Volhard

Volhard method Titration of Ag with NCS" in the presence of Fe. A deep red colour is formed at the end-point. 

Principle. An organic compound which contains chlorine is mixed with sodium peroxide and ignited in a closed metal bomb. The chlorine is thus converted to sodium chloride, and after acidification the chloride is estimated by the Volhard volumetric method. Bromine and iodine, when constituents of organic compounds similarly treated, are converted largely into sodium bromate and iodate respectively these ions are therefore subsequently reduced by hydrazine to bromide and iodide ions, and estimated as before. 

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 ... 

Haworth reaction Hell-Volhard-Zelinsky reaction Hoesch reaction Hofmaim reaction. ... 

In 1873, almost simultaneously, Maly (24), Volhard (38), and Nencki (42) studied the action of thiourea on chloroacetic acid. As mentioned previously, they believed the product to be the thioanalog of hydantoin and called it thiohydantoin with formula 34. 

In 1874, Volhard (39) showed that thiohydantoin resisted desulfuration, proving its sulfur atom to be more firmly bonded than formula 34 indicated. 

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 Hell-Volhard-Zehnsky reaction is of synthetic value m that the a halogen can be displaced by nucleophilic substitution... 

Section 19 16 Halogenation at the a carbon atom of carboxylic acids can be accom plished by the Hell-Volhard-Zehnsky reaction An acid is treated with chlorine or bromine m the presence of a catalytic quantity of phospho rus or a phosphorus trihalide... 

Nucleophilic substitution by ammonia on a halo acids (Section 19 16) The a halo acids obtained by halogenation of car boxylic acids under conditions of the Hell-Volhard-Zelinsky reaction are reac tive substrates in nucleophilic substitu tion processes A standard method for the preparation of a ammo acids is dis placement of halide from a halo acids by nucleophilic substitution using excess aqueous ammonia... 

The a halo acid is normally prepared by the Hell-Volhard-Zehnsky reaction (see Sec tion 19 16)... 

Hell-Volhard-Zelmsky reaction (Section 19 16) The phos phorus tnhahde catalyzed a halogenation of a carboxylic acid... 

A second end point is the Volhard method in which Ag+ is titrated with SCN in the presence of Fe +. The end point for the titration reaction... 

For Volhard methods identified by an asterisk ( ) the precipitated silver salt must be removed before carrying out the back titration. 

The analysis for I using the Volhard method requires a back titration. A typical calculation is shown in the following example. 

The %w/w K in a 0.6712-g sample was determined by a Volhard titration. After adding 50.00 mb of 0.05619 M AgNOa and allowing the precipitate to form, the remaining silver was back titrated with 0.05322 M KSCN, requiring 35.14 mb to reach the end point. Report the %w/w K in the sample. 

A 0.1093-g sample of impure Na2C03 was analyzed by the Volhard method. After adding 50.00 ml of 0.06911 M AgN03, the sample was back titrated with 0.05781 M KSCN, requiring 27.36 ml to reach the end point. Report the purity of the Na2C03 sample. 

The exact amount of silver oxide required may be determined by titrating a sample of the solution with silver nitrate by the Volhard method. 

Other direct methods for the sulfonation of the higher fatty acids are by the use of sulfur trioxide vapor or by the use of chlorosulfonic acid. Indirect methods are also available for the preparation of a-sulfo fatty acids and their salts from an a-bromo fatty acid made by the Hell-Volhard-Zelinsky reaction. The bromo compound may be converted directly to the sodium salt of a sulfonic acid through the Strecker reaction or may be converted to the mercaptan and oxidized to the sulfonate. Sulfonation of the lower fatty acids has been studied by Backer and co-workers. ... 

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

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