Halogenation of aldehydes

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. 

Halocarbons, ketone-alcohol reduction, 84 Halogenation, 4-methylbenzyl chloride [reductive halogenation of aldehyde to benzyl chloride], 124 Hemiacetals, reduction of, 97-99 Hemiaminals, reduction of, 99-100 Hemiketals, reduction of, 97-99 Heptene derivatives, alkene to alkane reductions, disubstituted alkenes, 36-38... 

In the latest development of his elegant work with hydrazone derivatives, Andrew Myers of Harvard reports (J. Am. Chem. Soc. 2004,126, 5436) that Sc(OTf), catalyzes the addition of l,2-bis(r-butyldimethylsilyl)hydrazine, to aldehydes and ketones to form the t-butyldimethylsilylhydrazones. Addition of tBuOH/tBuOK in DMSO to the crude hydrazone effects low temperature Wolff-Kishner reduction. Alternatively, halogenation of ketone hydrazones can lead to vinyl halides such as 11, or the 1,1-dihalo derivatives, depending on conditions. Halogenation of aldehyde hydrazones provides the 1,1-dihalo derivatives such as 13. 

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

Exercise 17-11 The direct halogenation of aldehydes under either acidic or basic conditions is complicated by side reactions involving either oxidation of the aldehyde —CHO group or additions to the—CH—0 double bond. Therefore the synthesis of a-halo aldehydes by the procedure described for ketones is not of much practical value. a-Haio aldehydes can be prepared indirectly from the enol ethanoate of the aldehyde. The enol ethanoate is made by treating the aldehyde with ethanoic anhydride and potassium ethanoate. The overall sequence follows ... 

Unlike ketones, aldehydes are easily oxidized, and halogens are strong oxidizing agents. Attempted halogenation of aldehydes usually results in oxidation to carboxylic acids. 

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. 

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