Organic chemistry Part 10: Reactivity and reaction mechanisms

10.7 The hydrolysis of an acid/acyl chloride reaction mechanism, reaction with water to yield a carboxylic acid

[Author ©  Dr Phil Brown PhD: Doc Brown's exam revision notes suitable for A level chemistry students of advanced pre-university/college advanced level organic chemistry courses:  a hydrolysis mechanism for acid/acyl chlorides [mechanism updated April 14th 2026 *]

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10.7 Hydrolysis of acid/acyl chlorides by water - a nucleophilic addition-elimination substitution reaction mechanism

 Examples are explained of the organic chemistry mechanisms for acid/acyl chlorides undergoing nucleophilic addition and elimination reactions.

They are described with diagrams and full explanation revision notes.

Hydrolysis with water to give a carboxylic acid The revision notes include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions are also explained.

The >C=O is polarised because of the difference in electronegativity of the carbon (2.5) and oxygen (3.0). The four reactions described all involve an initial nucleophilic addition at the positive carbon of the polarised bond of the carbonyl group >C^δ+=O^δ– of the acyl chloride. This is followed by the elimination of a small molecule e.g. HCl.

• Examples of nucleophilic addition of water to acid/acyl chlorides,  subsequent elimination on hydrolysis to give the carboxylic acid and hydrogen chloride/hydrochloric acid

  • (i) + H₂O ===> + HCl

  • ethanoyl chloride + water ==> ethanoic acid + hydrogen chloride/hydrochloric acid

  • (ii) + H₂O ===> + HCl

  • benzenecarbonyl chloride, benzoyl chloride + water ==> benenecarboxylic acid/benzoic acid + hydrogen chloride/hydrochloric acid

• What is the mechanism for the hydrolysis of acid chlorides?

• e.g. R–COCl + 2H₂O ==> R–COOH + H₃O⁺ + Cl^–    [see mechanism 14 below]

  • effectively overall a nucleophilic substitution of Cl by OH (via H₂O)

  • R = alkyl or aryl

• The organic hydrolysis product is a carboxylic acid.

  • The above equation applies to excess water but with limited water you do get fumes of hydrogen chloride gas.

  • R–COCl + H₂O ==> R–COOH + HCl 

  • Acid chlorides tend to fume rather nastily in air as hydrogen chloride fumes are formed, and these will form hydrochloric with any moisture, most noticeably in the eyes!

mechanism 14 – nucleophilic addition–elimination reaction for the hydrolysis of an acyl chloride

• [mechanism 14 above] The mechanism involves several rearrangements and assumes excess water.

  • Step (1) The >C^δ+=O^δ– carbonyl is highly polarised and the positive carbon is attacked by the nucleophilic water molecule, acting as an electron pair donor.

  • The water adds to form a highly unstable ionic intermediate via a C–O bond and simultaneously the π electron pair of the C=O double bond moves onto the oxygen atom to give it a full negative charge.

  • The water is the nucleophile - the electron pair donor to a partially positive carbon atom.

  • Step (2) The C–Cl bond pair moves onto the chlorine atom which leaves as a chloride ion and simultaneously one of the lone pairs of electrons from the negative oxygen atom shifts back to complete (reform) the C=O carbonyl bond.

  • Step (3) A water molecule abstracts a proton to form the oxonium ion and the carboxylic acid product.

    • If only limited water is available, e.g. like when the acid chloride liquid fumes in air, step (3) could be written as a chloride ion removing the proton to form hydrogen chloride i.e.

      • RCOOH₂⁺ + Cl^– ==> RCOOH + HCl

• FURTHER COMMENTS

  • The reaction is effectively, overall, the substitution of the –Cl chlorine atom with an –OH hydroxy group.

APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

See also INDEX of all revision notes on CARBOXYLIC ACIDS and DERIVATIVES