Question

A common illicit synthesis of methamphetamine involves an interesting variation of the Birch reduction. A solution of ephedrine in alcohol is added to liquid ammonia, followed by several pieces of lithium metal. The Birch reduction usually reduces the aromatic ring, but in this case, it eliminates the hydroxy group of ephedrine to give methamphetamine. Propose a mechanism, similar to that for the Birch reduction, to explain this unusual course of the reaction

Ephedrine Methamphetamine

Short answer

Mechanism for the formation of Methamphetamine

Step-by-step solution

Birch Reduction

Usually, the Birch reduction is used to convert arenes to cyclohexadiene but in this, in the presence of liquid ammonia and lithium metal and alcohol, it eliminates the hydroxyl group present.

Mechanism of the reaction

The ephedrine is treated with lithium metal and replaces hydrogen with Li which reacts with a proton to form the carbocation and then a radical. This radical on the addition of electron form a carbanion which reacts with ethyl alcohol to form the methamphetamine as the desired product.

Mechanism for the formation of Methamphetamine