Below is the mechanism from this link of the Beckmann Rearrangement.

Beckmann Rearrangement1

Beckmann Rearrangement2

In the alkyl migration step, the alkyl group migrates simultaneously as the $\ce{H2O}$ leaving group is expelled. This seems as though it is an $\mathrm{S_N2}$ reaction at the $\mathrm{sp^2}$-hybridized nitrogen atom, but I thought that it was geometrically impossible to have an $\mathrm{S_N2}$ reaction at an $\mathrm{sp^2}$ center, per this link.

What am I missing in my reasoning?

  • $\begingroup$ Please do not use \ce{…} expression in formatting other than chemical equations and formulas. $\endgroup$ May 28 '20 at 18:33
  • $\begingroup$ For the record, the link you provide in "per this link", contains an answer of mine where I argue that $\mathrm{S_N2}$ reactions at $\mathrm{sp^2}$ centers are quite feasible, not impossible. For example, the Burgi-Dunitz angle is in accord with such reactions at carbonyl carbons. Sometimes the Pauling "bent-bond" heuristic makes things more understandable than the equivalent Huckel "sigma-pi" approach. $\endgroup$
    – ron
    May 29 '20 at 1:29

When you were taught that we don't see $S_{\mathrm{N}}2$ reactions at $sp^{2}$ centers, what evidence or rationale was given? In organic chemistry, very few things are definite. When they are, it's usually because of being forbidden by quantum mechanics (e.g., symmetry in pericyclic reactions). Otherwise, they are just very unlikely, unless you somehow magically make the conditions right.

Greg Fu used to give this example: J. Am. Chem. Soc. 1998, 120, 2275-2282.

In it, a trans-substituted alkene undergoes nucleophilic substitution to become a cis-substituted alkene. Using that fact and other data, the authors conclude that the mechanism most consistent with the observed reaction data is a vinylic $S_{\mathrm{N}}2$. If you have a better mechanism, consider writing a paper and submitting it to JACS.

  • $\begingroup$ My textbook, Clayden et al., says: "To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction of course is not possible." It then generalizes this rule by saying: "SN2 at sp2 C does not occur". $\endgroup$ May 29 '20 at 2:47
  • $\begingroup$ That's for a benzene ring. What about an alkene? $\endgroup$
    – Zhe
    May 29 '20 at 12:04

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