Chapter 4: Alcohols and Alkyl Halides

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

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 3^o and 2^o are known. 1^o 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.

Structure:

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.

Reactivity:

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.

Rearrangements:
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 S_N1 mechanism
2. Eliminations via the E1 mechanism (Chapter 5)
3. Electrophilic Additions to alkenes (Chapter 6)

© Dr. Ian Hunt, Department of Chemistry