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

Reaction 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 CARBOXYLIC ACIDS and DERIVATIVES – Acyl/acid chlorides


10.7.1 Introduction to acid chloride/acyl chloride activity

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 in this esterification process is via the addition of the nucleophile alcohol.

 


10.7.3 Acyl chloride esterification by nucleophilic addition–elimination

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,  subsequent elimination on esterification to give the ester and hydrogen chloride/hydrochloric acid

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

    • (c) doc b + CH3-CH2-OH ==> (c) doc b + HCl

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

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

organic reaction mechanisms

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 >Cd+=Od 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 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.

 


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic addition elimination HCl  R–COCl + R'OH ==> R–COOR' + HCl


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