Hydrogen atom transfer from phenols to radicals

3 Hydrogen atom transfer from phenols to radicals 

According to this mechanism, the rate of formation of products is given by r = fc0[ArOH][Y ] + fe[Y-][HS+], 

An experimental rate law of the following form would be the consequence of this mechanistic rate expression  

In solvents where there is no formation of the phenolate anion ArO-, only the direct ko path is possible. Although simple second-order kinetics are expected in such solvents, first order in [ArOH] andin [Y ],the denominator of the expected rate law (Equation 4.21 derived from Equation 4.20) includes a term acknowledging the equilibrium formation of the hydrogen-bonded complex which depletes the concentration of the free phenol  

This model describes the observed behaviour when 2,2 -methylene-bis(4-methyl-6-terf-butylphenol) (ArOH in Fig. 4.7) reacts with 2,2-diphenyl-l-picrylhydrazyl (Y ), for example, in several solvents including CC14 and 1,4-dioxane [19]. 

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Scheme 4.9 Proposed mechanism for hydrogen atom transfer from phenols to radicals in non-aqueous solvents [19],...

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