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

The kinetics of chlori nation of the parent and thirteen substituted phenols (2-methyl , 2-chloro, 2-carboxy. 3-meth yl, 3chl oro, 3-carboxy, 4-methyl, 4-ethyl, 4-chloro. 4-bromo, 4-carboxy, 4-acetyl and 4-nitro phenols) by NaOel have been studied in aqueous alk aline medium under v,!rying conditions. The rates show first ord er kinetics each in J NaOCI] and [(X)C6H4(OH)] and inverse first order in [OW]. Variati on in ionic strength of the medium and addition of CI have no sig­ nificant effect on th e rates of reactions. The rates of the reactions are measured at different temperatures and the activation parameters for all th e phenols computed. A mechanism involving the e lectrophilic attack of th e phenoxide ions by NaOCI in the rate determining step has been considered. The values of the pre-equilibrium and the rate determining steps have been calcul ated for all th e phenols. The rates decrease in th e ord er: 3-CH3 >2-CH3 >4-C2HS == 4-CH3 >phenol >3-COO == 3-CI > 2-COO >4-COO >2-CI == 4-CI == 4-8r > 4-COCH3 >4-N02' Hammett plot of the type, log k ubs = -2.88 -3 .2980cr is found to be valid . The correlation between the enthalpies and th e free energies of activ atio ns is reasonably linear with an isok in etic temperature of 300 K. Further, th e e nerg if!s of activation of all th e phenols are optimised corresponding to the log A of the parent phenol through th e equati on, Ea = 2.303 RT (log A - log k Obs) ' Similarly log A values of all the phenols are optimised correspondin g to the Ea of PhOH th ro ugh th e equation, log A = log k obs + Ea 12.303RT. Ea increases with th e introducti on of e lectron-withdrawi ng groups into th e benzene ring, while the introduction of the electron-releasing groups lowers Eo for th e reaction. Similarly log A decreases with the substituti on of electron-withdrawi ng groups, while log A increases o n substitu­ tion with the e lectron-re leasin g groups. The halogens-fluorine, chlorine, bromine and io­ dine form an important group of elements which exist not only in their well-known diatomic molecular fo rms but also as atoms, ions and in covalent combi­ nation wi th many other elements 1.2. In a number of reactions halogen molecules act as sources of positive halogens for coordination with electron rich centres. NaOCI is a well characterised electrophilic reagent containing -O-Cl bond and is a good chlorinating agent. There are several reports on NaOCI oxida­ ti ons 3 . NaOCI has also been used in haemodialysis, in activation of infective agents in conj uctivitis, pho­ tographic material processing, blue print processing, rubber surface treatment etc., and as a preservative, bacteriocide in water treatment, and even to effect chromosome abberations and growth stimulation 4 . Phenol chemistry is dominated by the nucIeo­ phi licity of the system and the propensity for varied reactions with a wide range of nucIeophiles 5 - 22 . Both natural and ionised phenols are ambident nucIeophiles and may react at 0 or C centres with neutral or posi­ tively charged nucIeophiles. There are a number of reports on the halogenation of phenols. Products of halogenation depend on the nature of halogenating agent and the reaction conditions like acidity, solvent medium etc. The present paper reports our results on the substituent effect studies on chlorination of the parent phenol and thirteen substituted phenols by NaOCI in the aqueous alkaline medium.