Comparing the SN1 and SN2 Reactions

Introduction

Nucleophilic substitution reactions involve breaking a bond between carbon and a leaving group (CLG) and forming a new bond between carbon and a nucleophile (CNu).
SN1 and SN2 are the main mechanisms for nucleophilic substitution of alkyl halides.

Unimolecular rate-determining step: two-step process.
- Step 1: Formation of a carbocation by the leaving group departure (slow, rate-determining step).
- Step 2: Nucleophile attacks the carbocation (fast step).
- Often followed by an acid-base step if neutral nucleophiles like H2O or ROH are used.
Carbocation stability is crucial for SN1.
- Favored by tertiary alkyl halides: Stability order is tertiary > secondary > primary.
Results in retention and inversion of stereochemistry.

Bimolecular rate-determining step: Single-step, concerted mechanism.
- Nucleophile attacks the backside of the CLG bond, passing through a transition state.
- Results in inversion of configuration at the carbon atom.
Steric hindrance is a major barrier.
- Favored by methyl and primary alkyl halides.

SN1: Rate depends solely on the concentration of the alkyl halide.
- Rate = k[Concentration of alkyl halide].
SN2: Rate depends on both nucleophile and substrate.
- Rate = k[Concentration of alkyl halide][Concentration of nucleophile].

SN1: Polar protic solvents stabilize carbocations and often serve as nucleophiles.
SN2: Polar aprotic solvents are preferred as they increase nucleophile reactivity by not solvating it with hydrogen bonds.

SN1 and SN2 differ in their mechanisms, rate laws, stereochemistry, and solvent preferences.
SN1 involves carbocation intermediates and is favored by more substituted substrates.
SN2 is a concerted mechanism and is preferred with less hindered, primary substrates.

Understanding the differences between SN1 and SN2 reactions is crucial in predicting the outcomes of nucleophilic substitutions in organic chemistry.