What is the difference between SN1 and SN2 reactions?

Definition and Explanation

SN1 and SN2 are two types of nucleophilic substitution reactions in organic chemistry.

• SN1 (Substitution Nucleophilic Unimolecular):
• The rate-determining step involves only the substrate.
• Proceeds via a two-step mechanism: formation of a carbocation intermediate, then nucleophilic attack.
• Rate law:

$$ \text{Rate} = k[\text{R-L}] $$
where R-L is the alkyl halide.

• SN2 (Substitution Nucleophilic Bimolecular):
• The rate-determining step involves both the substrate and the nucleophile.
• Proceeds via a single concerted step: nucleophile attacks as the leaving group departs.
• Rate law:

$$ \text{Rate} = k[\text{R-L}][\text{Nu}^-] $$

Worked Example

Example: Reaction of 2-bromopropane with hydroxide ion ($ce{OH^-}$).

SN1 Mechanism

1. Carbocation Formation:

$$ ce{(CH_3)_2CHBr} \longrightarrow ce{(CH_3)_2CH^+} + ce{Br^-} $$

2. Nucleophilic Attack:

$$ ce{(CH_3)_2CH^+} + ce{OH^-} \longrightarrow ce{(CH_3)_2CHOH} $$

SN2 Mechanism

3. Single Step:

$$ ce{(CH_3)_2CHBr} + ce{OH^-} \longrightarrow ce{(CH_3)_2CHOH} + ce{Br^-} $$

• The nucleophile attacks from the opposite side of the leaving group (backside attack), causing inversion of configuration.

Takeaways

• SN1: Two-step, forms carbocation, rate depends only on substrate, leads to racemization.
• SN2: One-step, no carbocation, rate depends on both substrate and nucleophile, causes inversion of stereochemistry.
• Choice between SN1 and SN2 depends on substrate structure, nucleophile strength, and solvent.