Halogenation of aldehydes and ketones

Halogenation can be achieved by reaction with chlorine, bromine or iodine in acidic solution. 

The double bond of an enol is electron-rich and reacts with electrophilic reagents to give an a-substituted derivative of the parent aldehyde or ketone. 

Halogens are among the electrophiles that react with aldehydes and ketones in this way. The reaction is regiospecific for replacement of an a hydrogen and can be carried out in a variety of solvents such as water, chloroform, acetic acid, and diethyl ether. 

One of the products of the reaction, the hydrogen halide, is itself an enolization catalyst therefore, the reaction requires no additional catalyst—it is autocatalytic. 

Chlorination of 2-butanone yields two isomeric products, each having the molecular formula C4H7CIO. Identify these two compounds. 

In 1904 while carrying out one of the earliest mechanism studies in organic chemistry, Arthur Lapworth found that the rates of a chlorination and bromination of acetone were the same. He later showed that iodination proceeded at the same rate, and that all the rates were independent of the halogen concentration. Lapworth 

Halogenation of saturated aldehydes and ketones usually occurs exclusively by replacement of hydrogens alpha to the carbonyl group  

The reagents that commonly are used to balogenate carbonyl compounds are those that are used to halogenate alkanes (e.g., Cl2, Br2, S02C12, and rV-brornoamidcs see Sections 4-4 and 14-3). However, the characteristics of the two types of halogenation normally are very different. 2-Propanone has been particularly well studied, and the important features of the halogenation of this compound are summarized as follows  

2-Propanone reacts easily with chlorine, bromine, and iodine. 

2-Propanone reacts at the same rate with each halogen. Indeed, the rate of formation of the l-halo-2-propanone is independent of the concentration of the halogen, even at very low halogen concentrations. 

The halogenation of 2-propanone is catalyzed by both acids and bases. The rate expressions for formation of 1 -halo-2-propanone in water solution are  

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. 

In the presence of acid, halogenation usually stops after the first halogen has been introduced, as shown in the following example. 

Why is further halogenation retarded The answer lies in the requirement for enolization. To repeat halogenation, the halo carbonyl compound must enolize again by the usual 

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Other than nucleophilic addition to the carbonyl group the most important reac tions of aldehydes and ketones involve replacing an a hydrogen A particularly well stud led example is halogenation of aldehydes and ketones... 

As m the acid catalyzed halogenation of aldehydes and ketones the reaction rate is mde pendent of the concentration of the halogen chlorination brommation and lodmation all occur at the same rate Formation of the enolate is rate determining and once formed the enolate ion reacts rapidly with the halogen... 

A particularly common cr-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their a- positions by reaction with Cl2, Br2, or I2 in acidic solution. Bromine in acetic acid solvent is often used. 

Halogenation of Aldehydes and Ketones Halogenation or Halo-de-hydrogenation... 

This procedure avoids the difficulties we outlined earlier in the direct halogenation of aldehydes and ketones. It allows the preparation of haloketones on the less substituted side of the carbonyl group, for instance. 

A variety of methods are available for the halogenation of aldehydes and ketones, and rely on the ease of enolization of such compounds. Copper(II) chloride or bromide in ethyl acetate at reflux have been shown to be effective reagents and rely on the promotion of enolization by the copper ion prior to the transfer of halogen. Since these conditions tend to favor the thermodynanuc enol, unsymmetrical ketones preferentially halogenate at the more highly substituted a-carbon atom. Similar selectivity is observed withNBS. O- ... 

Halogenation of aldehydes and ketones occurs under both basic and aci( conditions. As you might expect, the base-promoted reaction occurs throi an enoiate ion intermediate. Even relatively weak bases such as hydro ion are effective for halogenation because it s not necessary to convert, ketone completely into its enoiate ion. As soon as a small amount of eno-late is generated, it reacts immediately with the halogen. 

Base-promoted halogenation of aldehydes and ketones is little used ir, practice because it s difficult to stop the reaction at the monosubstituted product. An -halogenated ketone is generally more acidic than the star ing, unsubstituted ketone because of the electron-withdrawing inductiv effect of the halogen atom. Thus, monohaiogenated products are themselvi rapidly turned into enoiate ions and further halogcnated. 

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