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 o f 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 sig­nificant 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 e lectrophilic attack of the phenoxide ions by NaOCI in the rate determining step has been considered. The values of the pre-equilibrium and the rate determi ning steps have been calcul ated for all the 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 isok inetic temperature of 300 K. Further, the energ if!s of activation of all the phenols are optimised corresponding to the log A of the parent phenol through the equation, Ea = 2.303 RT (log A - log k Obs) ' Similarly log A values of all the phenols are optimi sed corresponding to the Ea of PhOH through the equation, log A = log kobs + Ea 12.303RT. Ea increases with the introduction of e lec tron-withdrawi ng 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-withdrawi ng groups, while log A increases on subst itu­tion with the e lectron-re leasing groups.

The halogens-fluorine, chlorine, bromine and io­dine 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 combi­nation 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 oxida­tions3

. NaOCI has also been used in haemodialysis, inactivation of infective agents in conj uctivitis, pho­tographic 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 stimulation4

.

Phenol chemistry is dominated by the nucIeo­phi licity of the system and the propensity for varied reactions with a wide range of nucIeophiles5

-22

. Both natural and ionised phenols are ambident nucIeophiles and may react at 0 or C centres with neutral or posi­tively 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.

Materials and Methods Commercial sample of sodium hypochlorite (BDH­

GR grade) was used. A stock solution of NaOCI (0.03 mol dm-3

) in 0 .025 mol dm-3 NaOH was prepared, standardised and stored in dark coloured bottles. Sta­bility of the oxidant was checked at regular intervals by iodometric titration against standard sodium thio­sulphate. The concentration remained unchanged for sufficiently a long period of time.

Pure samples of the parent phenol and thirteen monosubstituted phenols-2-methy 1, 2-chloro, 2-carboxy, 3-methyl, 3-chloro, 3-carboxy, 4-methyl , 4-ethyl, 4-chloro, 4-bromo, 4-carboxy, 4-nitro and 4-acetyl phenols (Aldrich Chemie) were recrystallised or distilled before use. Stock solutions of the phenols (0.10 mol dm -3) in 0.10 m01 dm -3 of NaOH were pre­pared as and when required. Initial investigations re­vealed that the addition of chloride ion did not have 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 con­centrated aqueous solution of sodium nitrate.

Since the phenols were found to exist virtually as the phenoxide ions in alkaline solutions, the effective hydroxyl ion concentration in the reaction mixture was taken as the difference between the hydroxyl ion concentration and the concentration of phenol used.

Kinetic measurements The kinetic studies were \l1ade in glass stoppered

pyrex bottles under pseudo-first order reaction condi­tions with [phenol] »[NaOCI] (5-60 fold excess). The reactions were initiated by the rapid addition of requisite amounts of NaOCI (0.0003-0.003 mol dm-3

)

thermally pre-equilibrated at a desired temperature, to mixtures containing known amounts of phenol (0.005-0.Q5 mol dm-3

), sodium hydroxide (0.01-0.30 mol dm\ sodium nitrate and water, pre-equilibrated at the same temperature. The progress of the reactions was monitored for at least two half-lives by the iodometric determination of unreacted oxidant at regular intervals of time. The pseudo-first order rate constants (lcobs) were computed by the graphical methods and the val­ues were reproducible within ± 5%.

Stoichiometry and product analysis Stoichiometry of NaOCI-phenol reaction was de­

termined by allowing the reaction mixtures containing phenol and NaOCI in 1: 1 molar ratio in aqueous so­dium hydroxide to go to completion at room tem­perature. The observed stoichiometry may be repre­sented as

. .. (1)

The chlorinated phenoxide ion was characterised as follows: The reaction products were acidified with dilute sulphuric acid. The brown layers separated were removed, washed with water and distilled. The presence of chlorine in the product phenol was con­firmed by Lassaigne's tese3

. Further, the aqueous layer of the reaction mixture did not gi ve test for the free chloride ion. Ortho/para ratio of the chlorinated products is dependent on pH and the nature of sol­vent. But in the case of chlorination of ortho or para substituted phenols, the site of attack would be either para or ortho position respectively, while with meta substituted phenols, varying proportions of ortho and para products were expected, as in the case of the 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

103 NaOCllo

(mol dnh 102 [ArOHlo (mol d~3)

Effect of varying [NaOCllo

0.3 2.0 0.5 2.0 1.0 2.0 2.0 2.0 3.0 2.0

Effect of varying [ArOHlo

1.0 0.5 1.0 1.0 1.0 2.0 1.0 3.0 1.0 5.0

Effect of varying [OH··]

1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0

"[OW leff = [OW llOlal- [ArOH1; Temp. : b283 K. c298 K. d278 K

1020H l"eff

(mol d~3 )

