Alcohol to chloroalkane (thionyl chloride): practice and mechanism
Thionyl chloride swaps the OH group for Cl; SO₂ and HCl escape as gases.
Example
Thionyl chloride turns the OH group into a chlorosulfite ester, which chloride replaces; SO₂ and HCl escape as gases. Alcohols give chloroalkanes, carboxylic acids give acyl chlorides.
Mechanism step by step
- 1
The alcohol attacks the sulfur of SOCl₂ with its oxygen and Cl⁻ leaves.
- 2
A chlorosulfite ester R–O–S(=O)Cl forms; pyridine traps the HCl.
- 3
Chloride attacks the carbon (with pyridine: SN2, inversion); SO₂ and Cl⁻ leave.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That is only the intermediate (chlorosulfite ester). It falls apart: chloride replaces the group and SO₂ escapes.
- An alkene would come from elimination, e.g. with concentrated sulfuric acid and heat. SOCl₂ with pyridine substitutes.
Often confused with: Alcohol to haloalkane (HBr) · Acid-catalysed dehydration (E1)
Frequently asked questions
Why is SOCl₂ so handy?
The by-products SO₂ and HCl are gases and escape, so work-up is easy.
And for bromoalkanes?
Use PBr₃: it likewise converts primary and secondary alcohols via SN2.