🎓 Prepared by students from Boğaziçi University

What Are SN1 and SN2 Substitution Reactions?

Nucleophilic substitution reactions replace one group with another on a carbon atom. SN1 (unimolecular) goes through a carbocation intermediate, while SN2 (bimolecular) is a direct, concerted displacement. Understanding which mechanism dominates is key to predicting reaction outcomes.

Short answer

SN1 is a two-step mechanism through a carbocation intermediate (order-1 kinetics, favored by weak nucleophiles and polar solvents). SN2 is one-step, direct attack with inversion of configuration (order-2 kinetics, favored by strong nucleophiles and polar aprotic solvents).

SN1 vs SN2: Key Differences

SN1 (Unimolecular)

•Two-step mechanism
•Carbocation intermediate
•First-order kinetics
•Racemic mixture
•Weak nucleophiles
•Polar protic solvent

SN2 (Bimolecular)

•One-step mechanism
•No intermediate
•Second-order kinetics
•Inversion of configuration
•Strong nucleophiles
•Polar aprotic solvent

Step-by-step worked examples

Predict the mechanism for (CH₃)₃C-Br with CN⁻ in DMSO.

Substrate: tertiary alkyl halide (favors carbocation)
Nucleophile: CN⁻ (strong)
Solvent: DMSO (polar aprotic)
Conclusion: SN2 dominates due to strong Nu and polar aprotic solvent.
Product: (CH₃)₃C-CN with inversion.

What happens with CH₃-I and H₂O (protic solvent)?

Substrate: primary alkyl halide
Nucleophile: H₂O (weak in protic solvent)
Solvent: H₂O (polar protic)
Conclusion: SN2 is favored for primary.
Product: CH₃-OH (methanol) with inversion.

Explain the stereochemistry when (R)-2-bromobutane reacts via SN2.

SN2 requires backside attack.
Walden inversion occurs.
Configuration inverts from R to S.
Product: (S)-2-nucleophile-butane.

Flashcards

Quick quiz

Q1.Tertiary alkyl bromide + weak nucleophile + polar protic solvent → ?

Correct answer: B. SN1 is favored: bulky substrate (carbocation stability), weak Nu, polar protic solvent.

Q2.What does 'Walden inversion' mean?

Correct answer: B. SN2 backside attack causes inversion of stereochemistry.

Q3.Primary alkyl halide + CN⁻ in DMF → kinetics?

Correct answer: B. SN2 is bimolecular → rate = k[RX][Nu⁻] (second-order).

Q4.Which solvent favors SN2?

Correct answer: B. DMSO is polar aprotic — does not solvate nucleophile as strongly, keeping it reactive.

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Common mistakes

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SN1 always gives only one product. — Correct: SN1 gives a mixture of stereoisomers (racemic or partially inverted) because the carbocation can be attacked from both sides.

✗

Stronger nucleophile favors SN1. — Correct: Stronger nucleophile favors SN2 (direct attack is faster than waiting for carbocation formation).

✗

SN2 with tertiary halides is very fast. — Correct: SN2 is very slow with tertiary halides due to steric hindrance; SN1 dominates instead.

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Polar protic solvents increase the rate of SN2. — Correct: Polar aprotic solvents increase SN2 rate because they solvate cations, not anions, leaving Nu⁻ more reactive.

FAQ

What is a nucleophilic substitution reaction?

A nucleophile (electron-rich) attacks an electrophilic carbon, displacing a leaving group.

How do you decide between SN1 and SN2?

Consider substrate (1°→SN2, 2°→mixed, 3°→SN1), nucleophile strength (strong→SN2), and solvent (aprotic→SN2, protic→SN1).

What is a leaving group?

A stable, weakly basic anion that departs during substitution (e.g., Br⁻, I⁻, OTs⁻).

Can SN1 and SN2 happen simultaneously?

Yes, especially with secondary substrates. The dominant mechanism depends on conditions.