Chapter 5: Structure and Preparation of Alkenes. Elimination Reactions

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 p bond (elimination). 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-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:

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.

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

© Dr. Ian Hunt, Department of Chemistry