Among standard laboratory reagents, the competition between and relies heavily on the specific natur — Haloalkanes and Haloarenes Chemistry Question
Question
Among standard laboratory reagents, the competition between $S_N2$ and $E2$ relies heavily on the specific nature of the nucleophile/base. Which of the following negatively charged species acts as an exceptionally potent, highly penetrating nucleophile for $S_N2$ reactions but is practically useless for promoting $E2$ elimination reactions due to its remarkably low intrinsic basicity?
💡 Solution & Explanation
The intense kinetic competition between $S_N2$ and $E2$ relies heavily on the nature of the reagent. A chemical species that is intrinsically a strong base will actively and aggressively promote $E2$ elimination. Ethoxide (A), amide (C), and *tert*-butoxide (D) are all very strong bases (they are the highly unstable conjugate bases of the weak acids ethanol, ammonia, and *tert*-butanol). They heavily promote elimination, especially when reacting with sterically hindered $2^\circ$ and $3^\circ$ substrates. In contrast, the hydrosulfide ion ($HS^-$) is a relatively large, highly polarizable atom, making it an excellent, extremely fast nucleophile at attacking electrophilic carbon. However, it is fundamentally a very weak base (it is the stable conjugate base of $H_2S$, which is much more strongly acidic than water or alcohols). Therefore, it powerfully favors substitution ($S_N2$) and is structurally ineffective at abstracting strong $C-H$ protons to cause elimination.