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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 SN2?

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

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  • $\begingroup$ 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. $\endgroup$
    – Proscionexium
    Commented Jun 26, 2023 at 5:47
  • $\begingroup$ @Proscionexium Then what about $\ce{NH_2^-}$? It also prefer elimination to above compound, this time it isn't bulkier. $\endgroup$
    – Leibniz-Z
    Commented Jun 26, 2023 at 6:03
  • $\begingroup$ Are you talking about $\ce{R-NH_2^-}$ or just only $\ce{NH_2^-}$? $\endgroup$
    – Proscionexium
    Commented Jun 26, 2023 at 6:11
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    $\begingroup$ SN2 does occur. What do you think the minor product is? $\endgroup$
    – Waylander
    Commented Jun 26, 2023 at 8:14
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    $\begingroup$ @Proscionexium It's not bulky - compare with t-butanolate that actually is. $\endgroup$
    – Mithoron
    Commented Jun 26, 2023 at 12:04

1 Answer 1

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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:

nucleophilic substitution of chloromethane in ethoxide

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

substitution elimination chlorocyclohexane ethoxide nucleophile base

Reference

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