SN2 Thiol Formation [HS⁻]
SN2 Thiol Formation [HS⁻]: This reaction turns an alkyl halide into a thiol. Step-by-step mechanism, reagents and practice.
What happens
This reaction turns an alkyl halide into a thiol. The hydrosulfide ion (HS⁻) acts as a nucleophile. It attacks the carbon attached to the leaving group and replaces the halogen with SH. A thiol (R–SH) is similar to an alcohol, but it contains sulfur instead of oxygen.
Reaction summary
R–X + HS⁻ → R–SH + X⁻ (X = Cl, Br, or I); e.g. 1-bromopropane → propane-1-thiol
Reagents and conditions
- Starting material: 1-Bromopropane (primary alkyl halide)
- Reagents: Hydrosulfide ion (HS⁻)
- Products: Propane-1-thiol (thiol); Bromide ion (Br⁻)
Mechanism
- Concerted Backside Attack (SN2): The SN2 reaction happens in one step. HS⁻ 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 carbocation intermediate.
- Product: The final thiol is formed, and the halide has left as X⁻.
Selectivity
- Stereoselectivity: SN2 reactions involve backside attack. If the carbon being attacked is a stereocenter, its configuration is inverted (Walden inversion). The reacting carbon of 1-bromopropane is not a stereocenter, so no inversion shows in this example.
- Chemoselectivity: Which alkyl halides work best? SN2 works best when the reacting carbon is not crowded: methyl → fastest; primary (1°) → very good; secondary (2°) → slower; tertiary (3°) → essentially no SN2. The nucleophile needs enough space to attack from the backside.
Common mistakes
- Don't confuse a thiol with an alcohol. Alcohol: R–OH. Thiol: R–SH. So HO⁻ → alcohol, HS⁻ → thiol.
- Tertiary (3°) alkyl halides: essentially no SN2. The nucleophile needs enough space to attack from the backside.
- HS⁻ is a good nucleophile but a weaker base than HO⁻, which can make substitution more favorable relative to E2 in many cases.
Memory trick
HS⁻ brings the SH. R–X → R–SH. The halogen leaves, SH takes its place, and because it's SN2: backside attack → inversion 🔄