Acyl chloride preparation
Reaction with phosphorus(V) chloride replaces the carboxyl OH group with chlorine, retaining the acyl carbon and aromatic ring. The new acyl chloride is more susceptible to nucleophilic substitution, while POCl3 and HCl account for the remaining atoms.
- Reagents
- PCl5
- Conditions
- Dry conditions; controlled reaction
- Reaction class
- Acyl chloride preparation
- Equation
- C6H5COOH + PCl5 -> C6H5COCl + POCl3 + HCl
Overview
Reaction with phosphorus(V) chloride replaces the carboxyl OH group with chlorine, retaining the acyl carbon and aromatic ring. The new acyl chloride is more susceptible to nucleophilic substitution, while POCl3 and HCl account for the remaining atoms.
Transformation
Benzoic acid → Benzoyl chloride
- Equation
- C6H5COOH + PCl5 -> C6H5COCl + POCl3 + HCl
- Reactants
- Benzoic acid + Phosphorus pentachloride
- Products
- Benzoyl chloride + Phosphoryl chloride + Hydrogen chloride
- Reagents
- PCl5
- Environment
- Dry conditions; controlled reaction
- Reaction class
- Acyl chloride preparation
- Mechanism
- carboxylic-acid activation and substitution
- Evidence level
- source-backed molecular example
Scope and limitations
- Scope
- Carboxyl OH is replaced by Cl.
- Limitations
- Moisture hydrolyses benzoyl chloride; phosphorus-containing and HCl by-products must be included.
Related reactions
- Side chain oxidation: Toluene → Benzoic acid
Toluene gives benzoic acid through side-chain oxidation.
- Complete oxidation: Benzyl alcohol → Benzoic acid
Oxidation first changes the benzylic CH2OH group into an aldehyde and then into carboxylate under the alkaline conditions. Acid work-up gives benzoic acid; the ring is retained while the side-chain carbon becomes more oxidised.
- Aldehyde oxidation: Benzaldehyde → Benzoic acid
Water reversibly adds to the aldehyde to form a hydrate, which can then be oxidised. The aldehyde carbon becomes the acid carbonyl carbon; acid work-up converts the initially formed benzoate into benzoic acid.
- Amide formation: Benzoyl chloride → Benzamide
Ammonia adds to the acyl carbonyl to form a tetrahedral intermediate. Carbonyl re-formation expels chloride, and proton transfer gives benzamide; additional ammonia captures the acid as ammonium chloride.
- Ester formation: Benzoyl chloride → Ethyl benzoate
Ethanol oxygen attacks the acyl carbonyl carbon. Loss of chloride and proton transfer restore C=O and form the ester, with ethanol contributing the ethoxy group and HCl produced in the overall equation.
- Esterification: Benzoic acid → Ethyl benzoate
Acid activation allows ethanol to add to benzoic acid; proton transfers and water loss produce ethyl benzoate. The aromatic ring stays intact throughout the acyl substitution.
References
- Pearson Edexcel International Advanced Level Chemistry Scheme of WorkPearson Education Limited · Pearson qualifications · 2018
Official Pearson teaching guidance specifying chloroalkane-relevant conditions: aqueous alkali, ethanolic potassium hydroxide, alcoholic ammonia, alcoholic potassium cyanide, and PCl5 alcohol chlorination.