Acyl chloride from carboxylic acid: practice and mechanism
SOCl₂ replaces the OH of the carboxyl group with Cl and the C=O stays: R–COOH → R–COCl.
Example
Thionyl chloride replaces the OH group of the acid with chlorine; SO₂ and HCl escape. The acyl chloride is the most reactive acid derivative and the starting point for esters, amides and Friedel–Crafts acylations.
Mechanism step by step
- 1
The OH of the acid attacks the sulfur of SOCl₂ and Cl⁻ leaves.
- 2
A mixed anhydride (acyl chlorosulfite) forms, a very good leaving group.
- 3
Chloride attacks the carbonyl carbon (addition–elimination); SO₂ and HCl escape.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Why thionyl chloride
With SOCl₂ the only by-products besides the acid chloride are SO₂ and HCl, both gases. They escape, the equilibrium lies fully on the product side and workup is easy. Catalytic DMF is often added to speed things up.
Other reagents
Oxalyl chloride (with DMF) works more gently at room temperature; PCl₅ and PCl₃ also work. All of them turn the poor OH leaving group into a good one.
What acid chlorides are for
Acid chlorides are the most reactive carboxylic acid derivatives. With alcohols they give esters, with amines amides, with arenes and AlCl₃ ketones (Friedel–Crafts acylation). They react vigorously with water back to the acid, so work is done under dry conditions.
Typical mistakes
- The C=O group stays; only the OH is swapped for Cl. Chloroalkanes come from alcohols, not from acids.
- Chlorine binds to the carbonyl carbon, not to the oxygen. The OH oxygen leaves (as SO₂).
- The C=O double bond stays and only one Cl is added. The product is the acyl chloride R–C(=O)Cl.
Often confused with: Fischer esterification · Reduction with lithium aluminium hydride
Frequently asked questions
Why an acyl chloride as an intermediate?
It is the most reactive derivative: with alcohols, amines or arenes (AlCl₃) it reacts quickly and completely.
What is the reactivity order of the derivatives?
Acyl chloride > anhydride > ester ≈ acid > amide. You can only convert "downhill", e.g. acyl chloride → ester → amide.