Replacement of hydrogen by halogen in aldehydes and ketones

Direct chlorination or bromination of aldehydes can occur at the C of the CHO group or at the C atom next to the CHO group. The latter reaction is much the more important. Reaction of the lower aldehydes with chlorine is highly exothermic and must therefore be carried out with good cooling or in a diluent such as methylene dichloride, chloroform, carbon tetrachloride, water, hydrochloric acid, or preformed chloro aldehyde. 

Acetaldehyde derivatives chlorinated at the -position are interesting intermediates. Chloroacetaldehyde, CH2C1—CHO, b.p. 85-85.5°/748 mm, can be obtained by chlorinating acetaldehyde in aqueous HC1 at 18-20° 619 or by chlorinating anhydrous acetaldehyde below io° 140a 620 in the latter case there must be efficient cooling, particularly at the start, to avoid inflammation due to reaction of chlorine with the acetaldehyde vapor only a small amount of acetaldehyde is used at first, and the remainder, cooled in Dry Ice, is added later in portions the rate of chlorination (to d 1.31) is controlled by the rate at which the heat of reaction can be removed. 

When dry chlorine is led into acetaldehyde with cooling at 16-18° a product of composition 2CH2C1CH0, ICH3CHO, 1HC1 is formed which affords 60% of chloroacetaldehyde on distillation but if acetaldehyde is chlorinated whilst the temperature is raised gradually to about 80° di- and tri-chloroacetaldehyde are obtained according to the amount of chlorine introduced.621 

Anhydrous chloroacetaldehyde can be obtained in good yield by the following laboratory procedure 523 

A stream of chlorine is led into 3.44 moles of freshly distilled ethylene carbonate (1,3-dioxolan-2-one) under UV-irradiation at 63-70° until a weight increase of 119 g is attained (24 h). Fractional distillation at water-pump vacuum then gives 5.2% of 1,2-dichloroethylene carbonate (3,4-dichloro-l,3-dioxolan-2-one), b.p. 78-79°/l9-20 mm, followed by 69% of the monochloro derivative, b.p. 106-107°/10-11 mm.622 The latter product is treated with one drop of triethylamine and heated in a distillation apparatus slowly to 210° (oil-bath) C02 is split off and chloroacetaldehyde is formed, which collects in the receiver as a clear liquid with a green tinge this reaction requires about 1 hour redistillation through a short column gives the pure aldehyde (9 g), b.p. 84-86°. 

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