8.0 8.0 8.0 8.0 8.0

8.0 8.0 8.0 8.0 8.0

1.0 2.0 3.0 5.0 8.0

10.0 20.0 30.0

3.0 3.1 3.1 3.2 3.2

0.7 1.6 3.1 5.0 8.0

25.0

8.3 4.6 3.0 2.4

13.1 13.0 13.2 13.3

3.8 7.6

13.0 21.3 34.2

32.6 18.2 13.0

5.3 3.4

3.0 3.1 7.1 2.9 7.1 2.8 6.8 2.9 6.7

0.7 1.7 1.4 3.4 2.8 7.1 4.0 10.5 7.8 17.7

25.5 11.1 10.3 7.5 4.4 8.0 2.9 7.1 2.3 1.1 5.5

3-COOHc

10.1 23.0 9.2 9.9 24.0 9.5 9.8 23.0 9.8 8.9 21.1 10.6 8.9

6.2 2.3 2.3 12.4 4.5 4.4 25.0 9.5 9.8 35.3 12.9 13.6 60.8 25 .0 24.9

38.9 41.5 67.8 31.0 40.3 15.7 15.7 25.0 9.6 9.8

7.5 10.9 4.1 3.8

1198 INmAN 1. CHEM., SEC A, NOVEMBER 2001

Results The kinetic data on the chlorination of the parent

phenol and thil1een monosubstituted phenols: 2-methy 1, 2-chloro, 2-carboxy, 3-methyl, 3-chloro, 3-carboxy, 4-methyl, 4-ethyl, 4-chloro, 4-bromo, 4-carboxy, 4-nitro and 4-acetyl phenols, by NaOCI in aqueous al­kaline medium, under varying conditions of [ArOH] , [NaOCI] , [OH] , solvent composition and temperatures are shown in Tables 1-5 .

Effect of varying [NaOCl}o At constant [~rOH]o (5 -50 fold excess over

[NaOCl]o) and [OH] , first order plots of log [NaOCI] versus time were linear up to at least 75 % completion of the reactions. The pseudo-first order rate constants computed from the plots remained unaffected by the changes in [NaOCl]o (Tables I and 2), establishing the firs t order dependence of the rate on [NaOCI], in all the cases.

Effect ofvaryillg [ArOH}o At constant [NaOCl]o and [OH-] under substrate

excess conditions (5-50 fold) , the rates increased with increase in [ArOH] with first order dependences in lArOH] (Tables I and 2). The plots of kobs versus

[ArOH] were linear with zero intercepts on the ordi­nates.

Effect of varying other parameters of the medium The rates decreased with increase in [OH-] at fixed

[NaOCl]o and [ArOH]o with inverse first order kinet­ics in [OH-] for chlorination of all the phenols except salicylic acid (Tables 1 and 2). The plo ts of kobsversus I/[OH-] were linear passing through the origin .

Variation in ionic strength of the medium (0.08-0.8 mol dm-3

) or addition of cr (0.0-0.1 mol dm-3) had

no significant effect on the rate of chlorinations, but the decrease in dielectric constant of the medium ef­fected by the addition of t-BuOH increased the rates. The rates of reactions were measured at different temperatures and the activation parameters have been calculated (Table 3 and 4).

There were wide variations in the rates of chlorina­tion of phenols by NaOCI wi th the change of su bstitu­ents. The rates were higher for phenols with electron donating substituents in the benzene ri ng and lower for phenols with electron withdrawing subst ituents. Therefore the reactions had to be studied at different temperatures for different substi tuted phenols de­pending on their rates of reactions (Table I). How-

Table 2- Pseudo first order rate constants (kobs) for the chlorination of some para substituted phenols by NaOCI in aqueous alkaline medium

10' [NaOCI]o (mol dl~J )

102 [ArOI-l]o (mol dn;3)

Effect of varying [NaOCl]o

0.3 2.0 0.5 2.0 1.0 2.0 2.0 2.0 3.0 2.0

Effect of varying [ArOI-llo

1.0 0.5 1.0 1.0 1.0 2.0 1.0 3.0 1.0 5.0

Effect of varying [01-1]

1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0

' [OW ]cf f = [OW JIO(al - [ArOH] ; Temp. : b278 K, C 298 K, d3 18 K

102 [Ol-l]"cff (mol d~) 4_CI-I)h 4-C21-1 ,b 4-COCH/ 4-COOl-lc . 4_Clc 4-Brc

8.0 6.7 3. 1 4.0 0.5 8.0 6.6 7.3 5. 1 4.0 2.8 0.4 8.0 6.7 7.4 5.4 3.8 2.9 2.9 0.3 8.0 6.3 7.5 5.5 3.8 3.1 2.7 0.3 8.0 6.4 6.8 5.4 3.8 3. 1 2.6 0.3

