SolutionsFriedel-Crafts Alkylation
1. Alkylation with olefins
Predict the result of the following reactions:
- 1-Methylcyclohexene is protonated to the tertiary carbocation, which alkylates benzene: (1-methylcyclohexyl)benzene.
- With 1-methylcyclopentene, besides the direct product, cyclohexylbenzene is obtained: the carbocation rearranges with ring expansion.
- The carbocation from 1-methylcyclobutene rearranges completely: ring expansion from four to five members relieves strain and cyclopentylbenzene is obtained.
2. Polyalkylation
Why is a large excess of benzene used in Friedel-Crafts alkylation?
Because the alkylbenzene formed is more reactive than benzene (the alkyl group activates the ring) and tends to be alkylated again. With excess benzene the electrophile is more likely to meet a benzene molecule.
3. Limitations
Can nitrobenzene be alkylated by the Friedel-Crafts method? And aniline?
No. Nitrobenzene is too deactivated. Aniline coordinates to the Lewis acid through the nitrogen, and the resulting –NH2–AlCl3 group deactivates the ring.