I know that allyl halides are reactive towards SN1 reactions due to the stabilization of carbocation due to resonance but why are they reactive towards SN2 too?

  • $\begingroup$ I actually read that allyl halides are more reactive towards SN2 than primary alkyl halides, whom I thought were most reactive. $\endgroup$ – Kirti Agrawal Feb 26 '18 at 12:01
  • $\begingroup$ Can you add what you just said above to your question? $\endgroup$ – dr.drizzy Mar 4 '18 at 5:58

In a reaction, SN2 as well as SN1 reactions take place. It only matters that which reaction pathway gives the major product. The answer to your question depends on the type of base used, the substrate and the overall stability of the product. Allyl halides are also reactive to SN2' reactions, i.e allylic nucleophilic substitution reactions. There are a lot of possibilities.

Keep in mind that the electronegativity order of hybridized carbons is $\ce{sp > sp2 > sp3}$. So in an allylic halide, such as $\ce{CH2=CH-CH2Br}$, the alpha carbon is much more partially positive than the alpha carbon in $\ce{CH3-CH2-CH2Br}$. Therefore, the attacking nucleophile will be attracted to the alpha carbon more in the allylic halide compound than in the primary halide. Hence, allylic halides tend to follow SN2 pathways more than primary halides.

  • $\begingroup$ Can you explain why are they more reactive than primary halides in SN2 reactions? $\endgroup$ – Kirti Agrawal Feb 26 '18 at 12:03
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    $\begingroup$ @KirtiAgrawal I have edited my question with the answer. I hoped it helped you! But next time please give one example at least and describe your problem instead of making your question so broad. This will help us answer you much better! $\endgroup$ – dr.drizzy Feb 26 '18 at 12:15

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