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). The plots of kobsversus are shown in Tables 1-5 . I/[OH-] were linear passing through the origin. Variation in ionic strength of the medium (0.08-0.8 Effect of varying [NaOCl}o 3 3 mol dm- ) or addition of cr (0.0-0.1 mol dm- ) had At constant [~rOH] o (5 -50 fold excess over no significant effect on the rate of chlorinations, but [NaOCl]o) and [OH], first order plots of log [N aOCI] the decrease in dielectric constant of the medium ef­ versus time were linear up to at least 75 % completion fected by the addition of t-BuOH increased th e rates. of the reactions. The pseudo-first order rate constants The rates of reactions were measured at different computed from the plots remained unaffected by the temperatures and the activation parameters have been changes in [NaOCl]o (Tables I and 2), establishing calculated (Table 3 and 4). the firs t order dependence of the rate on [NaOCI], in There were wide variations in the rates of chlorina­ all the cases. tion of phenols by NaOCI with the change of su bstitu­ ents. The rates were hi gher for phenols with electron Effect ofvaryillg [ArOH}o donating substituents in the benzene ri ng and lower At constant [NaOCl]o and [OH-] under substrate for phenols with electron withdrawing substituents. excess conditions (5-50 fold), the rates increased with Therefore the reactions had to be studied at different increase in [ArOH] with first order dependences in temperatures for different substi tuted phenols de­ lArOH] (Tables I and 2). The plots of kobs versus pending on their rates of reactions (Table I). How-

Table 2-Pseudo first order rate constants (kobs) for th e chl orin ati on of some para substituted phenols by NaOCI in aqueous alkaline medium

' [OW ]cf f = [OW JIO(al - [ArOH] ; Temp. : b278 K, C 298 K, d3 18 K 10' [NaOCI]o 102 [ArOI-l]o 102 [Ol-l]"cff c (mol dl~ J ) (mol dn;3) (mol d~ ) 4_CI-I)h 4-C21-1 ,b 4-COCH/ 4-COOl-lc . 4_C lc 4-Br Effect of varyin g [NaOCl]o 0.3 2.0 8.0 6.7 3. 1 4.0 0.5 0.5 2.0 8.0 6.6 7.3 5. 1 4.0 2.8 0.4 1.0 2.0 8.0 6.7 7.4 5.4 3.8 2.9 2.9 0.3 2.0 2.0 8.0 6. 3 7.5 5.5 3.8 3.1 2.7 0.3 3.0 2.0 8.0 6.4 6.8 5.4 3.8 3. 1 2.6 0.3

Effect of va ryin g [ArOI-llo 1.0 0.5 8.0 1.7 1.8 1.4 0.95 0.8 0.7 0.1 1.0 1. 0 8.0 3.2 3.7 2.7 1.9 1.6 1.3 0.2 1.0 2.0 8.0 6.5 7.4 5.2 3.7 3. 1 2.6 0.3 1.0 3.0 8.0 9.4 11.7 7.3 5.6 4.8 4.0 0.5 1.0 5.0 8.0 16. 6 18.5 12.1 10.0 8.7 6.6 0.7 Effect of varying [01-1] 1.0 2.0 1.0 25.2 2. 1 1.0 2.0 2.0 27.0 29.2 20.0 15.4 2. 1 1. 1 1.0 1.0 2.0 3.0 18.3 20.0 15.5 9.6 8.0 0.7 1.0 2.0 5.0 11.1 11 .4 8.8 6.7 4. 1 4.6 0.5 1.0 2.0 8.0 6.8 7.4 5.2 3.3 3.0 2.6 0.3 1.0 2.0 10.0 2. 1 2.2 0.2 1.0 2.0 20.0 2.0 3.4 2.8 1.6 1.0 1.2 1.0 2.0 30.0 2.4 GOWDA et af. : KINETICS AND MECHANISM OF CHLORINATION OF PHENOLS 11 99 ever, for the purpose of comparison, the rates of all 1=1 .0 mol dm-3. Generally alkyl substitution in­ the substituted phenols were computed for 298 K creased the rate. Other groups like -Cl, -COOH, through the relationship. -N02 etc. decreased the rates of chlorination. The E. (298-T) substitution at the meta-position had the largest influ­ 100 k = log k +" s to obs 2.303 R 298 T ence on the rate. The energies of ac tivation computed by the Arrhenius plots were used. The calculated pseudo­ Discussion first order rate constants at 298 K are shown in Table In alkaline solutions, NaOCl exists as ocr 5. As may be seen, the rate constant for the 3-CH3 NaOCl ~ Na+ + OCl- ... (2) substituted phenol is the highest at 366.0 x 10-4 S- I and lowest at 0.032 x 10-4 sec- I for 4-N02 substituted Phenols exist as phenoxide ions in alkaline solutions

phenol at [N aOCI]o=50 [S]o= 12.5 [OH-]=3.33 XC6H40H+OH- · ' XC6H40 - + H20 ... (3)

