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):
• Proceeds in two steps.
• The rate-determining step is the formation of a carbocation intermediate.
• Rate depends only on the concentration of the substrate:

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

• SN2 (Substitution Nucleophilic Bimolecular):
• Proceeds in a single, concerted step.
• The nucleophile attacks as the leaving group leaves.
• Rate depends on both the substrate and nucleophile:

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

Worked Example

Example: Reaction of 2-bromopropane with hydroxide ion.

SN1 Mechanism (Tertiary Alkyl Halide Example)

1. Carbocation Formation:

$$ (\text{CH}_3)_3\text{CBr} \longrightarrow (\text{CH}_3)_3\text{C}^+ + \text{Br}^- $$

2. Nucleophilic Attack:

$$ (\text{CH}_3)_3\text{C}^+ + \text{OH}^- \longrightarrow (\text{CH}_3)_3\text{COH} $$

SN2 Mechanism (Primary Alkyl Halide Example)

3. Single Step:

$$ \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \longrightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^- $$

• The nucleophile attacks from the opposite side, causing inversion of configuration (Walden inversion).

Takeaways

• SN1: Two steps, forms a 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 of mechanism depends on substrate structure, nucleophile strength, and solvent.