An advanced organic chemist is heavily evaluating the following five distinct negatively charged ion — Haloalkanes and Haloarenes Chemistry Question
Question
An advanced organic chemist is heavily evaluating the following five distinct negatively charged ions to specifically explicitly maximize the final percentage yield of a concerted bimolecular substitution ($S_N2$) reaction rigorously on a secondary alkyl halide, without accidentally triggering massive, unwanted base-induced $E2$ elimination: 1. $F^-$ 2. $CH_3O^-$ 3. $I^-$ 4. $CN^-$ 5. $NH_2^-$ Exactly how many of these specific individual ions actively possess the precise, mathematically ideal chemical profile (being kinetically highly nucleophilic but decidedly exceedingly weak bases) to successfully completely fulfill this incredibly strict synthetic requirement?
💡 Solution & Explanation
To strictly physically maximize $S_N2$ on a moderately hindered $2^\circ$ substrate and entirely avoid $E2$, the chosen reagent must fiercely be a very fast, excellent nucleophile but a decidedly very weak base. Strong bases far too eagerly abstract exposed protons to form unwanted alkenes. Let's meticulously evaluate the available ions: 1) $F^-$: Noticeably poor nucleophile in typical protic solvents due to immense hydrogen bonding, and a weak/moderate base. Not ideal. 2) $CH_3O^-$: Highly strong nucleophile, remarkably strong base (highly unstable conjugate of the weak acid $CH_3OH$). This ion strongly, fiercely promotes $E2$ on $2^\circ$ halides. 3) $I^-$: Excellent, incredibly fast nucleophile (very large, highly polarizable), very incredibly weak base (highly stable conjugate of the very strong acid $HI$). Absolutely ideal for $S_N2$. 4) $CN^-$: Fast, excellent nucleophile, physically weak base (stable conjugate of $HCN$). Extremely ideal for $S_N2$. 5) $NH_2^-$: Massively, powerfully strong base (highly unstable conjugate of the incredibly weak acid $NH_3$). Massively promotes $E2$. Only $I^-$ and $CN^-$ perfectly physically fit the highly precise chemical profile required to strictly force substitution on a secondary halide. Total count = 2.