Transition state free-radical halogenation

We know that bromine is less reactive than chlorine in the rate-determining step of halogenation of methane. We also recall that bromine is more selective than chlorine. Both the rate of reaction and the selectivity of free radical halogenation are related to the first propagation step, so let s look at the relationship between reactivity and selectivity in terms of the structure of the transition states for chlorination and bromination. 

The halogenation reaction of ethylene has been modeled by many researchers [170, 172-176], For chlorination in apolar solvents (or in the gas phase), the formation of two radical species requires the use of flexible CASSCF and MRCI electronic structure methods, and such calculations have been reported by Kurosaki [172], In aqueous solution, Kurosaki has used a mixed discrete-continuum model to show that the reaction proceeds through an ionic mechanism [174], The bromination reaction has also received attention [169,170], However, only very recently was a reliable theoretical study of the ionic transition state using PCM/MP2 liquid-phase optimization reported by Cammi et al. [176], These authors calculated that the free energy of activation for the ionic bromination of the ethylene in aqueous solution is 8.2 kcalmol-1, in good agreement with the experimental value of 10 kcalmol-1. 

These results suggest that the transition state features an incipient free radical (note the trends with R), and that the metal atom has begun to make a bond to the halogen of appreciable strength. The trend with halogen substitution is a particularly pronounced one, since the trends in BDE(R-X) and BDE(M-X) are in the opposite direction (that is, the low-valent metal center is a soft acid). 

Apart from the free-radical mode of decomposition, many halogen compounds decompose by unimolecular mechanisms, the most common of these being the direct unimolecular elimination of hydrogen halide. There is evidence that these types of unimolecular reactions involve charge separation of the carbon-halogen bond in the transition state, and they have received considerable attention in recent years. 

The dormant species in ATRP arises from the polymer chain being capped with a halogen atom (P -X), while in the active state the halogen is chelated to a metal complex, thus allowing monomer to add. This takes advantage of the Kharasch reaction in which halo-genated alkanes add to vinyl monomers by a free-radical reaction that is catalysed by transition-metal ions in their lower-valent state (Fischer, 2001). 

The alternative way to initiate ATRP is via a conventional free-radical initiator, which is used in conjunction with a transition-metal complex in its higher oxidation state. Typically one would use AIBN in conjimction with a Cu(II) complex. Upon formation of the primary radicals and/or their addncts with a monomer unit, the Cu(II) complex very efficiently transfers a halogen to this newly formed chain. In doing so the copper complex is rednced, and the active chain is deactivated. It is easily envisaged that the system will arrive at the same equilibrium as depicted in equation 2, but now approaching it from the right-hand side. This alternative way of initiation was termed reverse ATRF (65,66). 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

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