Return to Contents Chapter 4: Alcohols and Alkyl Halides Ch 4 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 π bond. 

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-hydride or 1,2-alkyl shifts if it generates a more stable carbocation. These effects will be discussed in more detail later (Chapter 8)

Examples of 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