7.10 Reduction

Catalytic hydrogenation of benzene to cyclohexane takes place at high temperatures and pressures. Platinum, palladium, nickel, ruthenium or rhodium is used as catalyst. The reduction cannot be stopped at an intermediate stage as these alkenes are reduced faster than benzene (Scheme 17).

Scheme 17

Benzene and its derivatives can be reduced to nonconjugated cyclic dienes by treating sodium or lithium liquid ammonia (Scheme 18). This reduction is called Birch reduction.

Scheme 18

A solution of sodium or lithium in liquid ammonia contains solvated electrons that can add to benzene, forming a radical anion. The strongly basic radical anion abstracts a proton from the alcohol, giving a cyclohexadienyl radical. The radical quickly adds another solvated electron to form a cyclohexadienyl anion which is then protonated to give the reduced product (Scheme 19).

Scheme 19