Lakhasly

trend with the Hammett substituent constants, but the logarithms of the rate constants do not give straight lines as a function of the substituent constants, because the kOHs are controlled by both the chemical reactivity and the diffusion.rPhXOH d16TH In aromatic compounds, electron-donating substituent directs OH into the orthoand para-positions; in the case of the electron withdrawing substituents, metaaddition is preferred [45, 46].In the case of phenol, ipso-, ortho-, meta- and para-additions take place with probabilities of 0.08, 0.50, 0.08 and 0.34, respectively8), however, show linear correlation with rp for monosubstituted benzenes (Fig.

trend with the Hammett substituent constants, but the logarithms of the rate
constants do not give straight lines as a function of the substituent constants,
because the kOHs are controlled by both the chemical reactivity and the diffusion.
The logarithms of rate constants corrected for diffusion (Eq. 8), however, show
linear correlation with rp for monosubstituted benzenes (Fig. 1) and parasubstituted phenols [13]. The negative slope in Fig. 1, q = -0.82, shows
electrophilic reaction. The absolute value of q is smaller than that found in Cl2

(-1.5 [42]) and Br2

• (-1.1 [43]) reaction with aromatic molecules or in
  NH2 (-3.3
  [44]) reaction with phenoxide ions. The smaller negative q is due to the low
  selectivity and high reactivity of
  OH.
  As the first step, the reaction between the hydroxyl radical and the aromatic
  molecule (PhX) leads to the formation of a loosely bound p-complex in a reversible
  reaction [12]. The complex may dissociate, or transform to a r-complex
  (hydroxycyclohexadienyl radical).

OH þ PhX pPhXOH ! rPhXOH ð16Þ
In aromatic compounds, electron-donating substituent directs
OH into the orthoand para-positions; in the case of the electron withdrawing substituents, metaaddition is preferred [45, 46]. The regioselectivity of hydroxyl radical addition
reaction may occur in the transition from the p-complex to the r-complex [21].
Addition to the ipso-position, presumably due to steric reasons generally has low
weight [47]. In the case of phenol, ipso-, ortho-, meta- and para-additions take place
with probabilities of 0.08, 0.50, 0.08 and 0.34, respectively