SN1 versus and SN2 reactions comparing the key features:
| Feature | SN1 Reaction | SN2 Reaction |
| Kinetics | First-order kinetics: Rate = k [alkyl halide] | Second-order kinetics: Rate = k [alkyl halide] [nucleophile] |
| Reaction mechanism | Two-step mechanism with carbocation intermediate | One-step concerted mechanism |
| Order of reactivity of alkyl halides | 3° > 2° > 1° > methyl halide | methyl halide > 1° > 2° > 3° |
| Stereochemistry | Racemization (formation of racemic mixture) | Inversion of stereochemistry (Walden inversion) |
| Carbocation rearrangement | Possible, if it leads to a more stable carbocation | No rearrangement, as no carbocation intermediate is formed |
| Nucleophile | Weak nucleophiles, often neutral | Strong nucleophiles, often negatively charged |
| Effect of steric hindrance | Less important, as the reaction proceeds through a carbocation intermediate | Crucial, as steric hindrance affects the nucleophile’s approach to the reaction center |
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
There are typically 5 Factors affecting SN1 and SN2 reactions which are explained below in detail:
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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.
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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.
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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.
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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.
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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).
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