Phosphorus tribromide, reaction with carboxylic acids

In the following the reaction is outlined for an a-bromination. The reaction mechanism involves formation of the corresponding acyl bromide 3 by reaction of carboxylic acid 1 with phosphorus tribromide PBr3. The acyl bromide 3 is in equilibrium with the enol derivative 4, which further reacts with bromine to give the a -bromoacyl bromide 5 ... 

Acid chlorides are prepared from carboxylic acids by reaction with thionyl chloride (SOCl2), as we saw in the previous section. Similar reaction of a carboxylic acid with phosphorus tribromide (PB ) yields the acid bromide. 

For many years, the reaction between carboxylic acids and bromine in the presence of red phosphorus was known to produce the corresponding a-bromo-carboxylic acid. The process (the Hell-Yolhard-Zeliiisky reaction) can also be effected with phosphorus trihahdes too, and so it is believed that bromine first reacts with phosphorus to produce phosphorus tribromide (PBrs), and the latter converts... 

The conversion of an aliphatic carboxylic acid into the a-bromo- (or a-chloro ) acid by treatment with bromine (or chlorine) in the presence of a catal3rtic amount of phosphorus tribromide (or trichloride) or of red phosphorus is known as the Hell-Volhard-Zelinsky reaction. The procedure probably involves the intermediate formation of the acyl halide, since it is known that halogens react more rapidly with acyl haUdes than with the acids themselves ... 

This method of a bromination of carboxylic acids is called the Hell-Volhard-Zelinsky reaction This reaction is sometimes carried out by using a small amount of phosphorus instead of phosphorus trichloride Phosphorus reacts with bromine to yield phosphorus tribromide as the active catalyst under these conditions... 

The formulated mechanism is supported by the finding that no halogen from the phosphorus trihalide is transferred to the a-carbon of the carboxylic acid. For instance, the reaction of a carboxylic acid with phosphorus tribromide and chlorine yields exclusively an a-chlorinated carboxylic acid. In addition, carboxylic acid derivatives that enolize easily—e.g. acyl halides and anhydrides—do react without a catalyst present. 

An analogue in which a carboxylic acid attached to the 3 position takes the place of the polar pyridone ring in the previous examples also exhibits antiandrogenic activity. The synthesis starts by reaction of the intermediate (44-1) common to the preceding antiandrogens with phosphorus tribromide. This reagent converts the... 

The Hell-Volhard-Zelinsky (HVZ) reaction replaces a hydrogen atom with a bromine atom on the a carbon of a carboxylic acid. The carboxylic acid is treated with bromine and phosphorus tribromide, followed by water to hydrolyze the intermediate a-bromo acyl bromide. 

Hell-Volhard-Zelinskii reaction (Section 22.4) the a-bromination of a carboxylic acid by treatment with bromine and phosphorus tribromide. 

Carboxylic acids can be brominated at the a-position by reaction with bromine and phosphorus tribromide (PBr3) in the Hell-Volhard-Zelinsky reaction. The reaction, which proceeds via an acid bromide, leads to the substitution of an a-hydrogen atom by a bromine atom. 

In Summary With trace amounts of phosphorus (or phosphoms tribromide), carboxylic acids are brominated at C2 (the Hell-Volhard-Zelinsky reaction). The transformation proceeds through 2-bromoacyl bromide intermediates. 

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