De la Mare, P. B. D.; Harvey, J. T. published the artcile< Kinetics and mechanisms of aromatic halogen substitution. III. Partial rate factors for the acid-catalyzed bromination of tert-butylbenzene by hypobromous acid>, Category: bromides-buliding-blocks, the main research area is .
The products of acid-catalyzed reaction of tert-BuPh (I) with HOBr in 50% aqueous dioxane were determined, by isotopic dilution, as 37.7% o- (II), 7.2%, m- (III), and 53.2% p-BrC6H4Bu-tert (IV), and 1.9% PhBr (V). Negligible side chain substitution accompanied the reaction, the kinetic form of which was -d[BrOH]/dt = k[ArH][BrOH][H+], with k = 435 1.2 mole-2 min.-1 at 25°, 12 times greater than for C6H6. These results showed that, in a comparison of I with PhMe, the p-position was less reactive, and the m-position was about equally reactive. Significant electrophilic displacement of the tert-Bu group by Br occurred during the reaction. The results were discussed with special reference to the hyperconjugative power of alkyl groups, and to the effect of steric hindrance in determining the reactivity of the o-position. Some nitro derivatives of II, III, and IV were described. III b10 95-7°, nD25 1.5335 and II, b14 102°, nD25 1.5410. II heated at 80° with 1.5 equivalents 98% HNO3 in a mixture of 35% volume/volume H2SO4 and AcOH and the product chromatographed in C6H6 on Al2O3 gave 2,5-Br(O2N)C6H3Bu-tert (VI). The position of the NO2 group was proved as follows: VI (7.5 g.) heated 50 hrs. at 160° with 30% HNO3 in 3 sealed tubes gave about 3% 2,5-Br(O2N)C6H4CO2H, m. 181°. Model experiments showed that 2,4-Br(O2N)C6H3Bu-tert was similarly oxidized to 2,4-Br(O2N)C6H4CO2H, m. 166°. III heated at 80° with 2 equivalents HNO3 in H2SO4435AcOH similarly gave 30% 5,2 (?),4-Br(O2N)2C6H2Bu-tert, m. 92° (from C5H12). IV (5 g.) similarly heated 6 hrs. at 80° under the same nitrating conditions gave a small yield of p-BrC6H4NO2, m. 124°. In the study of the kinetics of bromination the following was a typical run: from a mixture of I (0.00564M), 0.0028M HOBr, and 0.0221M HClO4 in 50% dioxane, aliquot parts were removed at intervals for titration. A blank run with I omitted was made side by side with this measurement. The mean value of k was found to be 435 l.2 mole-2 min.-1 Labeled IV was prepared, NH4Br82 being first shaken with the Br used in the reaction. The product had f.p. 15.7°. To a solution of 10 l. HOBr and 0.05M HClO4 in 50% dioxane was added 19.18 g. I, then 2.37 g. labeled IV was added to the mixture, the mixture extracted with Et2O, and the IV crystallized to give by calculation 9.19 g. IV. Similarly it was shown that 0.308 g. II had been produced in the reaction. III was treated as for II. The mixture contained 0.843 g. active bromo-tert-butylbenzenes, to which was added 11.45 g. inactive III, and the mixture counted, showing that 0.0606 g. III was produced in the reaction. To a mixture which should have contained 1.36 g. mixed active bromo-tert-butylbenzenes was added 12.74 g. V; the mixture extracted, the product nitrated for 2-3 hrs. at 80°, and the product recovered and chromatographed showed that the reaction mixture contained 0.0197 g. V. The conclusion that there was greater hindrance to the entry of NO2 than of Br o- to an alkyl group, was confirmed by this investigation. For I the rate ratios were 0.37 and 0.07, resp., and the free energy difference corresponding to these figures was 0.87 kcal./mole. The effects of hyperconjugation in PhMe and I were considered in some detail. The small proportion of V (1.9 mole-%) determined in the products of bromination was considered significant. The absolute rate of displacement of a tert-Bu group from I was faster than that of displacement of a proton from C6H8.
Journal of the Chemical Society published new progress about Bromination. 16426-64-5 belongs to class bromides-buliding-blocks, and the molecular formula is C7H4BrNO4, Category: bromides-buliding-blocks.
Referemce:
Bromide – Wikipedia,
bromide – Wiktionary