Hammett substituent constant, effect phenyl esters

Hammett substituent constant, effect of cyclo-amyloses on, in hydrolysis of phenyl esters, 23 222... 

Hammett (and related sigma) relationships have been applied to aquatic reactions of several classes of aromatic contaminants. For example, alkaline hydrolysis of triaryl phosphate esters fits a Hammett relationship (Table 3) is t he sum of the substituent constants for the aromatic groups and k0 is the hydrolysis rate constant for triphenyl phosphate (0.27 M 1 s-1 t1/2 = 30 days at pH 8). Triaryl esters thus hydrolyze much more rapidly than trialkyl or dialkyl-monoaryl esters under alkaline conditions. Rates of photooxidation of deprotonated substituted phenols by singlet oxygen have been found to be correlated with Hammett a constants (Scully and Hoigne, 1987). The electronic cllects of substituents on pKa values of substituted 2-nitrophenols also fit a I lammett relationship this, of course, is not a kinetic LFER. Two compounds (4-phenyl-2-NP and 3-methyl-2-NP) did not fit the relationship and were not included in the regression. Steric effects may account for the discrepancy for the latter compound. Nitrophenols are used as intermediates in synthesis of dyes and pesticides and also used directly as herbicides and insecticides. 

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