SN1 and SN2 Reactions of Allylic Halides and Tosylates

Allyl Halides and tosylates undergo substitution reactons.  Please review Nucleophilic Substitution: S_N1 and S_N2 Mechanisms before going on.

S_N1 Reactions of Allyl Halides

Secondary and tertiary allyl halides and tosylates have the potential to undergo Nucleophilic substitution by SN1 mechanism.

Can you write a mechanism to explain how the same two products are obtained in each reaction?

Can you speculate on why the above example has added Na₂CO₃?  What might happen if the reaction was run without Na₂CO₃ base?

SN2 reactions of Allyl Halides

Primary allyl halides can undergo nucleophilic substitution by an S_N2 concerted single step mechanism.

Notice that only the allylic chloride reacted NOT the vinylic chloride.   Recall that sp² halides and tosylates do not undergo SN2 reactions.

Its noteworthy that allyl haldes react by nucleophilic substitution reactions much faster than similar alkyl halides.  For example allyl chloride reacts over 800 times faster than propyl chloride.

This has been attributed to two factors; 1) Sterics and 2) Electronics.

1. Sterics - This is similar to the β-effect discussed in SN2 reactions.  The allyl group is less crowded than analogous alkyl halides.
   
   
   
   
2. Electronics - Because the delocalization of electrons is greater in the allyl group, the LUMO of ally chloride is more stable and lower in energy than the LUMO of propyl chloride.  This allows the LUMO of the allyl chloride to be more in line with the HOMO of the nucleophile.