Substitution Reactions:Explain the mechanism of nucleophilic substitution reactions (SN1 and SN2) in haloalkanes with suitable examples.

Nucleophilic substitution reactions in haloalkanes can occur via two different mechanisms: SN1 and SN2.

1. SN1 (Substitution Nucleophilic Unimolecular) Reaction:

The SN1 reaction is a two-step process involving the formation of a carbocation intermediate.

Step 1: Ionization The bond between the carbon and the halogen breaks to form a carbocation and a halide ion. This is the slow, rate-determining step. For example, in the reaction of 2-chloro-2-methylpropane with water, the carbocation is a tertiary carbocation, which is relatively stable.

(CH3)3C-Cl → (CH3)3C+ + Cl-

Step 2: Nucleophilic Attack The carbocation then reacts with the nucleophile to form the substitution product. In our example, water acts as the nucleophile and attacks the carbocation.

(CH3)3C+ + H2O → (CH3)3C-OH + H+

2. SN2 (Substitution Nucleophilic Bimolecular) Reaction:

The SN2 reaction is a one-step process where the nucleophile attacks the carbon atom and the leaving group leaves simultaneously.

For example, in the reaction of bromoethane with hydroxide ions, the hydroxide ion attacks the carbon atom from the side opposite to the bromine atom, leading to the inversion of configuration at the carbon atom.

CH3CH2-Br + OH- → CH3CH2-OH + Br-

In this reaction, the hydroxide ion is the nucleophile, bromine is the leaving group, and ethanol is the substitution product.

In both SN1 and SN2 reactions, the nucleophile replaces the halogen in the haloalkane. However, the rate of reaction and the stereochemistry of the product can vary depending on the type of haloalkane and the conditions of the reaction.