J. CHEM. RESEARCH (S), 1997 225 J. Chem. Research (S), 1997, 225 J. Chem. Research (M), 1997, 1601–1614 Kinetics of Bromination of Some Aromatic Oxocarboxylic Acids M. Krishna Pillay*a and K. Banumathib aDepartment of Chemistry, Bharathidasan University, Tiruchirapalli 620 024, India bDepartment of Chemistry, Seethalakshmi Ramaswami College, Tiruchirapalli 620 002, India Rates of enolization, monitored by bromine scavenging, are determined for some aromatic carboxylic acids, ArCO[CH2]nCO2H.It was first suggested by Lapworth1 that ketones undergo a-halogenation only through their enol form. This was followed by extensive investigation into the halogenation of dialkyl and alkyl aryl ketones by Dawson and co-workers,2–4 Nathan and Watson,5 and Zucker and Hammett.6 Bell and co-workers studied the a-halogenation of alicyclic mono- and di-ketones,7 and oxo esters,8 etc. So far a systematic kinetic study on the bromination of aromatic oxocarboxylic acids has not been carried out.This prompted us to undertake the title investigation to understand the steric and electronic effects of substituents on the rate of bromination of aryloxocarboxylic acids. A kinetic study on the acid-catalysed bromination of ArCO[CH2]nCO2H (1) reveals that the order with respect to [Br2] is zero and that with respect to [1] and [H+] is one. Thus the rate of disappearance of bromine is determined solely by the rate at which the oxocarboxylic acid is transformed into the enolic form.The rates of the tautomeric change of ArCO[CH2]nCO2H are presented in Table 4. Kinetic data reveal that electronreleasing substituents in the benzene ring accelerate the rate while electron-withdrawing groups retard it. Hammett correlation of this substituent effect is obtained with s values and the reaction constant is found to be µ0.78 at 303 K. 3p-Nitro- 4p-methoxy- and 3p-bromo-4p-methoxy-substituted oxoacids deviate considerably from the linear Hammett plot obtained with other substituents.The reduced reactivity of these two compounds may not be due to steric effects17 but may be ascribed to hydrogen bonding interactions between OMe and H3O+ or other acidic solvent species.18 The rate of enolization is found to be enhanced to a greater extent by the introduction of a methyl group ortho to the carbonyl moiety. The enhanced reactivity is explained by considering their preferred conformations. 1H and 13C NMR spectral data of these compounds reveal that the carbonyl moiety is twisted out of the benzene ring plane.These compounds which are in a higher energy level in the ground state due to reduced conjugative interaction pass through the energy barrier in a facile manner. The reactivity of 1p-naphthyloxobutanoic acid is very high compared to the 2p-naphthyl analogue. This may be ascribed to the conformation-dependent conjugation effect as in ortho-substituted compounds. Lengthening of the alkyl chain leads to an increase in the reaction rate.This suggests that the opposing effect caused by coordination of H3O+ with carbonyl oxygen by CO2H decreases as it is far removed from the reaction centre. Enolization rate constants of 4p-OMe and 2p,4p,6p-Me3 substituted oxoacids could not be measured owing to ring bromination. Techniques used: 1H and 13C NMR, IR References: 28 Tables 1–3: Kinetic data Table 5: 1H and 13C NMR data Received, 29th January 1997; Accepted, 26th March 1997 Paper E/7/00665I References cited in this synopsis 1 A.Lapworth, J. Chem. Soc., 1904, 30. 2 H. M. Dawson and H. Ark, J. Chem. Soc., 1911, 1740. 3 H. M. Dawson and F. Powis, J. Chem. Soc., 1913, 2135. 4 H. M. Dawson, C. R. Hoskins and J. E. Smith, J. Chem. Soc., 1929, 1884. 5 W. S. Nathan and H. B. Watson, J. Chem. Soc., 1933, 217. 6 L. Zucker and L. P. Hammett, J. Am. Chem. Soc., 1939, 61, 2779. 7 R. P. Bell and O. M. Lidwell, Proc. R. Soc. London, A, Math.Phys. Sci., 1940, 176, 88. 8 R. P. Bell and H. F. F. Ridgewell, Proc. R. Soc. London, A, Math. Phys. Sci., 1967, 298, 178. 17 M. Krishna Pillay and S. Palanivelu, J. Indian Chem. Soc., 1986, 63, 1055. 18 R. G. Coombes, J. G. Golding and P. Hadjigeorgiou, J. Chem. Soc., Perkin Trans. 2, 1979, 1451. *To receive any correspondence. Table 4 Enolization rate constants of 4-oxo 4-substituted phenylbutanoic acids at 303 Ka Entry Substituent 106 k2/dm3 molµ1 sµ1 123456789 10 11 12 13 14 15 16 17 18 19 20 H4 p-Me 4p-Et 4p-Ph 4p-Cl 4p-Br 4p-I 3p-NO2 3p,4p-Me2 3p-Me-4p-Cl 3p-NO2-4p-Me 3p-NO2-4p-OMe 3p-Br-4p-OMe 2p,4p-Me2 2p-Me-4p-Cl 3p-Br-2p,4p,6p-Me3 3-Oxo-4-(1p-naphthyl)butanoic acid 4-Oxo-4-(2p-naphthyl)butanoic acid 5-Oxo-5-phenylpentanoic acid 6-Oxo-6-phenylhexanoic acid 6.89�0.15 9.49�0.26 8.44�0.22 7.48�0.14 4.56�0.11 4.60�0.12 5.73�0.23 1.94�0.06 10.2�0.24 5.32�0.12 2.38�0.06 2.17�0.04 4.56�0.11 21.9�0.58 13.3�0.23 22.1�0.39 18.0�0.31 5.51�0.07 21.4�1.4 29.7�0.67 a[Substrate] =1.2Å10µ2 mol dmµ3, [Br2] =1.0Å10µ3 mol dmµ3, [HClO4] =1.0 mol dmµ3, [NaBr] =0.2 mol dmµ3, m=1.2