2.4.6- Tribromobenzoic acid

Tribromobenzoic acid has been prepared by the deamination of 2,4,6-tribromo-3-aminobenzoic acid (reagents not specified), by hydrolysis of 2,4,6-tribromobenzonitrile, " and by oxidation of the tribromotoluene, the benzyl chloride, the aldehyde,and the glyoxylic acid.i The present method is a modification of that of Bunnett, Robison, and Pennington.i ... 

In general, however, reduction by ethanol is recommended only in cases where one has a reliable analytical method for distinguishing between products resulting from hydrogen- or ethoxy-substitution. For all other cases we recommend Kornblum s dediazoniation in an aqueous solution of hypophosphorous acid, in some cases in the presence of a catalyst, e.g., 0.05-0.10 mol% CuS04. The procedure is notable for its simplicity of operation. In Organic Syntheses the diazotization and hydro-de-diazoniation of 3,3 -dimethyl- and 3,3 -dimethoxybenzidine are described by Kornblum (1955) and the formation of 2,4,6-tribromobenzoic acid by Robison and Robison (1963). 

A combination of steric and electrostatic factors is presumably decisive with regard to the form of the acid most stable in sulfuric acid solution. The simple protonated form XX of benzoic acid is stabilized by resonance structures sterically prohibited in mesitoic acids. The ortho methyl groups of mesitoic acid would interfere with a coplanar dihydroxymethylene group. On the other hand, the inductive and resonance effects of the methyl groups help stabilize the acylium ion form of mesitoic acid as in the formulae XXI. In the case of 2,4,6-tribromobenzoic acid the steric effect and its abetting electronic effects are not sufficient, and this acid behaves like benzoic acid.17 >177... 

B. 2,4,6-Tribromobenzoic acid. A 5-1. three-necked flask, equipped with a mechanical stirrer and thermometer and surrounded by an ice-salt bath, is charged with a precooled mixture of 1.02 1. of concentrated sulfuric add and 500 ml. of water. Cooling during the reaction period is assisted by the ice-salt bath, but is effected chiefly by periodic additions of large amounts... 

In the preparation of 2,4,6-tribromobenzoic acid from m-aminobenzoic acid, this substance (0.2 mole) is brominated in solution in dilute hydrochloric acid and the tribromo compound which precipitates is collected and washed and used in the next step without drying. A diazotizing solution is prepared by stirring with ice... 

Tribromobenzoic acid is similarly obtained from 3-amino-2,4,6-tribromobenzoic acid the NH2 group is replaced by H when the amino acid is diazotized in the presence of hypophosphorous acid. For details see Organic Syntheses 12... 

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