Acid Chloride Formation with Thionyl Chloride [SOCl₂]
Acid Chloride Formation with Thionyl Chloride [SOCl₂]: A carboxylic acid (–COOH) reacts with thionyl chloride (SOCl₂) to form an acid chloride (–COCl): the…
What happens
A carboxylic acid (–COOH) reacts with thionyl chloride (SOCl₂) to form an acid chloride (–COCl): the OH is replaced by Cl. SOCl₂ is especially useful because the byproducts are gases, SO₂ and HCl, and their removal helps drive the reaction toward the acid chloride. The carbonyl is retained and the carbon count is unchanged. Acid chlorides are much more reactive than carboxylic acids, so they are useful activated derivatives: R–COCl → ester, amide or anhydride.
Reaction summary
Carboxylic acid + SOCl₂ → acid chloride + SO₂ + HCl (COOH → COCl)
Reagents and conditions
- Starting material: Propanoic acid (carboxylic acid)
- Reagents: Thionyl chloride (SOCl₂)
- Products: Propanoyl chloride (acid chloride); Sulfur dioxide (gas); Hydrogen chloride (gas)
Mechanism
- Activate the Carboxylic Acid: The oxygen of the carboxylic acid reacts with sulfur in SOCl₂. This converts the poor leaving group –OH into a much better leaving group, and a reactive sulfur-containing intermediate forms.
- Chloride Attacks: Cl⁻ acts as a nucleophile and attacks the carbonyl carbon, creating a tetrahedral intermediate.
- Intermediate Collapses: The tetrahedral intermediate collapses and the activated oxygen-containing group leaves. The C=O is re-formed.
- Acid Chloride Forms: The final organic product is R–COCl, with gaseous byproducts ultimately including SO₂ and HCl.
Common mistakes
- Don't confuse this with the reaction of alcohols with SOCl₂. An alcohol gives an alkyl chloride (R–OH → R–Cl), but a carboxylic acid gives an acid chloride (R–COOH → R–COCl). Same reagent, different functional-group transformation.
Memory trick
SOCl₂ puts Cl where the acid OH was.