Which of the following correctly describes the mechanistic sequence in the Birch reduction of an aromatic ring?

• A. Single-electron transfer to the aromatic ring, protonation, second single-electron transfer, then final protonation
• B. Protonation of the aromatic ring, followed by two consecutive single-electron transfers, then final protonation
• C. Two single-electron transfers occur simultaneously, followed by double protonation
• D. Direct hydride addition to the aromatic ring, followed by two protonations

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the Birch reduction mechanism involves the reduction of an aromatic ring using alkali metals (like sodium or lithium) in liquid ammonia, typically with an alcohol as a proton source.
2. Step 2: The first key step is a single-electron transfer (SET) from the metal to the aromatic ring, generating a radical anion intermediate.
3. Step 3: This radical anion is then protonated by the alcohol, adding a proton to one of the carbons in the ring.
4. Step 4: A second single-electron transfer occurs, further reducing the intermediate to a dianion species.
5. Step 5: Finally, a second protonation takes place, resulting in the 1,4-cyclohexadiene product characteristic of the Birch reduction.