NCERT · Class 12 · Chemistry · Haloalkanes and HaloarenesExplain the mechanism of $SN1$ and $SN2$ nucleophilic substitution reactions taking tertiary butyl chloride and primary butyl chloride as respective examples. Discuss the stereochemical aspects and factors affecting these reactions.

1. Introduction to Nucleophilic Substitution Reactions\nNucleophilic substitution reactions are fundamental reactions in haloalkanes where a nucleophile replaces a halogen atom attached to a carbon atom. These generally proceed via two major mechanisms: $S_N1$ (Substitution Nucleophilic Unimolecular) and $S_N2$ (Substitution Nucleophilic Bimolecular).

2. $S_N2$ Mechanism (Substitution Nucleophilic Bimolecular)

• Reaction and Kinetics: Primary alkyl halides like primary butyl chloride typically react via the $S_N2$ mechanism. The rate of the reaction depends on the concentration of both the reactant and the nucleophile, following second-order kinetics.
• Step-by-Step Mechanism: It is a single-step concerted reaction. There are no intermediates formed. The incoming nucleophile attacks the carbon atom from the side opposite to the halogen atom (backside attack).
• Transition State: As the bond between the nucleophile and carbon starts forming, the bond between carbon and the halogen starts breaking simultaneously, passing through an unstable transition state where all three remaining bonds are partially coplanar.
• Stereochemistry: This mechanism leads to complete inversion of configuration, known as the Walden inversion. If the reactant is optically active, the product will have the opposite configuration.
• Factors Affecting $S_N2$: Steric hindrance plays a crucial role. Primary halides have minimum steric hindrance, hence they react fastest via $S_N2$. Polar aprotic solvents (like acetone, DMSO) favor this reaction.

3. $S_N1$ Mechanism (Substitution Nucleophilic Unimolecular)

• Reaction and Kinetics: Tertiary alkyl halides like tertiary butyl chloride react via the $S_N1$ mechanism. The rate of the reaction depends only on the concentration of the alkyl halide, following first-order kinetics.
• Step-by-Step Mechanism: It occurs in two steps:
  1. Step 1 (Slow step): The polarized C-X bond breaks heterolytically to form a carbocation intermediate and a halide ion. This is the rate-determining step.
  2. Step 2 (Fast step): The nucleophile rapidly attacks the planar carbocation to form the final substitution product.
• Stereochemistry: Since the carbocation intermediate is $sp^2$ hybridized and planar, the nucleophile can attack from either side with equal probability. This leads to racemization (formation of a racemic mixture containing equal amounts of both enantiomers) if the starting material is chiral.
• Factors Affecting $S_N1$: Stability of the carbocation determines the reactivity. Since tertiary carbocations are the most stable due to hyperconjugation and inductive effects, tertiary halides react fastest via $S_N1$. Polar protic solvents (like water, alcohol) stabilize the carbocation intermediate through hydrogen bonding and favor this pathway.