Carefully consider the following highly substituted organic molecules: 1. -Nitro-(2-bromoethyl)benze — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

The $E1cB$ mechanism fundamentally requires a relatively poor leaving group (like F) AND a powerful electron-withdrawing group ($\beta$ -EWG) on the adjacent carbon to safely stabilize the intermediate carbanion. 1) $p$ -Nitro-(2-bromoethyl)benzene: The $p$ -nitrophenyl group is a massive EWG that beautifully stabilizes a carbanion at the benzylic position, making E1cB highly probable. 2) 2-Fluoro-1,1,1-trifluoroethane ($CF_3-CH_2F$): The extreme inductive pull of the three fluorine atoms strongly stabilizes the $CF_3-C^-HF$ carbanion, and the remaining lone fluorine is a notoriously poor leaving group. Heavily favors E1cB. 3) 3-bromo-3-methylpentane: Simple aliphatic, no EWG. Strictly undergoes E2/E1. 4) 1-chloro-1-phenyl-2-propanone: While containing an EWG, the acidic protons are completely misaligned for standard $\beta$ -elimination (it's an $\alpha$ -halo ketone). 5) 2-chloro-2-methylpropane: Standard $3^\circ$ halide, strictly E2/E1. Therefore, exactly 2 molecules strongly favor E1cB.