Kinetics and mechanism of chlorination of phenol and substituted phenols by sodium hypochlorite in aqueous alkaline medium

The kinetics of chlorination of the parent and thirteen substituted phenols (2-methyl, 2-chloro, 2-carboxy, 3-methyl, 3-chloro, 3-carboxy. 4-methyl, 4 -ethyl, 4-chloro, 4-bromo, 4-carboxy, 4-acetyl and 4-nitro phenols) by NaOC l have been studied in aqueous alkaline medium under varying conditions. Th e rates show first order kinetics each in [NaOCl] and [(X)C6H4(OH)] and inv erse first order in [OH-]. Variation in ionic strength of the medium and ad dition of Cl have no Significant effect on the rates of reactions. The rate s of the reactions are measured at different temperatures and the activatio n parameters for all the phenols computed. A mechanism involving the electr ophilic attack of the phenoxide ions by NaOCl in the rate determining step has been considered, The values of the pre-equilibrium and the rate determi ning steps have been calculated for all the phenols. The rates decrease in the order: 3-CH3 > 2-CH3 > 4-C2H5 approximate to 4-CH3 > phenol > 3-COO sim ilar or equal to 3-Cl > 2-COO > 4-COO > 2-Cl approximate to 4-Cl approximat e to 4-Br > 4-COCH3 > 4-NO2. Hammett plot of the type, log k(obs) = -2.88 - 3.2980 sigma is found to be valid. The correlation between the enthalpies a nd the free energies of activations is reasonably linear with an isokinetic temperature of 300 K. Further, the energies of activation of all the pheno ls are optimised corresponding to the log A of the parent phenol through th e equation, E-a = 2.303 RT (log A - log k(obs)). Similarly log A values of all the phenols are optimised corresponding to the E-a of PhOH through the equation, log A = log k(obs) + E-a/2.303RT. E-a increases with the introduc tion of electron-withdrawing groups into the benzene ring, while the introd uction of the electron-releasing groups lowers E. for the reaction. Similar ly log A decreases with the substitution of electron-withdrawing groups, wh ile log A increases on substitution with the electron-releasing groups.