In my class notes, it says: "The Nu will NEVER displace the leaving group directly in an SN2 reaction."

We have previously been taught that SN2 reaction only occur at tetrahedral carbons, so I can see why it wouldn't occur here (where the carbon is trigonal planar), but I'm seeking an answer that is a little deeper than this heuristic. Why is it exactly that nucleophilic acyl substitutions reactions are mechanistically referred to as "an addition-elimination" rather than a substitution reaction?

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    $\begingroup$ Because it's a different mechanism that happens via separate addition and elimination step, whereas an Sn2 reaction is strictly concerted by definition. $\endgroup$ – Zhe Apr 6 at 23:53

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