See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Nucleophilic addition of organolithium reagents (PhLi) to fulvene-type (cross-conjugated) systems depends on the aromatic stabilization of the carbanion formed upon addition. Step 1 - Identify the structures: Compound 1 is 6,6-dimethylfulvene (or more precisely 5-isopropylidenecyclopenta-1,3-diene): a cyclopentadienyl ring with an exocyclic isopropylidene (=CMe2) double bond. This is a fulvene. Compound 2 is 8,8-dimethylheptafulvene (or 7-isopropylidenecyclohepta-1,3,5-triene): a cycloheptadienyl ring with an exocyclic isopropylidene double bond. This is a heptafulvene. Step 2 - Reactivity toward nucleophiles (PhLi): When a nucleophile adds to the exocyclic carbon of a fulvene (compound 1), the resulting carbanion is delocalized into the cyclopentadienyl ring, generating a cyclopentadienyl anion (6 pi electrons, aromatic, highly stable). This strong thermodynamic driving force makes nucleophilic addition to fulvene (compound 1) very favorable. Step 3 - Reactivity toward compound 2: When a nucleophile adds to the exocyclic carbon of heptafulvene (compound 2), the resulting carbanion would be delocalized into the cycloheptadienyl ring, generating a cycloheptadienyl anion (8 pi electrons, antiaromatic, highly destabilized). Because this carbanion intermediate/product is antiaromatic, nucleophilic addition to heptafulvene is strongly disfavored. PhLi does not add to compound 2. Step 4 - Why other options fail: (a) Incorrect: the two compounds differ dramatically in reactivity due to aromaticity vs. antiaromaticity of the resulting anions. (b) Incorrect: PhLi does react with compound 1 because of the aromatic cyclopentadienyl anion formed. (d) Incorrect: it is compound 1 (fulvene), not compound 2 (heptafulvene), that reacts readily with PhLi. Conclusion: PhLi adds readily to compound 1 (forming a stabilized aromatic cyclopentadienyl anion) but does not add to compound 2 (which would form a destabilized antiaromatic cycloheptadienyl anion). Therefore, the correct answer is C.