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I've often herd that aryl halides are very less reactive towards nucleophilic substitution reactions. One of the reasons for this I found was that there is resonance effect in the compound, but how is that responsible for it to less reactive towards S.N. reactions and in addition to resonance effect are there other factors also responsible ?

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No, that is a very incorrect explanation for why (unsubstituted) aryl halides do not participate in nucleophilic substitution reaction. Conjugating effect between the halide substituent and the benzene ring is not responsible for this. The problem is that there is simply no available mechanism to perform the substitution:

  • $\ce{S_{N}2}$ is impossible since inversion would give an incredibly weird strained ring (even more weird than a bridgehead carbon that is already forbidden by Bredt's rule).
  • $\ce{S_{N}1}$ would leave a very unstable empty $\ce{sp^2}$ orbital. This can happen, but only for the very best leaving groups, like $\ce{N2}$.
  • There is no electron stabilizing group that is conjugated with the halide group. This renders an addition-elimination type mechanism impossible.
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  • $\begingroup$ What this weird strained ring would like? Thanks in advance. $\endgroup$ – ado sar Nov 2 at 19:39

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