Induced Bromination of Aromatic Hydrocarbons by Alkali Metals Bromides and Sodium Hypochlorite

Abstract

Induced bromination of aromatic compounds in a system MBracidNaOCl was studied. Optimum conditions of the process were developed, kinetics of the reactions were investigated, and the process mechanism was suggested. The bromination occurs both with the bromine in statu nascendi and with the hypobromous acid by hydrogen substitution exclusively in the aromatic ring.The bromine introduction into an aromatic ring is one of the most important organic reactions widely used both in laboratory and in industry. Bromine-substituted aromatic hydrocarbons are applied as key compounds to the synthesis of drugs [1], specialty epoxy resins, flameretardants, flame-resistant polymer materials [24]. The halogenation is performed mainly by two procedures: by direct halogenation with free halogens, and by substitution with the use of halogen-containing reagents. The vast majority of recent publications deals with the arenas bromination with the molecular bromine [5], chlorinebromine mixtures [6], with bromine in the presence of zeolites [7, 8], potassium bromate in the 65% H 2 SO 4 [9, 10], KBr and H 2 O 2 in acetic acid in the presence of NaBO 3 ·4H 2 O or (NH 4 ) 6 Mo 7 O 24 ·4H 2 O [11], and zeolite H-ZSM-5 [12], with hydrobromic acid with an oxidant sulfoxide (Me 2 SO) [13], H 2 O 2 [14, 15], or tert-butyl hydroperoxide [16], with N-bromosuccinimide [1719], and with other bromine-containing reagents [20].The alternative methods of the oxidative halogenation of aromatic compounds were treated in detail in review [21].We showed formerly that the bromination of aromatic compounds [22,23] and polystyrene [24] might be carried out in water solutions of alkali metals bromides involving various oxidants. No published information exists on the induction of arene bromination in a system MBrHX NaOCl.This study was performed with a goal to reveal the regular trends in the induced bromination of benzene and its derivatives in the system NaBr(KBr)HXNaOCl employed as a brominating agent.The reaction of induced bromination proceeds in succession through stages (14). 0H%U +; +%U 0H; +%U 1D2&O +2%U 1D&O +2%U +%U %U + 2 5 5 5 5 5 5 %U B +%U +2%U B + 2 , B ,; %U R 1 = R 2 = R 3 = H (I); R 1 = R 2 = H, R 3 = CH 3 (II), C 2 H 5 (III), i-C 3 H 7 (IV), C 2 H 5 O (VIII); R 1 = H, R 2 = R 3 = CH 3 (1, 2) (V), (1, 3) (VI), (1, 4) (VII); R 1 = R 2 = R 3 = CH 3 (1, 2, 4) (IX); X = Cl , HSO 4 , RSO 3 .The yield of brominated products and the bromination selectivity depended on the molar ratio of the reagents, on the temperature and time of the process. With the temperature growing from 293 to 323 K the yield of the monobromobenzene and monobromoalkylbenzenes increased from 54.3 to 98.6 wt % (Table 1). Note that in going from toluene to isopropylbenzene at the growing bulk of the alkyl group a partial dealkylation takes place affording bromobenzene. As seen from Table 1, no bromobenzene formed from toluene, but in the case of ethylbenzene or isopropylbenzene the yield of the bromobenzene reached 6.5 wt %. In the induced bromination of di-and trisubs...