MP Board · Class 12 · Chemistry · Haloalkanes and HaloarenesExplain the mechanism of $SN1$ and $SN2$ reactions with respect to alkyl halides. Discuss the stereochemical aspects of both mechanisms in detail.

Introduction to Nucleophilic Substitution Reactions\nNucleophilic substitution reactions are among the most important reactions of haloalkanes. These reactions broadly proceed via two primary mechanisms: $S_N1$ (Substitution Nucleophilic Unimolecular) and $S_N2$ (Substitution Nucleophilic Bimolecular).

1. $S_N2$ Mechanism (Substitution Nucleophilic Bimolecular)

• Kinetic Characteristics: The rate of the reaction depends upon the concentration of both the alkyl halide and the nucleophile. Therefore, it is a second-order reaction.
• Mechanism Steps: It is a single-step concerted process where bond formation and bond breaking occur simultaneously. There is no intermediate formed.
• Transition State: During the reaction, the incoming nucleophile attacks from the side opposite to the leaving group (backside attack). This forms an unstable transition state where both the nucleophile and leaving group are partially bonded to the carbon atom.
• Stereochemical Outcome: This backside attack results in the inversion of configuration, commonly known as the Walden inversion. If the starting material is optically active, it leads to the inversion of the optical rotation.
• Reactivity Order: Primary alkyl halides > Secondary alkyl halides > Tertiary alkyl halides, mainly due to steric hindrance.

2. $S_N1$ Mechanism (Substitution Nucleophilic Unimolecular)

• Kinetic Characteristics: The rate of the reaction depends only upon the concentration of the alkyl halide and is independent of the concentration of the nucleophile. Thus, it is a first-order reaction.
• Mechanism Steps: It occurs in two steps:
  1. Step 1: The polarized carbon-halogen bond breaks heterolytically to form a planar carbocation intermediate. This is the slowest and rate-determining step.
  2. Step 2: The nucleophile rapidly attacks the planar carbocation from either side to form the final substitution product.
• Stereochemical Outcome: Since the carbocation intermediate is planar ($sp^2$ hybridized), the incoming nucleophile has an equal probability of attacking from the front or the back. This leads to racemization, yielding a racemic mixture (50% inversion and 50% retention of configuration).
• Reactivity Order: Tertiary alkyl halides > Secondary alkyl halides > Primary alkyl halides, due to the greater stability of tertiary carbocations.