See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Birch reduction rules: 1. For electron-donating groups (EDG, e.g. alkyl, -OR): The double bonds remain on the carbon bearing the substituent; substituent ends up on sp2 carbon of the product diene. 2. For electron-withdrawing groups (EWG, e.g. -CO2H, -COOR): The double bonds remain conjugated with the substituent; substituent ends up on sp2 carbon. 3. For unsubstituted benzene: Birch reduction gives 1,4-cyclohexadiene (unconjugated), NOT 1,3-cyclohexadiene. 4. For alkynes (dissolving metal): Na/liq. NH3 gives trans-alkene (anti addition of two H atoms), NOT cis-alkene. 5. For naphthalene: One ring is reduced preferentially giving 1,2-dihydronaphthalene (Delta1,9 or the non-aromatic ring gets reduced keeping one aromatic ring intact). 6. For anthracene: The central ring is reduced giving 9,10-dihydroanthracene, keeping two outer aromatic rings. Now analyzing each reaction: (i) Para-disubstituted benzene with EDG groups: Birch reduction of para-disubstituted benzene with alkyl groups - the double bonds should remain on the substituted carbons. The product shown has double bonds between substituted carbons, which is correct for EDG. This appears correct. (ii) Benzene --[Birch]--> shown product is 1,3-cyclohexadiene (conjugated). The actual major product of Birch reduction of benzene is 1,4-cyclohexadiene (unconjugated). So (ii) is WRONG - this does NOT represent the major product. (iii) Toluene (EDG = CH3): For EDG, double bonds remain conjugated with/on the carbon bearing the substituent - substituent on sp2 carbon. The product shown has CH3 on sp2 carbon with appropriate diene pattern. Looking at the structure: 1-methyl-1,4-cyclohexadiene where CH3 is on double-bond carbon. This is correct for EDG rule. So (iii) is correct. (iv) Benzoic acid (EWG = CO2H): For EWG, double bonds remain in conjugation with the group; substituent stays on sp2 carbon. The product shown has CO2H on sp2 carbon with double bonds at C2,3 and C5,6 (i.e., 2,5-cyclohexadien-1-carboxylic acid). This IS the correct product for EWG. However, looking more carefully at the image, the product shown for (iv) appears to have the CO2H on a carbon NOT bearing double bonds (sp3 carbon), which would be WRONG for EWG. If (iv) shows the substituent on sp3 carbon, it does not represent the major product. (iv) is INCORRECT. (vi) Anthracene Birch reduction: The correct major product is 9,10-dihydroanthracene (middle ring reduced). The product shown appears to show the terminal ring reduced rather than the central ring, making (vi) INCORRECT. (vii) 2-butyne + Na/liq. NH3: Dissolving metal reduction of an internal alkyne gives trans-alkene (via radical anion mechanism, anti addition). The product shown is cis-2-butene, which is WRONG. The correct product is trans-2-butene. So (vii) does NOT represent the major product. Thus the reactions that do NOT represent the major product are: (iv), (vi), and (vii). Therefore, the correct answer is B.