Replacement of hydrogen by halogen in aliphatic monocarboxylic acids

Preparation of 2-chlorobutyric acid by chlorination of the acid chloride at the b.p. in the presence of iodine has long been known.536 

A systematic study537 was devoted to -chlorination of medium-chain fatty acids in the presence of polarizing catalysts that form acid chlorides as intermediates (I2, S, or P with SOCl2 or PC15) I2/PC15 was the most effective combination. One mole of Cl2 is used per mole of acid, with a streaming velocity of 25-501 per hour at 100-160° for butyric but not more than 100 to 120° for hexanoic and longer-chain acids. Since side reactions are much favored 

Acid chlorides up to C8 give good yields of a-chloro acid too great concentrations of chlorine decrease the yields.537 

The acid chloride (1 mole) is heated in a bath at 100° and chlorine (1.1 mole) is led in, with stirring, at a rate of 25 1 per hour, the solution being irradiated throughout by a 200-W lamp. The various chlorination products must then be separated by distillation. Thus are obtained (yields based on acid that reacted) 92% of 2-chlorobutyryl chloride, b.p. 130-131°, or 89% of 2-chloro-3-methylbutyryl chloride, b.p. 155-156°. 

Addition of a polarizing catalyst, e.g., I2, is recommended. Thus 2-chloro-propionyl or -butyryl chloride is prepared by passing dry Cl2 into the boiling acid chloride (30 g) containing a little (0.5 g) iodine until the theoretical increase in weight is achieved.538 

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