Hydrogen bonding and the formation of free radicals

Molecules of hydroperoxide in solution are linked by hydrogen bonds, forming dimers 

The heat of formation of one hydrogen bond was estimated as 2 kcal mole 1 for cumene hydroperoxide in CC14 [247] and as 2.8 kcal mole 1 for f-butyl hydroperoxide [248]. Hydroperoxide dimers decompose to free radicals by the reaction [150] 

This mode of decay is more rapid than unimolecular decomposition with scission of the O—O bond as it is energetically more favorable. When the concentration of hydroperoxide is low, [ROOH] [(ROOH)2], and the rate of decomposition is 

If fe2X[ROOH] fei, Wj = fe2IiL[ROOH]2 and so long as the rate of oxygen uptake (Wq2) is proportional to /Wi, Wo2 [ROOH], Such a dependence has been established for the oxidation of a number of hydrocarbons [249—251] and interpreted as the result of bimolecular decomposition of hydroperoxide [150]. If this bimolecular decomposition occurs via preliminary formation of a dimer, then at a high concentration of hydroperoxide (when almost all molecules are associated) the rate of initiation will increase linearly with the concentration of hydroperoxide (taken as ROOH), i.e. Wj = fe2[(ROOH)2] = 0.5 ft2[ROOH]z. This dependence, has been established for f-butyl hydroperoxide in n-heptane [252] (Fig. 5). The rate coefficient for decomposition of the dimer to radicals appears to be ft2 = 1.14 X 10s exp(—23,000/RT) s 1. The equilibrium constant of t-butyl hydroperoxide association estimated from kinetic data (0.8 1 mole 1 at 90°C) is very close to K found from spectroscopic measurements (0.741 mole 1 at 90°C) [253]. 

Such adducts decompose to free radicals [252], as do dimers of hydroperoxides, probably by the reaction 

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