Table 3-Acti va ti on parameters for the chlorinati on of phenol and some ortho and meta substituted phenols by NaOCI in aqueous alkaline medium

X-C6 H4-OH, where X = Parameter -H 2-CH3 2-Cl 2-COOH 3-CH3 3-Cl 3-COOH E,,(kJ mol- I) 73.0 78. 5 80.5 64.1 91.3 80.8 76.5 log A 10.0 11.6 10.6 8.2 14.6 11.1 10.4 I1 H# (kJ mol- I) 70.5 76.0 78.0 6 1.7 88.8 78.3 74. 1 l l I1S(JK moi - ) -62.5 -3 1.0 -5 1.0 -98.3 -25.5 -39.9 -53.8 l I1C#( kJmol- ) 89.2 85.2 93.2 90.9 8 1. 2 90.2 90.2

Optimised va lues corresponding to 10gA va lue of PhOH"

E,,(k J mol- I) 73 .0 69. 1 77.0 74.8 65.0 74.0 74.0 l I1 H\ kJmoi- ) 70.5 60.7 74.6 72.3 62.5 71.6 71.5 l I1C#(kJmoi- ) 89.2 85.3 Y3.2 90.9 81.2 90.2 90.1

Optimised values corresponding to E" va lue of PhOH"

0 10 <> A . 10.0 10.6 9.3 9.7 11.4 9.8 9.8 I1S#(J K I mol- I) -62.7 - 49.5 - 76.1 -68.5 - 36.7 -66.1 -66.0 l I1C#(kJmoi- ) 89. 1 85.3 93.2 90.9 81.2 90.2 90.2 "Please see tex t

Table 4-Activation parameters for the chl orin ation of some para-substituted phenols by NaOCI in aqueous alkaline medium

Parameters X-C6H4-0H, where X = 4-CH, 4-C2HS 4-Cl 4-8r 4-COOH 4-COCH3 4-N02 E,,(kJ mo'I- I) 72.0 66.4 78.3 75 .6 79.8 80.0 91.9 log A 10.1 9.1 10.2 9.7 10.6 9.9 10.6 t:,. H# (kJ moi- I) 69.5 63.9 75.8 73.2 77.3 77.5 89.4 l l I1S' (JK mol - ) - 58.8 -78.6 -57.3 -68.0 -5 1.2 -64.7 -50.0 !:J.C#( kJ 111 0',- 1) 87.3 87.3 92.9 93.5 92.6 %.8 104.3

Opti mised val lies corresponding to 10gA va lue of PhOH"

E,, (Kj mol- I) 7 1.2 30.4 76.4 77.3 76.4 80.7 88.2 l M I#(kJ mul- ) 68.7 68. 7 73.9 74.8 73.9 78.2 85 .7 l I1C#( kJmol- ) 87.3 87.3 92.7 93.4 92.6 96.8 104.3

Optimi sed va lues correspond ing tu E" va lue of PhOH"

0 10 <> A - 10.3 10.3 9.3 9.2 9.4 8.6 7.3 l l I1S#(JK mol- ) - 56.4 -56.4 -75.2 -76.9 -74.0 -88.4 - 11 3.5 l !:J.C#(kJmol ) 87.3 87.3 92.9 93.4 92.7 96.9 104.3 "Pl ease see text 1200 TNDIAN 1. CHEM., SEC A, NOVEMBER 2001

Hence, ocr and phenoxide ions ArO- were used to K,

represent NaOCI and phenols, respectively. OCl- + H20 E :> HOCI + OH- . .. (8) The observed kinetics of first order each in [NaOCI] was calculated from the dissociation constant, Ka 8 and [ArOH] and inverse first order in [OH-] may be (2.95 X 10- )24 of HOCI and the ionic product of wa­ explained by the mechanism shown in Scheme 1. ter, Kw (10- '4) as K,

