organic reaction mechanisms

Doc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

GCE A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

10.7 Acid (Acyl) Chlorides - hydrolysis

 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.


10.7 CARBOXYLIC ACIDS and DERIVATIVES – Acyl/acid chlorides


10.7.1 Introduction to acid chloride/acyl chloride reactivity

Only acid chloride reactions are considered at the moment and the high reactivity of acyl chlorides is towards nucleophilic attack is due to the highly polarised situation of the carbon–oxygen/chlorine bonds i.e.

δCl–Cδ+=Oδ–

The ensuing mechanism is called a nucleophilic addition elimination (because of the two principal stages of the mechanism i.e. an addition followed by an elimination), but overall it amounts to a nucleophilic substitution mechanism (see further comments).

The initial point of attack is via the addition of the nucleophile water.


10.7.2 The hydrolysis of acyl chloride by nucleophilic addition–elimination

The >C=O is polarised because of the difference in electronegativity of the carbon (2.1) 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 >Cδ+=Oδ– 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) (c) doc b + H2O ==> (c) doc b + HCl

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

    • (ii) (c) doc b + H2O ==> (c) doc b + 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)

  • 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!

organic reaction mechanisms

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 >Cδ+=Oδ– 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 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.


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic addition elimination R–COCl + 2H2O ==> R–COOH + H3O+ + Cl– R–COCl + H2O ==> R–COOH + HCl


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