SolutionsAlkylarenes
1. Side-chain reactions
Give the product of the reaction of ethylbenzene with: a) NBS and light; b) hot KMnO4 and then H3O+; c) Br2/FeBr3.
- 1-Bromo-1-phenylethane: radical bromination takes place at the benzylic position.
- Benzoic acid: strong oxidation cleaves the chain and leaves a COOH at the benzylic position.
- A mixture of 1-bromo-2-ethylbenzene and 1-bromo-4-ethylbenzene (major): with the catalyst the reaction is an SEAr on the ring.
2. Solvolysis
Why does benzyl chloride, a primary halide, react with water much faster than 1-chloropropane?
Because the benzyl carbocation is stabilized by resonance with the ring: its positive charge is spread over the ortho and para positions. Moreover, the SN2 transition state at the benzylic position is also stabilized by conjugation with the ring.
3. Acidity of toluene
The pKa of toluene is 41 and that of methane about 50. Why is toluene more acidic? Which base can deprotonate it?
The benzyl carbanion is stabilized because the negative charge is delocalized over the ortho and para positions of the ring. A very strong base is needed, such as butyllithium (butane has pKa ≈ 50).