocations are prone to rearrangement via 1,2-hyride or 1,2-alkyl shifts provided it generates a more stable carbocation (these effects will be discussed in more detail later in Chapter 8)

For example:

3,3-dimethyl-2-butanol dehydrates to give mainly 2,3-dimethyl-2-butene
Notice that the "predicted" product is only formed in 3% yield, and that products with a different carbon skeleton dominate. 
The reaction proceeds via protonation to give the better leaving group which departs to give the 2o carbocation shown.  A methyl group rapidly migrates taking its bonding electrons along, giving a new skeleton and a more stable 3o carbocation which can then lose H+ to give the more stable alkene as the major product. 
1,2-alkyl shift
2o carbocation to 3o carbocation

This is an example of a 1,2-alkyl shift.  The numbers indicate that the alkyl group moves to an adjacent position.
Similar migrations of H atoms, 1,2-hydride shifts are also known.

Reactions involving carbocations:

  1. Substitutions via the SN1 mechanism
  2. Eliminations via the E1 mechanism (Chapter 5)
  3. Electrophilic Additions to alkenes (Chapter 6)

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary