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Kinetics and Mechanism of Chlorination of Phenol and Substituted Phenols by Sodium Hypochlorite in Aqueous Alkaline Medium

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Kinetics and Mechanism of Chlorination of Phenol and Substituted Phenols by Sodium Hypochlorite in Aqueous Alkaline Medium Indian Journal of Chemistry Vol. 40A, November 2001, pp. 1196-1202 Kinetics and mechanism of chlorination of phenol and substituted phenols by sodium hypochlorite in aqueous alkaline medium B Thimme Gowda* & M C Mary Department of Post-Graduate Studies and Research in Chemistry, Mangalore University, Mangalagangothri 574 199, India Received 6 February 2001; revised 3 July 2001 The kinetics of chlori nation 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 NaOel have been studied in aqueous alkaline medium under v,!rying conditions. The rates show first order kinetics each in J NaOCI] and [(X)C6H4(OH)] and inverse first order in [OW]. Variation in ionic strength of the medium and addition of CI have no sig­ nificant effect on the rates of reactions. The rates of the reactions are measured at different temperatures and the activation parameters for all the phenols computed. A mechanism involving the electrophilic attack of the 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 the order: 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 kubs = -2.88 -3 .2980cr is found to be valid. The correlation between the enthalpies and the free energies of activations is reasonably linear with an isokinetic temperature of 300 K. Further, the energif!s of activation of all the phenols are optimised corresponding to the log A of the parent phenol through the equati on, Ea = 2.303 RT (log A - log k Obs) ' Similarly log A values of all the phenols are optimised corresponding to the Ea of PhOH through the equation, log A = log kobs + Ea 12.303RT. Ea increases with the introducti on of electron-withdrawing groups into the benzene ring, while the introduction of the electron-releasing groups lowers Eo for the reaction. Similarly log A decreases with the substitution of electron-withdrawing groups, while log A increases on substitu­ tion with the electron-releasing groups. The halogens-fluorine, chlorine, bromine and io­ medium etc. The present paper reports our results on dine form an important group of elements which exist the substituent effect studies on chlorination of the not only in their well-known diatomic molecular parent phenol and thirteen substituted phenols by fo rms but also as atoms, ions and in covalent combi­ NaOCI in the aqueous alkaline medium. nation wi th many other elements 1.2. In a number of reactions halogen molecules act as sources of positive Materials and Methods halogens for coordination with electron rich centres. Commercial sample of sodium hypochlorite (BDH­ NaOCI is a well characterised electrophilic reagent GR grade) was used. A stock solution of NaOCI (0.03 containing -O-Cl bond and is a good chlorinating 3 3 mol dm- ) in 0.025 mol dm- NaOH was prepared, agent. There are several reports on NaOCI oxida­ 3 standardised and stored in dark coloured bottles. Sta­ tions . NaOCI has also been used in haemodialysis, bility of the oxidant was checked at regular intervals inactivation of infective agents in conjuctivitis, pho­ by iodometric titration against standard sodium thio­ tographic material processing, blue print processing, sulphate. The concentration remained unchanged for rubber surface treatment etc., and as a preservative, sufficiently a long period of time. bacteriocide in water treatment, and even to effect 4 chromosome abberations and growth stimulation . Pure samples of the parent phenol and thirteen Phenol chemistry is dominated by the nucIeo­ monosubstituted phenols-2-methy 1, 2-chloro, 2- phi licity of the system and the propensity for varied carboxy, 3-methyl, 3-chloro, 3-carboxy, 4-methyl, 4- 5 22 reactions with a wide range of nucIeophiles - . Both ethyl, 4-chloro, 4-bromo, 4-carboxy, 4-nitro and 4- natural and ionised phenols are ambident nucIeophiles acetyl phenols (Aldrich Chemie) were recrystallised and may react at 0 or C centres with neutral or posi­ or distilled before use. Stock solutions of the phenols tively charged nucIeophiles. There are a number of (0.10 mol dm-3) in 0.10 m01 dm-3 of NaOH were pre­ reports on the halogenation of phenols. Products of pared as and when required. Initial investigations re­ halogenation depend on the nature of halogenating vealed that the addition of chloride ion did not have agent and the reaction conditions like acidity, solvent any effect on the reaction rates. Ionic strength of the GOWDA et at.: KINETICS AND MECHANISM OF CHLORINATION OF PHENOLS 1197 medium was maintained at 0.30 mol dm-3 using con­ Stoichiometry and product analysis centrated aqueous solution of sodium nitrate. Stoichiometry of NaOCI-phenol reaction was de­ Since the phenols were found to exist virtually as termined by allowing the reaction mixtures containing the phenoxide ions in alkaline solutions, the effective phenol and NaOCI in 1: 1 molar ratio in aqueous so­ hydroxyl ion concentration in the reaction mixture dium hydroxide to go to completion at room tem­ was taken as the difference between the hydroxyl ion perature. The observed stoichiometry may be repre­ concentration and the concentration of phenol used. sented as . .. (1) Kinetic measurements The kinetic studies were \l1ade in glass stoppered The chlorinated phenoxide ion was characterised as pyrex bottles under pseudo-first order reaction condi­ follows: The reaction products were acidified with tions with [phenol] »[NaOCI] (5-60 fold excess). dilute sulphuric acid. The brown layers separated The reactions were initiated by the rapid addition of were removed, washed with water and distilled. The 3 requisite amounts of NaOCI (0.0003-0.003 mol dm- ) presence of chlorine in the product phenol was con­ 3 thermally pre-equilibrated at a desired temperature, to firmed by Lassaigne's tese . Further, the aqueous mixtures containing known amounts of phenol (0.005- layer of the reaction mixture did not gi ve test for the 3 0.Q5 mol dm- ), sodium hydroxide (0.01-0.30 mol free chloride ion. Ortho/para ratio of the chlorinated dm\ sodium nitrate and water, pre-equilibrated at the products is dependent on pH and the nature of sol­ same temperature. The progress of the reactions was vent. But in the case of chlorination of ortho or para monitored for at least two half-lives by the iodometric substituted phenols, the site of attack would be either determination of unreacted oxidant at regular intervals para or ortho position respectively, while with meta of time. The pseudo-first order rate constants (lcobs) substituted phenols, varying proportions of ortho and were computed by the graphical methods and the val­ para products were expected, as in the case of the ues were reproducible within ± 5%. parent phenol. Table 1-Pseudo first order rate constants (kobs) for the chlorination of phenol and some ortho and meta substituted phenols by NaOCI in aqueous alkaline medium "[OW leff = [OW llOlal- [ArOH1; Temp. : b283 K. c298 K. d278 K 3 2 10 NaOCllo 10 [ArOHlo 1020Hl"eff (mol dnh (mol d~3) (mol d~ 3 ) 3-COOHc Effect of varying [NaOCllo 0.3 2.0 8.0 3.0 3.0 10.1 0.5 2.0 8.0 3.1 13.1 3.1 7.1 23.0 9.2 9.9 1.0 2.0 8.0 3.1 13.0 2.9 7.1 24.0 9.5 9.8 2.0 2.0 8.0 3.2 13.2 2.8 6.8 23.0 9.8 8.9 3.0 2.0 8.0 3.2 13.3 2.9 6.7 21.1 10.6 8.9 Effect of varying [ArOHlo 1.0 0.5 8.0 0.7 3.8 0.7 1.7 6.2 2.3 2.3 1.0 1.0 8.0 1.6 7.6 1.4 3.4 12.4 4.5 4.4 1.0 2.0 8.0 3.1 13.0 2.8 7.1 25.0 9.5 9.8 1.0 3.0 8.0 5.0 21.3 4.0 10.5 35.3 12.9 13.6 1.0 5.0 8.0 8.0 34.2 7.8 17.7 60.8 25.0 24.9 Effect of varying [OH··] 1.0 2.0 1.0 25.0 25.5 1.0 2.0 2.0 11.1 10.3 38.9 41.5 1.0 2.0 3.0 8.3 32.6 7.5 67.8 31.0 1.0 2.0 5.0 4.6 18.2 4.4 8.0 40.3 15.7 15.7 1.0 2.0 8.0 3.0 13.0 2.9 7.1 25.0 9.6 9.8 1.0 2.0 10.0 2.4 2.3 7.5 1.0 2.0 20.0 5.3 1.1 5.5 10.9 4.1 3.8 1.0 2.0 30.0 3.4 1198 INmAN 1. CHEM., SEC A, NOVEMBER 2001 Results [ArOH] were linear with zero intercepts on the ordi­ The kinetic data on the chlorination of the parent nates. phenol and thil1een monosubstituted phenols: 2-methy 1, 2-chloro, 2-carboxy, 3-methyl, 3-chloro, 3-carboxy, Effect of varying other parameters of the medium 4-methyl, 4-ethyl, 4-chloro, 4-bromo, 4-carboxy, 4- The rates decreased with increase in [OH-] at fixed nitro and 4-acetyl phenols, by NaOCI in aqueous al­ [NaOCl]o and [ArOH]o with inverse first order kinet­ kaline medium, under varying conditions of [ArOH], ics in [OH-] for chlorination of all the phenols except [NaOCI], [OH], solvent composition and temperatures salicylic acid (Tables 1 and 2).
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