The intense competition between bimolecular substitution and bimolecular elimination is strongly inf — Haloalkanes and Haloarenes Chemistry Question
Question
The intense competition between bimolecular substitution and bimolecular elimination is strongly influenced by the specific choice of attacking reagent. Which of the following analytical statements accurately describes how the precise size and intrinsic basicity of the attacking species dictate the overwhelmingly dominant reaction pathway for a secondary alkyl halide?
💡 Solution & Explanation
A) True. Secondary substrates are somewhat sterically crowded. When reacting with strong bases/nucleophiles like methoxide, $E2$ elimination becomes the major pathway because abstracting an exposed peripheral $\beta$ -proton is kinetically much easier than penetrating the steric shield for a backside $S_N2$ attack. B) True. Bulky bases like $t-BuO^-$ are physically too massive to perform $S_N2$ backside attacks and therefore act exclusively as bases, driving $E2$ elimination. Due to their extreme bulk, they are forced to abstract the most accessible peripheral proton, usually yielding the less substituted Hofmann product. C) True. Reagents like $I^-$ and $HS^-$ are large, highly polarizable, and very fast nucleophiles but are extremely weak bases (conjugate bases of very strong acids). They readily perform $S_N2$ attacks but cannot thermodynamically abstract protons, effectively suppressing E2. D) False. Cyanide ($CN^-$) is relatively small, an excellent nucleophile, and a weak base, powerfully favouring classic $S_N2$ substitution.