Halogenation at the a carbon

The first substitution reaction we examine is halogenation. Treatment of a ketone or aldehyde with halogen and either acid or base results in substitution of X for H on the a carbon, forming an a-halo aldehyde or ketone. Halogenation readily occurs with CI2, Br2, and I2. 

The mechanisms of halogenation in acid and base are somewhat different. 

Halogenation is often carried out by treating a carbonyl compound with a halogen in acetic acid. In this way, acetic acid is both the solvent and the acid catalyst for the reaction. 

The mechanism of acid-catalyzed halogenation consists of two parts tautomerization of the carbonyl compound to the enol form, and reaction of the enol with halogen. Mechanism 23.3 illustrates the reaction of (CH3)2C=0 with Br2 in CH3COOH. 

In Part [1], the ketone is converted to its enol tautomer by the usual two-step process protonation of the carbonyl oxygen, followed by deprotonation of the ot carbon atom. 

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Section 19 16 Halogenation at the a carbon atom of carboxylic acids can be accom plished by the Hell-Volhard-Zehnsky reaction An acid is treated with chlorine or bromine m the presence of a catalytic quantity of phospho rus or a phosphorus trihalide... 

Aldehydes and ketones react with chlorine, bromine or iodine in acidic solution, this is called halogenation at the a-carbon ... 

The comparative effect of the F atom in HF elimination under the same experimental conditions reveals that on increasing the number of halogens at the a-carbon, the energy of activation tends to increase, while the rate coefficient decreases in the following sequence ... 

Figure 23.3 summarizes the three possible outcomes of halogenation at the a carbon, depending on the chosen reaction conditions. 

Tautomerization in Acid 883 Tautomerization in Base 884 Acid-Catalyzed Halogenation at the a Carbon 893 Halogenation at the a Carbon in Base 894 The Haloform Reaction 895... 

This section examines a reaction of the carbonyl gronp that can proceed through the intermediacy of either enols or enolate ions—halogenation. Aldehydes and ketones react with halogens at the a-carbon. In contrast with deuteration, which proceeds to completion with either acid or base, the extent of halogenation depends on whether acid or base catalysis has been nsed. 

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