OCl- + H20 E " HOCI + OH- (fast) k2

HOCI + ArO- ---7 ClArO- + H20 (slow) where CIArO- is the chlorinated product. 4.0 ·Scheme 1 The rate law in accordance with Scheme 1 is given by equation (4) d[OCI - ] K,k [OCI - ][ArO - ][H 0] 3 . 0 2 2 .. . (4) dt Equation (4) may be rearranged as 1 d[OCI - ] K,k [ArO- ][H 0] ------=------=. = ----'--=------=-2 1 . .. (5) 2. 0 III [OCI- ] dt [OH - ] .D 0 But we have .:>t: rn 1 d [OCI - ] = k 0 1.0 [OCI- ] dt obs + <.J) Equation (5) therefore takes the form

k b = K,k2 [ArO- ][H 2 0] ... (6) 0.0 os [OH - ] \4 ~N O' -1 . 0~. ______L- ____-L ____ ~__ L- ____~ (7) - 0.2 0.2 0.6 1.0 The plots of k obs versus [ArO-] and k obs versus lI[OH- ] were linear with zero intercepts on the ordi­ Fig. 1-Plot of kobs VS. Op 3 nates, in accordance with the rate law (7). Further, K, 10 [NaOCllo = 50 [ XC6H40H)o = 12.5 [OHl = 3.33 1= 1.0 mol. for the equilibrium, d~ - 3 , T= 298K

Table 5- Pseudo first order rate constants at 298 K for the chlorination of phenol and some substituted phenols by NaOCI in aqueous alkaline medium 3 10 3[NaOCllo = 12.5 [OHlerr = 3.33 1= 1.0 mol dm- 2 4 10 [ArOHl 10 kobs (s- ') for X-C6H4-0H, where X = (mol d~ ·3) -H 2-CH3 2-CI 2-COOH 3-CH3 3-CI 3-COOH

0.5 3.4 20.2 0.70 1.7 87.7 2.2 2.3 1.0 7.6 40.7 1.39 3.4 189.0 4.4 4.4 2.0 14.6 69.7 2.86 7.1 366.0 9.5 9.8 3.0 28. 1 114.1 4.00 10.5 499.0 13.6 13.6 5.0 38.1 183.3 7.80 17.7 860.0 25.0 24.9

4-CH3 4-C2H5 4-COCH3 4-COOH 4-CI 4-Br 4-N02 0.5 7.2 7.9 0.18 0.95 0.84 0.68 (J.OI 1.0 15.1 15.0 0.34 1. 86 1.69 1.26 0.02 2.0 30.6 30.4 0.66 3.70 3.20 2.60 0.03 3.0 48.4 43.9 0.96 5.56 6.90 4.00 0.045 5.0 76.5 77.0 1.60 10.0 8.67 6.60 0.07 1 GOWDA ef at.: KINETICS AND MECHANISM OF CHLORINATION OF PHENOLS 1201

3 3 plots are (10 k2 , dm mort s-') = 3.8, 16.0, 0.65,58.5, Kw = [HOCl][OH- ][H 2 0] = K [H 0] 1.9, 2.2, 8.7,7.8,0.85,0.14,0.65,0.57 and 0.004 for Ka [OCl][H 0] I 2 2 the parent and 2-CH3.2-Cl, 3-CH3 , 3-Cl, 3-COOH, 4- 14 = 1.0xlO- = 3.39xlO-7 ... (9) CH3. 4-C2HS. 4-COOH, 4-COCH3. 4-Cl, 4-Br and 4- 8 2.95 x lO - N02 substituted phenols, respectively. K =3.39xlO-7 =3.39xlO-7 3.39xl0-7 Validity of Hammett equation for the chlorination Hence = of phenol by NaOCI has been tested (Figs 1 and 2). I [H 0] 1000118 55.56 2 Equations (13 and 14) were found to be valid for the 9 =6.1xI0- ... (10) chlorination of para substituted phenols The values of k2 for all the phenols were calculated log kobs = 2.88 - 3.4 (J (r = 0.988) (13) from the slopes of either kobs versus [ArOH] or kobs versus lI[OH-] plots. log k2 = 3.5 - 3.25 (J (r= 0.985) (14) K1k , [H , O] The larger values of 3.4 and 3.25 for the reaction con­ Slope = - - orslope=K k, [H 0][ArO- ], [OH - ] 1 - 2 stant support ionic type of reactions. respectively ... (II) The enthalpies and the free energies of activation for reactions of all the substituted phenols have been or k, = slopex [OH - ] or k = slope correlated. The correlation was reasonably linear with - K [H 0] 2 K [H 0]{ArO- ] 1 2 1 2 an isokinetic temperature of 300 K. Further, the ener­ ... (12) gies of activation of all the phenols were optimised where [OH-] and [ArO-] are the standard run concen­ corresponding to the log A of the parent phenol using trations. Two sets of k2 values were calculated for all the equation, Ea = 2.303 RT (log A -log kobs). Simi­ the phenols from the slopes of the plots, kobs versus larly log A values of all the phenols were optimised [ArO-] and kobs versus II[OH-] and the values agree corresponding to Ea of PhOH through the equation, very well. The k2 values calculated from the former log A = log kobs + Ea/2.303 RT.

' ,8

5.0 1·6

4.0

\Il .0 ;,co 1·2 I.-Cl I.- Br g' 3 -CI + .... 1'0 /' 01 ° 2 .0 ,.0 06V o OL-____-L ______~ ____-L ____ ~ 4-CI -0.4 0.0 0·4 0·8 o 5 10 15 20 GP 0'0 oft-BuOH (v/v)

Fig. 2- Plot of kz vs. a Fig. 3-Plol of log k obs VS . % i-BuOH 1202 INDIAN 1. CHEM., SEC A, NOVEMBER 2001

stabilised in alkaline solutions, electrophilic substitu­ tion takes place at both ortho and para positions.

References I De La Mare P B D, Electrophilic halogenation (University Press, Cambridge) 1976. 2 Cotton F A & Wilkinson G, Advanced illorganic chemistrv (Wiley, New York) 1988. 3 Amin G C, Wadekar S D & Mehta H lJ, Ind ] Tex Res, 2 ....- (1997) 20 and references therein. Scheme 2 4 Cunningham H M, Am] Vet Res" 41 ( 1980) 295. As may be seen from Tables 3 and 4, Ea increased 5 Ogata Y, Kimura M, Ko ndo Y, Katoh H & C hen F C.] Chem with the introduction of electron-withdrawing groups Soc Perkin Trans, 2 ( 1984) 451. 6 Alexander M & 1anathan T A, Tetrahedron, 43 (1987) 1753. into the benzene ring, while the introduction of the 7 Kalachandra S, Z phys Che11l , 268 ( 1988) 8. electron-releasing groups lowered Ea. Similarly log A 8 Rao S V. Oxidn Cmmull , I I (1988) 173. decreased with the introduction of electron­ 9 Singh A K, Sangeeta S, Madhu S, Rajanna G & Mishra R K, withdrawing groups into the benzene ring, while log A Illdian ] Chem, 27 A (1988) 438. increased on substitution with the electron-releasing 10 Derek H R B , Pierre F 1 & Thomas M, Tetrahedroll, 44 ( 1988) 6397. groups into the benzene ring. II Seok W K & Thomas J M, ] Am chem Soc, 110 (1988) 7358. The free energies of activation remain almost the 12 Perumal P, Bhatt T & Vivekananda M, Proc Indiall Acad Sci same in both the optimisations indicating the opera­ (Ch em Sci), 101 (1989) 25. tion of similar mechanisms in all the cases. 13 Minisel F, Attilio C & Fontana V E, ] org Chem, 54 (1989) The observed increase in rate with decrease in di­ 738. 14 Pope K D E & Michael T, ] chem Soc Daltoll TrailS, 8 ( 1989) electric constant of the medium (Fig. 3) may be ex­ 1483. plained by the Laider and Eyring25 equation. 15 Tee 0 S, Martino P, & 1anice B M. ] Am chem Soc, III (1989) 2233 . In kD = In k ~ + Z ~e2 (~-~) ... (15) 16 Pure P G, Sudalai A & Satish S, Tetrahedron lell, 30 ( 1989) _.- 2k BTD rB r~ 5929. 17 Mukhopadhyaya, Aloka S & Bhakuni S N D, Indiall ] Clwn. where kD and k~ are the rate constants in the media of 29B (1990) 1060. dielectric constants D and infinity respectively, rB and 18 Tee 0 S & Iyenger N R, Can] Ch em, 6H (1990) 1769. r" refer to the radii of the reactant species and the ac­ 19 Ganapathy K & Palaniappan A, lilt] chem Kinet, 22 ( 1990) tivated complex respectively. It is expected that the 415. rate should be greater in a medium of lower dielectric 20 Gowda B T, Rao P 1 M & Quine S D, J Indian chem Soc, 69 (1992) 830. constant when r" > rB. It is probable that the radii of 21 Vibhhute Y B & Dasharath D, ] Illdian chem Soc, 69 ( 1992) the activated complexes in the present cases are 835. greater than the reactant molecules, as the reactions 22 Vogel A I. A text book of practical orgallic chemist/yo 5 th cd involve the interaction between the n,egative ions and (E.L.B.S, U.K.) (1989), 1205. the dipolar molecule. 23 CRC Hand book of chemistry and physics, 61 st ed. (CRC Press, Florida) 1980. A detailed mechanism of chlorination is shown in 24 Laidler K 1, Chemical kinetics, 3rd ed (Harper & Row, New Scheme 2. As the phenoxide ion is highly resonance York) 1987.