See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Birch reduction involves dissolving metal reduction (Na or Li in liquid NH3 with an alcohol as proton source) of aromatic rings. The regioselectivity depends on whether the substituent is electron-donating or electron-withdrawing. Step 1 - General Birch reduction rule: - Electron-withdrawing groups (EWG): The double bonds remain conjugated with the EWG in the product; reduction occurs at the unsubstituted positions (ipso and para to the substituent are reduced, leaving double bonds that include the carbon bearing the EWG). In other words, for EWGs, the unreduced double bonds are those conjugated with the group. - Electron-donating groups (EDG): The double bonds remain on the unsubstituted carbons; the substituted carbon gets reduced. Step 2 - Classify COOH: COOH is an electron-withdrawing group. Step 3 - Apply the rule to benzoic acid: For an EWG like COOH, Birch reduction leaves the double bonds in conjugation with the COOH group. The product retains double bonds at C1-C2 and C5-C6 (i.e., the positions adjacent to and including the carbon bearing COOH), giving 2,5-cyclohexadien-1-carboxylic acid. Specifically, the product is 1-carboxyl-2,5-cyclohexadiene, where the COOH-bearing carbon (C1) is part of a double bond (C1=C2) and another double bond is at C5=C6, leaving C3 and C4 as the sp3 (reduced) carbons. Step 4 - Match to option (a): Option (a) shows a cyclohexadiene ring with COOH where the double bonds are positioned such that they include the carbon bearing COOH (conjugated arrangement consistent with EWG rule). This corresponds to 2,5-cyclohexadiene-1-carboxylic acid, the known Birch reduction product of benzoic acid. Step 5 - Why other options fail: - Option (b): Fully saturated cyclohexane with COOH - Birch reduction only partially reduces the ring (adds 2H, leaves 2 double bonds), not full saturation. - Option (c): Double bonds conjugated but in a 1,3-diene pattern adjacent to COOH - this would be the EDG product pattern, not EWG. - Option (d): Similar unconjugated pattern but structural arrangement does not match the known product; option (a) is the established correct regiochemistry. Therefore, the correct answer is A.