Bromination of carboxylic acids

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... 

I Alpha Bromination of Carboxylic Acids The Hell-Volhard-Zelinskii Reaction... 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

Treatment with bromine and a catalytic amount of phosphorus leads to the selective a-bromination of carboxylic acids. 

Fig. 12.7. Bromination of carboxylic acids in the presence of catalytic amounts of phosphorus tribromide ("Hell-Volhard-Zelinsky reaction I") the reaction product is an a-bromocarboxylic acid.

Fig. 12.8. Bromination of carboxylic acids in the presence of stoichiometric amounts of phosphorus tribromide ("Hell-Volhard-Zelinsky reaction II"). The actual reaction product is an a-bromocarboxylic acid bromide (B), which often undergoes further in situ reaction to afford an a-bromocarboxylic acid (C) or an a-bromocarboxylic acid ester (D).

Racemic or achiral a-azido acids are synthesized by direct azide substitution on commercially available a-bromo carboxylic acids or by radical bromination of carboxylic acids followed by azide substitution. In general, azido acids are stored in the dark to avoid photolytic degradation with loss of nitrogen temperatures above 50 °C should be avoided. Radical a-bromination of a-branched carboxylic acids as required for the synthesis of a,a-dialkyl or a,a-diaryl amino acids is performed with A-bromosuccinimide. This is followed by nucleophilic substitution with sodium azide or other azide donors, e.g. tetrabutylannmonium azide, to produce achiral or racemic a-azido-a,a-diaIkyl or a-azido-a,a-diaryl carboxylic acids (Scheme 74).Synthesis of more sterically hindered a,a-disubstituted azido acids leads to hydroxy compounds when prolonged reaction times are required and not sufficient care is taken to operate under dry conditions and an inert atmosphere.t ... 

Kwart, H., Scaizi, F. V. Observations regarding the mechanism and steric course of the a-bromination of carboxylic acid derivatives. An electrophilic substitution reaction in nonpolar media. J. Am. Chem. Soc. 1964, 86, 5496-5503. 

HELL-VOLHARDT-ZELINSKI Bromination a-Bromination of carboxylic acids (see 1st edition). 

HELL - VOLHAROT - ZE LIN SKI Bromination o-Bromination of carboxylic acids. 

A new method for the a-bromination of carboxylic acids has been described, based on sequential bromination and hydrolytic cleavage of the related /5-keto-esters. 

Synthetic Transformation 25.5 a-Bromination of Carboxylic Acids (Hell-Vollhard-Zelinsky)... 

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