Overall Details of The Reaction Mechanism
The solution of metal in ammonia provides electrons which are taken up by the aromatic ring to form the corresponding radical anion B in the first step of the reaction. This is followed by protonation by the alcohol to form a cyclohexadienyl radical C. Next, a second electron is transferred to the radical to form a cyclohexadienyl carbanion D. In the last step a second proton leads the cyclohexadienyl carbanion to the unconjugated cyclohexadienyl product. These steps are outlined below for the case of anisole.
The reaction is known to be third order – first order in aromatic, first order in the alkali metal, and first order in the alcohol. This requires the rate-limiting step to be the conversion of radical anion B to the cyclohexadienyl radical C.
Read more about this topic: Birch Reduction
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