Q.(i) Which of the following compounds will give faster SN2 reaction: benzyl chloride (ring-CH2Cl) or chlorobenzene (ring-Cl)?

Aryl C-Cl bonds resist SN2 due to resonance and sp2 hybridisation; benzylic C-Cl bonds react readily. The reaction sequence given tests Grignard carboxylation, and separately, nucleophilic substitution and E2 elimination.

1. Chlorobenzene's C-Cl bond has partial double-bond character: the lone pair on Cl delocalises into the aromatic pi system by resonance, and the carbon is sp2 hybridised (shorter, stronger bond, more electron-rich/hindered towards backside attack). This makes chlorobenzene essentially inert to SN2 conditions. In benzyl chloride, the -CH2Cl carbon is sp3 and NOT part of the ring; there is no such resonance stabilising the C-Cl bond, and moreover, the SN2 transition state (developing partial positive/negative charge) is stabilised by the adjacent phenyl ring (benzylic stabilisation). So benzyl chloride undergoes SN2 substitution readily and much faster than chlorobenzene.
2. Tracing the reaction sequence:

• CH3Br + Mg / dry ether -> CH3MgBr (A, a Grignard reagent)
• A + dry ice (solid CO2), then aqueous acid workup (H3O+) -> CH3COOH (B, acetic acid) — the Grignard adds to CO2 to give a magnesium carboxylate, which is hydrolysed to the carboxylic acid. …