Question

What is the nucleophilic substitution SN reaction of alkyl halides?

Answer 1

The nucleophilic substitution (SN) reaction of alkyl halides involves the substitution of a halogen atom (X) in an alkyl halide with a nucleophile (Nu). This reaction occurs via the attack of the nucleophile on the carbon atom bonded to the halogen, leading to the displacement of the halogen atom and the formation of a new bond between the carbon and the nucleophile.

There are two common mechanisms for nucleophilic substitution reactions of alkyl halides: SN1 (unimolecular) and SN2 (bimolecular).

1. SN1 (Unimolecular Nucleophilic Substitution):

   • The reaction proceeds in two steps.
   • Initially, the alkyl halide undergoes heterolysis (polar bond cleavage), resulting in the formation of a carbocation intermediate and a halide ion.
   • In the second step, the nucleophile attacks the carbocation, leading to the substitution product.
   • The rate of the reaction is influenced by the concentration of the alkyl halide only.
   • The reaction proceeds through a carbocation intermediate, so it is more likely to occur with tertiary alkyl halides due to the stability of the resulting carbocation.
   • The nucleophile can attack from either side of the planar carbocation, resulting in the formation of a racemic mixture (if a chiral center is present) or rearrangement products (if applicable).

2. SN2 (Bimolecular Nucleophilic Substitution):

   • The reaction proceeds in a single step, where the nucleophile attacks the alkyl halide at the same time the leaving group departs.
   • The nucleophile approaches the carbon atom from the opposite side of the leaving group, leading to an inversion of stereochemistry (Walden inversion) if a chiral center is present.
   • The reaction rate depends on the concentrations of both the alkyl halide and the nucleophile.
   • Primary alkyl halides are more reactive in SN2 reactions compared to secondary or tertiary alkyl halides.
   • Bulky substituents hinder the nucleophile's approach, so sterically hindered alkyl halides undergo SN2 reactions less readily.

It's important to note that the actual reaction mechanism and outcome can be influenced by various factors such as the nature of the alkyl halide, the strength of the nucleophile, the solvent used, and the reaction conditions.