SOCl2
Summary
SOCl₂ is thionyl chloride, a reagent that converts carboxylic acids to acid chlorides and alcohols to alkyl chlorides. The OH becomes a good leaving group, and SO₂ and HCl leave as gases. Testable point: with a chiral alcohol and pyridine, substitution typically goes by SN2 with inversion.
Detail
Meaning: SOCl₂ replaces an OH group with Cl. The oxygen of the acid or alcohol attacks sulfur and a chloride is lost, giving a chlorosulfite intermediate (R–O–S(=O)Cl) that is an excellent leaving group. Chloride then attacks the carbonyl carbon of an acid (nucleophilic acyl substitution) or the alkyl carbon of an alcohol (substitution). The products SO₂ and HCl escape as gases, which drives the reaction forward.
Equation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl, and ROH + SOCl₂ → RCl + SO₂ + HCl. R is an alkyl or aryl group. These are molecular species, so no SI units apply.
On the MCAT: SOCl₂ appears in multistep syntheses. A carboxylic acid is activated to an acid chloride, which is then treated with an alcohol, amine or carboxylate to give an ester, amide or anhydride. With alcohols, it makes alkyl chlorides from primary and secondary alcohols; with pyridine present, a stereocenter is typically inverted. You may be asked to compare it with PBr₃, which makes alkyl bromides.
Common confusion: Students treat it like HCl and expect a carbocation and rearrangement. With primary and secondary alcohols, SOCl₂ typically avoids rearrangement because it does not go through a free carbocation. Also, acid chlorides react readily with water, so conditions must be dry or the acid re-forms.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.