Bond dissociation energy aryl halides

In aprotic solvents, the radical anion, RX , for aryl halides has been detected as intermediate. In cyclic voltammetry of aryl halides, though an irreversible two-electron reduction occurs at low scan rate, a reversible one-electron reduction occurs at high scan rate. Thus, it is possible to get the values of the standard potential ( °) for the RX/RX couple and the rate constant (k) for RX -> R (therefore, the lifetime of RX ). In Fig. 8.18, the relation between ° and log k for aryl bromides in DMF is linear with a slope of 0.5 [5If], It is apparent that the lifetime of RX , obtained by 1/k, increases with the positive shift of E0. In contrast, the existence of RX for alkyl monohalides has never been confirmed. With these compounds, it is difficult to say whether the two processes, i.e. electron transfer and bond cleavage, are step-wise or concerted (RX+e -> R +X ). According to Sa-veant [5le], the smaller the bond dissociation energy, the larger the tendency for the concerted mechanism to prevail over the step-wise mechanism. 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

Figure 1. Homolytic C-X bond dissociation energies (X = F, Cl, Br, I) of aryl halides (data taken from Ref. [38 b]). 1 kcal mol = 4184 kJ mol . ...

Bond dissociation energy, 13, 151—153, 155 acetylene, 343 aryl halides, 918 benzene, 918 ethane, 151, 343, 918 ethylene, 171, 343, 918 ethyl halides, 918 and halogenation of methane, 155 2-methylpropane, 151,152, 414 peroxides, 220 propane, 151 propene, 370, 414 table, 151 vinyl hahdes, 918... 

The aminations of aryl halides were the first to be studied in palladium-catalyzed C—N bond formations, and are thus well developed nowadays. As the oxidative addition of the C—X bond of the aryl halide often constitutes the rate-limiting step in catalytic aminations, a relative reactivity is usually observed that is comparable to that obtained in most transition metal-catalyzed cross-couphngs. Thus, due to the decreasing bond dissociation energies, the reactivity order for hahdes is as follows Ph-Cl > Ph-Br > Ph-I [149]. 

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