11.4 The SN1 Reaction

SN1 Reaction Mechanism and Factors

The SN1 reaction is a two-step substitution where the leaving group departs before the nucleophile attacks. That distinction from SN2 (where bond-breaking and bond-forming happen simultaneously) drives every difference in kinetics, stereochemistry, and substrate preference you need to know.

Mechanism of SN1 Reactions

The "S" stands for substitution, "N" for nucleophilic, and "1" for unimolecular, meaning only one molecule is involved in the rate-determining step.

Step 1 (slow, rate-determining): The leaving group departs on its own, generating a planar carbocation intermediate. Because this step is slow and involves only the substrate, the reaction follows first-order kinetics:

$\text{rate} = k[\text{substrate}]$

Notice that the nucleophile concentration doesn't appear in the rate law. Doubling the nucleophile concentration has no effect on how fast the reaction goes.

Step 2 (fast): The nucleophile attacks the carbocation. Because the carbocation is $sp^2$-hybridized with an empty p orbital, it's flat. The nucleophile can approach from either face (top or bottom), which has major consequences for stereochemistry.

Stereochemistry in SN1 Reactions

If the substrate has a stereocenter at the carbon bearing the leaving group, the SN1 reaction produces a racemic mixture (roughly 50:50 R and S enantiomers). Here's why:

• The flat, $sp^2$ carbocation has no "memory" of the original configuration.
• Nucleophilic attack from the top face gives one enantiomer; attack from the bottom face gives the other.
• Both faces are equally accessible, so you get near-equal amounts of each.

This is a sharp contrast with SN2, which proceeds through backside attack and gives clean inversion of configuration at the stereocenter. If an exam question asks you to predict stereochemistry, the mechanism tells you everything: SN1 → racemization, SN2 → inversion.

SN1 vs. SN2 Reaction Factors

Substrate structure

• SN1 favored: Tertiary (3°) substrates and others that form stable carbocations. For example, tert-butyl bromide ionizes readily because three alkyl groups stabilize the carbocation through hyperconjugation and inductive electron donation. Allylic and benzylic substrates also favor SN1 because the resulting carbocations are resonance-stabilized.
• SN2 favored: Methyl and primary (1°) substrates, where the carbon bearing the leaving group is relatively unhindered. Steric crowding around a 3° carbon makes backside attack by the nucleophile nearly impossible.
• Secondary (2°) substrates are the borderline case and can go either way depending on solvent, nucleophile strength, and temperature.

Solvent effects

• SN1 favored by polar protic solvents (water, methanol, ethanol). These solvents stabilize the carbocation intermediate through solvation and help "pull off" the leaving group via hydrogen bonding. Stabilizing both the cation and the departing anion lowers the energy of the rate-determining step.
• SN2 favored by polar aprotic solvents (DMSO, acetone, DMF). These solvents solvate the cation of the nucleophile's salt (e.g., $\text{Na}^+$) but leave the nucleophile relatively "naked" and highly reactive. They don't stabilize carbocations well, so SN1 is disfavored.

Additional Factors Affecting SN1 Reactions

Leaving group ability directly controls Step 1. A better leaving group departs more easily, speeding up carbocation formation. The general trend for common leaving groups:

$\text{tosylate} > \text{I}^- > \text{Br}^- > \text{Cl}^- \gg \text{F}^-$

Weak bases are good leaving groups because they're stable once they depart.

Carbocation stability is the single most important factor. The easier it is to form the carbocation, the faster the SN1 reaction. Stability order:

$3° > 2° > 1° > \text{methyl}$

Resonance-stabilized carbocations (allylic, benzylic) can also undergo SN1 even if they aren't tertiary.

Solvolysis is a specific type of SN1 reaction where the solvent itself acts as the nucleophile. For example, dissolving tert-butyl bromide in ethanol gives an SN1 reaction where ethanol attacks the carbocation. Solvolysis reactions are common on exams because they clearly illustrate SN1 conditions: no strong nucleophile is added, the substrate is tertiary, and the solvent is polar protic.