Addition of halogen to unsaturated carboxylic acids

The rate of addition of halogen to unsaturated fatty acids decreases in the order Cl2 Br2 I2. Unsaturated acids and their esters and amides usually give good yields of bromine adducts at temperatures as low as —5 to —25° in the usual solvents — esters also without solvents — irradiation having considerable accelerating effect. 

Crotonic acid adds bromine in CC14 under irradiation at 20°, finally at 40°, giving a 90% yield of 2,3-dibromobutyrIc add.79 

When the theoretical amount of bromine is dropped into ethyl acrylate with stirring at 5-10° (ice cooling) and the reaction is allowed to proceed for 5 h at room temperature, 98 % of ethyl 2,3-dibromopropionate is obtained.80 

This ester, and also 2,3-dibromopropionitrile (obtained from acrylonitrile under similar conditions), exchange their 3-bromine atoms readily for alkoxyl groups, and the resulting a-bromo-/ -alkoxy esters or nitriles are intermediates in the synthesis of oc-amino- -hydroxy acids — in the above cases for serine.81 

Addition of bromine to multiply unsaturated fatty acids such as linoleic, linolenic, and arachidic acid, is used for quantitative or approximate analysis of fatty acid mixtures.82 

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