Alkyl Halides & Nucleophilic Substitution

Introduction

This section covers the fundamental mechanisms of nucleophilic substitution reactions in organic chemistry, focusing on alkyl halides. Two primary mechanisms, SN2 (bimolecular) and SN1 (unimolecular), are discussed, along with their kinetic features, stereochemical outcomes, and factors influencing their rates.

Features of SN2 Mechanism

Overview of SN2

The SN2 mechanism (Substitution, Nucleophilic, Bimolecular) is a concerted process where bond breaking and bond making occur simultaneously. The reaction rate depends on both the alkyl halide (electrophile) and the nucleophile concentrations.

• Rate Law:

• Kinetics: Second-order overall (first order in each reactant)

• Mechanism: Single-step, concerted transition state

Example: Substitution of methyl bromide by acetate ion.

Degree of Substitution

The rate of SN2 reactions is highly sensitive to the degree of substitution at the carbon bearing the leaving group due to steric hindrance.

• Order of Reactivity: methyl > 1° alkyl > 2° alkyl >> 3° alkyl

• Reason: Increasing substitution increases steric hindrance, which slows down the backside attack required for SN2.

Example: Methyl bromide reacts much faster than tert-butyl bromide in SN2 reactions.

Backside Attack and Stereochemistry

SN2 reactions proceed via a backside attack, where the nucleophile approaches the electrophilic carbon from the side opposite the leaving group. This leads to a specific stereochemical outcome known as Walden inversion.

• Backside Attack: Nucleophile attacks at a 180° angle to the leaving group.

• Inversion of Configuration: The product has the opposite configuration at the stereocenter compared to the starting material.

• Stereochemical Outcome: A single enantiomer is formed if the starting material is optically pure.

Example: Substitution of (R)-2-bromobutane by a nucleophile yields (S)-2-substituted butane.

Features of SN1 Mechanism

Overview of SN1

The SN1 mechanism (Substitution, Nucleophilic, Unimolecular) is a stepwise process involving the formation of a carbocation intermediate. The rate depends only on the concentration of the alkyl halide.

• Rate Law:

• Kinetics: First-order overall

• Mechanism: Two steps: (1) formation of carbocation, (2) nucleophilic attack

Example: Substitution of tert-butyl bromide by acetate ion.

Degree of Substitution

SN1 reactions are favored by more substituted alkyl halides due to increased carbocation stability.

• Order of Reactivity: methyl < 1° alkyl << 2° alkyl < 3° alkyl

• Reason: Tertiary carbocations are stabilized by hyperconjugation and inductive effects.

Example: Tert-butyl bromide reacts much faster than methyl bromide in SN1 reactions.

Stereochemical Outcome

SN1 reactions lead to racemization at the stereocenter because the nucleophile can attack the planar carbocation from either side.

• Racemization: Both enantiomers are formed in equal amounts if the starting material is optically pure.

• Intermediate: Trigonal planar carbocation allows attack from both sides.

Example: Substitution of (R)-2-bromobutane by a nucleophile yields a racemic mixture of (R)- and (S)-2-substituted butane.

Experimental Evidence: Rate Laws

Observed Changes in Rate Law

Experimental studies show that methyl bromide undergoes substitution with acetate via a second-order rate law, while tert-butyl bromide follows a first-order rate law. This indicates two distinct mechanisms:

• SN2:

• SN1:

Conclusion: SN2 is concerted (bimolecular), SN1 is stepwise (unimolecular).

Summary Table: SN2 vs SN1 Mechanisms

Key Terms and Concepts

• Alkyl Halide: An organic compound containing a halogen atom bonded to an sp3 carbon.

• Nucleophile: A species that donates an electron pair to form a new covalent bond.

• Leaving Group: An atom or group that departs with an electron pair in a substitution or elimination reaction.

• Carbocation: A positively charged carbon species, intermediate in SN1 reactions.

• Walden Inversion: The stereochemical inversion that occurs in SN2 reactions.

Additional info:

• These notes are based on lecture slides and summarize the main features, mechanisms, and experimental evidence for SN2 and SN1 nucleophilic substitution reactions of alkyl halides.

• For further study, refer to recommended textbook problems (e.g., Smith 7.19, 7.21, 7.23, 7.27, 7.28, 7.29, 7.34).