SN2 Alcohol Formation [HO⁻]
SN2 Alcohol Formation [HO⁻]: This reaction turns an alkyl halide into an alcohol. Step-by-step mechanism, reagents and practice.
What happens
This reaction turns an alkyl halide into an alcohol. The hydroxide ion (HO⁻) acts as a nucleophile. It attacks the carbon attached to the leaving group and replaces the halogen with OH.
Reaction summary
R–X + HO⁻ → R–OH + X⁻ (X = Cl, Br, or I); e.g. 1-bromobutane → butan-1-ol
Reagents and conditions
- Starting material: 1-Bromobutane (primary alkyl halide)
- Reagents: Hydroxide ion (HO⁻)
- Products: Butan-1-ol (alcohol); Bromide ion (Br⁻)
Mechanism
- Concerted Backside Attack (SN2): This reaction happens in one step. HO⁻ attacks the carbon attached to the leaving group from the backside, and at the same time the C–X bond breaks and X⁻ leaves. There is no intermediate.
- Product: The alcohol is formed, and the halide has left as X⁻.
Selectivity
- Stereoselectivity: SN2 reactions always involve backside attack. If the reacting carbon is a stereocenter, its configuration is inverted (Walden inversion): think of an umbrella flipping inside out. The reacting carbon of 1-bromobutane is not a stereocenter, so no inversion shows in this example.
- Chemoselectivity: Which alkyl halides work best? SN2 reactions need room for the nucleophile to attack: methyl → fastest; primary (1°) → good; secondary (2°) → slower / may compete with E2; tertiary (3°) → essentially no SN2. The more crowded the carbon becomes, the harder it is for HO⁻ to attack from behind. HO⁻ is both a strong nucleophile and a strong base, so with more substituted substrates E2 elimination can compete.
Common mistakes
- Don't think that OH is simply added to the molecule. It replaces the leaving group (R–Br → R–OH), so this is substitution, not addition.
- Tertiary (3°) alkyl halides: essentially no SN2. The more crowded the carbon, the harder it is for HO⁻ to attack from behind.
- HO⁻ is both a strong nucleophile and a strong base, so with more substituted substrates, E2 elimination can compete.
Memory trick
SN2 = Swap in 1 Step. HO⁻ comes in while X⁻ goes out: R–X → R–OH. And always remember: backside attack → inversion 🔄