Carboxylic acids Hell-Volhard-Zelinsky reaction

Of Carboxylic Acids (Hell-Volhard-Zelinski Reaction)... 

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... 

OL HALOGENATION OF CARBOXYLIC ACIDS THE HELL-VOLHARD-ZELINSKY REACTION... 

Section 19.16 Halogenation at the a-carbon atom of carboxylic acids can be accomplished by the Hell-Volhard-Zelinsky reaction. An acid is treated with chlorine or bromine in the presence of a catalytic quantity of phosphorus or a phosphorus trihalide ... 

Nucleophilic substitution by ammonia on a-halo acids (Section 19.16) The a-halo acids obtained by halogenation of carboxylic acids under conditions of the Hell-Volhard-Zelinsky reaction are reactive substrates in nucleophilic substitution processes. A standard method for the preparation of a-amino acids is displacement of halide from a-halo acids by nucleophilic substitution using excess aqueous ammonia. 

Hell-Volhard-Zelinsky reaction (Section 19.16) The phosphorus trihalide-catalyzed a halogenation of a carboxylic acid ... 

Carboxylic acids having an a-hydrogen are halogenated at the a-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction. 

Reversing the order of steps is not appropriate. It must be the carboxylic acid that is subjected to halogenation because the Hell-Volhard-Zelinsky reaction is a reaction of carboxylic acids, not esters. 

Instead, recall that o-bromo acids are prepared from carboxylic acids by the Hell-Volhard-Zelinsky reaction ... 

Chlorination must precede esterification, because the Hell-Volhard-Zelinsky reaction requires a carboxylic acid, not an ester, as the starting material. The remaining step is a nucleophilic substitution reaction. 

The reaction of aliphatic carboxylic acids with bromine in the presence of phosphorous produces a halo acids. This reaction is the Hell-Volhard-Zelinski reaction. 

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).

The bromine is introduced onto the a-carbon by treating the carboxylic acid with Br2 and a catalytic amount of PBr, in a process known as the Hell-Volhard-Zelinsky reaction. This reaction proceeds through an enol intermediate. Because carboxylic acids form enols only with difficulty, a catalytic amount of PBr3 is added to form a small amount of the acyl bromide, which enolizes more readily than the acid. Addition of bromine to the enol produces an a-bromoacyl bromide (see Section 20.2). This reacts with a molecule of the carboxylic acid in a process that exchanges the Br and OH groups to form the... 

The Hell-Volhard-Zelinsky reaction (Section 22-6) is an effective method for introducing bromine at the a position of a carboxylic acid. The racemic a-bromo acid is converted to a racemic a-amino acid by direct amination, using a large excess of ammonia. 

In the presence of a small amount of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which a-hydrogen has been replaced by halogen. This is the Hell-Volhard-Zelinsky reaction. Because of its specificity—only alpha halogenation-- the readiness with which it takes place, it is of considerable importance in synthesis. 

Hell-Volhard-Zelinsky reaction. a-Haloge-nation of carboxylic acids with halogen and phosphorus, presumably involving the enol form of the intermediate acyl halide. 

COBrj is reported to be a catalyst for the cu-bromination of saturated carboxylic acids, in a modified Hell-Volhard-Zelinsky reaction [e.g. 1320b]. Thus, treatment of octadecanoic acid with bromine in the presence of catalytic amounts of COBrj gave a 99.5% yield of 2-bromooctadecanoic acid [2029]. 

PROBLEM 19.10 a-lodo acids are not normally prepared by direct iodination of carboxylic acids under conditions of the Hell-Volhard-Zelinsky reaction. Show how you could convert octadecanoic acid to its 2-iodo derivative by an efficient sequence of reactions. 

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. 

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