Why can't ethoxide ion act as nucleophile?

Question

Consider the reaction of chlorocyclohexane with ethoxide in a suitable inert solvent. The major product according to my textbook is cyclohexene, which implies E2 reaction pathway. Why not S_N2?

$\ce{EtO^-}$ is an unstable anion, so why can it act as nucleophile rather than acting as a base?

Ethoxide is a bulky species. So it would undergo elimination rather than substitution. Perhaps you might get a better insight to this by this question. — Proscionexium, Jun 26 '23 at 05:47
@Proscionexium Then what about $\ce{NH_2^-}$? It also prefer elimination to above compound, this time it isn't bulkier. — Leibniz-Z, Jun 26 '23 at 06:03
Are you talking about $\ce{R-NH_2^-}$ or just only $\ce{NH_2^-}$? — Proscionexium, Jun 26 '23 at 06:11
I think maybe because abstracting $\ce{H^+}$ by $\ce{NH_2^-}$ would also release $\ce{Cl^-}$ (forming cyclohexene) just as $\ce{Cl^-}$ could be released by substitution (but remaining only cyclohexane). — Proscionexium, Jun 26 '23 at 06:42
@Proscionexium But $\ce{ \Delta H}$ whould be higher for elimination (Removing two $\sigma$ bonds). Also $\ce{EtO-}$ should be a good nucleophile due to its unstablity. — Leibniz-Z, Jun 26 '23 at 07:36
@Proscionexium It's not bulky - compare with t-butanolate that actually is. — Mithoron, Jun 26 '23 at 12:04
@Leibniz-Z There is enough hindrance of SN2 by the cyclohexane ring to favour proton abstraction by ethoxide — Waylander, Jun 26 '23 at 15:00
@Waylander then according to you $\ce{NH2-},\ce{OH-}$ would guve major product according to SN2. — Leibniz-Z, Jun 26 '23 at 15:49
Not in the case of amide anion. It is a very much stronger base and likely to produce more elimination — Waylander, Jun 26 '23 at 20:08
@Waylander but why? Since $\ce{\Delta H}$ is higher for elimination. And what about hydroxide ion? — Leibniz-Z, Jun 26 '23 at 20:35

ananta · Answer 1 · 2023-06-27T09:54:03.513

Ethoxide acts as a base only in presence of $\beta$-hydrogens. For secondary and primary alkyl halides, usually, $\text{S}_\text{N}\text{2}$ is favored over $\text{E}_\text{2}$ mechanism.

$\text{E}_\text{2}$ mechanism does not occur in the absence of $\beta$-hydrogens:

$\text{S}_\text{N}\text{2}$ reaction is slow for secondary alkyl halides, further stearic hinderance is present due to the cyclic nature of chlorocyclohexane.

Reference

Solomons, T. W. G., Fryhle, C. B. (2011). Organic Chemistry, 10th ed. John Wiley & Sons.

Why can't ethoxide ion act as nucleophile?

1 Answers1

Reference