Toluene competitive bromination

Scheme 4.12 Bromination of toluene competition between core and side-chain substitution.

These reactions occur on the benzylic hydrogens because these hydrogens are much more reactive. Competition experiments show, for example, that at 40°C a benzylic hydrogen of toluene is 3.3 times as reactive toward bromine atoms as the tertiary hydrogen of an alkane and nearly 100 million times as reactive as a hydrogen of methane. 

In 1994, the free-radical bromination of toluene (and other alkylaro-matics) in sc C02 was reported (Tanko and Blackert, 1994 Tanko et al., 1994). Product yields were similar to those obtained with conventional solvents. The relative reactivity of the secondary hydrogens of ethylbenzene versus the primary hydrogens of toluene on a per hydrogen basis, r(2°/l°), were assessed via competition experiments and (a) did not vary with pressure, (b) were nearly identical to what is observed in conventional solvents. 

Furthermore, competition experiments show that, under conditions where 3", 2, and P hydrogens show relative reactivities of 5.0 3.8 1.0, the relative rate per benzylic hydrogen of toluene is only 1.3. As in its attack on alkanes (Sec. 3.28), the more reactive chlorine atom is less selective than the bromine atom less selective between hydrogens in a single molecule, and less selective between hydrogens in different molecules. 

In order to demonstrate that uncomplexed bromine atoms act as chain propagators, toluene and ethylbenzene were photobromina-ted in a competition study at pressures of 75 to 423 bar and at 40 °C. Over the entire pressure range, the reactivity of the benzylic secondary C-H bond in ethylbenzene was found to be about 30 times greater than that of the corresponding primary C-H bond in toluene. The analogous value for the reactivity in CCI4 at 40 °C is 36. The bromine atoms in SC-CO2 are therefore particularly free. It would be important to determine quantum yields (chain lengths) at various pressures to learn more about mechanistic aspects and other details of the reaction. Local solvent structures on model free-radical reactions in SC-CO2 have been analyzed in some detail. [33]... 

Competition experiments (ethylbenzene versus toluene) confirm the role of Br as chain carrier in the Ziegler bromination in SCCO2 [57]. The role of NBS in this reaction is to maintain a low, steady-state concentration of Br2 (Scheme 4.4-9). 

A competition experiment pitting ethylbenzene vs toluene in SC-CO2 (40 C, 342 bar) yielded a relative reactivity, r(2 /lO) = 29 2. This result is identical to that observed with direct bromination with molecular bromine (Table I) and suggests that Br is the chain carrier for the Ziegler bromination in SC-CO2. Consequently, there does not appear to be a change of mechanism with the conversion from CCI4 to SC-CO2 as solvent for the Ziegler bromination. 

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