SN1 versus SN2 reactions, Factors affecting SN1 and SN2 reactions

Comparing the key features of SN1 and SN2 reactions:

This table summarizes the main differences between SN1 and SN2 reactions in terms of kinetics, reaction mechanisms, reactivity of alkyl halides, stereochemistry, carbocation rearrangement, nucleophile preferences, and the effect of steric hindrance.

Factors affecting SN1 and SN2 reactions

1) Structure of Alkyl Halide

• SN1: Tertiary (3°) alkyl halides are most reactive due to stable carbocation formation.

• SN2: Methyl and primary (1°) alkyl halides are most reactive due to minimal steric hindrance.

2) Nucleophile Strength

• SN1: Favors weak nucleophiles, often neutral, to minimize competing elimination reactions.

• SN2: Prefers strong nucleophiles, often negatively charged, for their readiness to attack the alkyl halide.

3) Solvent Polarity

• SN1: Polar protic solvents like water and methanol are favored to stabilize the carbocation intermediate and leaving group.

• SN2: Polar aprotic solvents such as acetone and DMSO are favored because they do not solvate the nucleophile, increasing its reactivity.

4) Steric Hindrance

• SN1: Less significant, as the carbocation intermediate is planar and accessible.

• SN2: Extremely important; high steric hindrance prevents the nucleophile's access to the reaction center, thus impeding the reaction.

5) Leaving Group Ability

• SN1 and SN2: Both mechanisms favor efficient leaving groups that can easily dissociate from the alkyl halide. Ideal leaving groups are weak bases like halide ions (I-, Br-, Cl-) and sulfonate ions (e.g., tosylate, mesylate).