|Chapter 8: Nucleophilic Substitution|
The general stability order of simple alkyl carbocations is: (most stable) 3o > 2o > 1o > 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 present.
||Alkyl carbocations are sp2 hybridised, planar
systems at the cationic C centre.
The p-orbital that is not utilised in the hybrids is empty and is often shown bearing the positive charge since it represents the orbital available to accept electrons.
|As they have an incomplete octet, carbocations are
excellent electrophiles and react readily with nucleophiles. Alternatively,
loss of H+ can generate a p bond.
The electrostatic potential diagrams clearly show
the cationic center in blue, this is where the nucleophile will attack.
Carbocations are prone to rearrangement via 1,2-hyride or 1,2-alkyl shifts provided it generates a more stable carbocation. For example:
|Notice that the "predicted" product is only formed
in 3% yield, and that products with a different 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 SN1 (see also Chapter 4)
2. Eliminations via E1 (Chapter 5)
3. Additions to alkenes (e.g. HX, H3O+) (Chapter 6)
4. Additions to alkynes (e.g. HX, H3O+) (Chapter 9)
5. Friedel-Crafts alkylations (Chapter 12)
|© Dr. Ian Hunt, Department of Chemistry|