All my advanced A level organic chemistry notes

10.7 Reaction of acid/acyl chlorides with alcohols

 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.

Esterification with alcohols to give an ester.

The revision notes include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions are also explained.

10.7.3 Acid/acyl chloride esterification of an alcohol by a nucleophilic addition–elimination substitution reaction mechanism

The organic synthesis of esters from the reaction of acid/acyl chloride with alcohols

• Examples of nucleophilic addition of an alcohol to acid/acyl chlorides,  followed by elimination to give the ester and hydrogen chloride.

  • (i) ethanoyl chloride + ethanol ==> ethyl ethanoate + hydrogen chloride

  • + CH₃-CH₂-OH ===> + HCl

  • (ii) ethanoyl chloride + phenol ==> phenyl ethanoate + hydrogen chloride

  • + ===> + HCl

• What is the mechanism for the formation of an ester from an alcohol and an acid chloride?

• e.g. R–COCl + R'OH ==> R–COOR' + HCl   [see mechanism 15 below]

  • effectively overall a nucleophilic substitution of Cl by OR (via ROH)

  • R = alkyl or aryl

mechanism 15 – nucleophilic addition–elimination reaction for the esterification of an acyl chloride

• [mechanism 15 above] The mechanism involves several rearrangements and is essentially the same mechanism as for water, i.e. one of the H's is replaced by R'.

  • Step (1) The >C^d+=O^d– carbonyl is highly polarised and the positive carbon is attacked by the nucleophilic alcohol molecule, acting as an electron pair donor. The alcohol 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 alcohol 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 and leaves as a chloride ion and simultaneously one of the lone pairs of electrons from the negative oxygen atom shifts to complete (reform) the C=O carbonyl bond.

  • Step (3) The previously formed chloride ion abstracts a proton to form the oxonium ion and the ester product.

• FURTHER COMMENTS

  • The reaction is effectively, overall, the substitution of the –Cl chlorine atom with an –OR group where R'=alkyl or aryl.

  • The acyl chloride and alcohol usually readily react at room temperature, especially if both are aliphatic. Phenols may require the presence of aqueous sodium hydroxide to facilitate the reaction, especially if the acyl chloride itself is itself a less reactive (than aliphatic) aromatic e.g. C₆H₅COCl. The alkali generates a negative phenoxide ion (e.g. C₆H₅O^– from phenol C₆H₅OH), which is a more powerful nucleophile than the original neutral phenol molecule.

APPENDIX

COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)