Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo

22 • Summary

22 • Summary

The α-substitution reaction of a carbonyl compound through either an enol or enolate ion intermediate is one of the four fundamental reaction types in carbonyl-group chemistry.

A flowchart shows two reactions of a carbonyl compound, one via an enolate intermediate and one via an enol, both forming an alpha-substituted carbonyl compound upon addition of an electrophile.

Carbonyl compounds are in an equilibrium with their enols, a process called keto–enol tautomerism. Although enol tautomers are normally present to only a small extent at equilibrium and can’t usually be isolated in pure form, they nevertheless contain a highly nucleophilic double bond and react with electrophiles in an α-substitution reaction. An example is the α halogenation of ketones on treatment with Cl2, Br2, or I2 in acid solution. Alpha bromination of carboxylic acids can be similarly accomplished by the Hell–Volhard–Zelinskii (HVZ) reaction, in which an acid is treated with Br2 and PBr3. The α-halogenated products can then undergo base-induced E2 elimination to yield α,β-unsaturated carbonyl compounds.

Alpha hydrogen atoms of carbonyl compounds are weakly acidic and can be removed by strong bases, such as lithium diisopropylamide (LDA), to yield nucleophilic enolate ions. The most useful reaction of enolate ions is their SN2 alkylation with alkyl halides. The Malonic ester synthesis converts an alkyl halide into a carboxylic acid with the addition of two carbon atoms (RX → RCH2CO2H). Similarly, the acetoacetic ester synthesis converts an alkyl halide into a methyl ketone with the addition of three carbon atoms (RX → RCH2COCH3). In addition, many carbonyl compounds, including ketones, esters, and nitriles, can be directly alkylated by treatment with LDA and an alkyl halide.

Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution-NonCommercial-ShareAlike License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at
Citation information

© Jan 9, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.