SN2 reaction of hydrosulfide ion with alkyl halides to give thiols

by James

Description: Alkyl halides (or tosylates) will react with the hydrosulfide ion (HS–) to give thiols.

Notes: X here is a halide (Cl, Br, I) or sulfonate (OTs, OMs). Although Na is shown here as the counter-ion for HS(-), any alkali metal can be used (e.g. Li, Na, K)


Notes: Note that since this is an SN2 reaction and proceeds via backside attack, there will be inversion of configuration at the carbon (note the last two examples)

Mechanism: In the SN2 reaction the nucleophile (HS-) attacks the carbon with the good leaving group, forming a C–S bond and breaking the C–Br bond (Step 1, arrows A and B).

Notes: For more information on the SN2 see the summary sheet on this reaction. It isn’t crucial to show the sodium (Na) here.


{ 3 comments… read them below or add one }

Sachin Thakur.

The explanations given here are point perfect and there is no need for other references.


Pablo Guerrero

Don’t you have to use DMSO as the solvent for the second example?



A polar aprotic solvent like DMSO or acetonitrile would certainly be helpful for these reactions. It’s unlikely to have any competition with the E2 since the sulfide anion is not particularly basic (good threshold is the E2 doesn’t compete if the pKa of the conjugate acid is 12 or below. Thiols have a pKa of 12).


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