5- Bromo-2,4-dimethylbenzoic acid

Substituent effects on the A,u I reaction have been studied by Bender and Chen55. These authors measured the rates of hydrolysis of a series of 4-substituted 2,6-dimethylbenzoates in 9.70 M sulphuric acid at 25°C, and found that the values for the first-order coefficients with 4-methoxy, 4-methyl, 4-unsubstituted and 4-bromo-compounds (5.0, 0.37, 0.033 and 0.01 x I0 4 sec-1, respectively) are satisfactorily correlated by the Hammett equation, following cr+ with a slope p = —3.22. Since the esters are not fully protonated in 9.70 M H2SOj, part of this factor is due to the effects of the 4-substituent on the protonation equilibrium, p for the protonation of substituted benzoic acids is about — l35, but is likely to be considerably smaller for di-ortho-substituted compounds, since the conjugative interaction of the p-substituents with the protonated carboxyl group requires coplanarity with the ring. 

Bromination of 2-methyl-,447, 481 7-methyl-,78,478 5-bromo-,76 2-fluoro-,482 or 2-phenylbenzo[6]thiophene483 in either chloroform or carbon tetrachloride gives the 3-bromo compound in each case. 2-(2-Naphthyl )benzo[6]thiophene is brominated,54 chlorinated, and iodinated484 in the 3-position. In the cases of 3-methyl-485 and 3-bromobenzo[6]thiophene,107 bromination takes place in the 2-position. Bromination of 2,3-dimethylbenzo[6]thiophene in dilute chloroform solution at 0° gives mainly nuclear substitution in the 6-position,81 but chlorination in acetic acid causes substitution in the 2-methyl group, to give 2-chloromethyl-3-methvlbenzo[6]thiophene.419... 

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