8.0 1.7 1.8 1.4 0.95 0.8 0.7 0.1 8.0 3.2 3.7 2.7 1.9 1.6 1.3 0.2 8.0 6.5 7.4 5.2 3.7 3. 1 2.6 0.3 8.0 9.4 11.7 7.3 5.6 4.8 4.0 0.5 8.0 16.6 18.5 12.1 10.0 8.7 6.6 0.7

1.0 25.2 2. 1 2.0 27.0 29.2 20.0 15.4 2. 1 1. 1 1.0 3.0 18.3 20.0 15.5 9.6 8.0 0.7 5.0 11.1 11 .4 8.8 6.7 4. 1 4.6 0.5 8.0 6.8 7.4 5.2 3.3 3.0 2.6 0.3

10.0 2. 1 2.2 0.2 20.0 2.0 3.4 2.8 1.6 1.0 1.2 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 the substituted phenols were computed for 298 K through the relationship.

E. (298-T) 100 k = log k +" s

to obs 2.303 R 298 T

1=1 .0 mol dm-3. Generally alkyl substitution in­creased the rate. Other groups like -Cl , -COOH, -N02 etc. decreased the rates of chlorination. The substitution at the meta-position had the largest influ­ence on the rate.

Discussion In alkaline solutions, NaOCl exists as ocr

NaOCl ~ Na+ + OCl- ... (2)

The energies of ac tivation computed by the Arrhenius plots were used. The calculated pseudo­first order rate constants at 298 K are shown in Table 5. As may be seen, the rate constant for the 3-CH3 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 phenol at [NaOCI]o=50 [S]o= 12.5 [OH-]=3.33

Phenols exist as phenoxide ions in alkaline solutions

XC6H40H+OH- · ' XC6H40 - + H20 ... (3)

Table 3-Acti va ti on parameters for the chlorination 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 I1S(JK l moi - l

) -62.5 -3 1.0 -5 1.0 -98.3 -25.5 -39.9 -53.8 I1C#( kJmol- l

) 89.2 85.2 93 .2 90.9 8 1.2 90.2 90.2

Optimised va lues correspond ing to 10gA value of PhOH"

E,,(kJ mol- I) 73 .0 69. 1 77.0 74.8 65.0 74 .0 74.0

I1H\ kJmoi- l) 70.5 60.7 74.6 72.3 62.5 71.6 71.5

I1C#(kJmoi- l) 89.2 85.3 Y3.2 90.9 81.2 90.2 90.1

Optimised values corresponding to E" value of PhOH"

10 0 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 I1C#(kJmoi- l

) 89. 1 85.3 93.2 90.9 81.2 90.2 90.2

"Please see tex t

Table 4-Activation parameters for the chlorinat ion of some para-substituted phenols by NaOCI in aqueous alkaline medium

Parameters X-C6H4-0 H, 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 I1S' (JK l mol - l

) - 58.8 -78.6 -57.3 -68.0 -5 1.2 -64.7 -50.0 !:J.C# (kJ 1110',- 1) 87.3 87.3 92.9 93.5 92 .6 %.8 104.3

Opti mised val lies corresponding to 10gA value of PhO H"

E,,(Kj mol- I) 7 1.2 30.4 76.4 77.3 76.4 80.7 88.2 M I#(kJ mul- l

) 68.7 68.7 73.9 74.8 73.9 78.2 85 .7 I1C#( kJmol- l

) 87.3 87.3 92.7 93.4 92.6 96.8 104.3

Optimised va lues correspond ing tu E" va lue of PhOH"

10 0 A <> - 10.3 10.3 9.3 9.2 9.4 8.6 7.3 I1S#(JK l mol- l ) - 56.4 -56.4 -75.2 -76.9 -74.0 -88.4 - 11 3.5 !:J.C#(kJmol l

) 87.3 87.3 92.9 93.4 92.7 96.9 104.3

"Please see text

1200 TNDIAN 1. CHEM., SEC A, NOVEMBER 2001

Hence, ocr and phenoxide ions ArO- were used to represent NaOCI and phenols, respectively.

The observed kinetics of first order each in [NaOCI] and [ArOH] and inverse first order in [OH-] may be explained by the mechanism shown in Scheme 1.

K, OCl- + H20 E " HOCI + OH- (fast)

k2 HOCI + ArO- ---7 ClArO- + H20 (slow) where CIArO- is the chlori nated product.

·Scheme 1

The rate law in accordance with Scheme 1 is given by equation (4)

d[OCI - ] K,k 2 [OCI - ][ArO - ][H 20]

dt .. . (4)

Equation (4) may be rearranged as

1 d[OCI - ] K,k 2 [ArO - ][H 10] --- - --=------=. = ----'--=-------=-[OCI- ] dt [OH - ]

. .. (5)

But we have

1 d [OCI - ] = k [OCI- ] dt obs

Equation (5) therefore takes the form

k b = K,k 2 [ArO- ][H 20] ... (6) os [OH - ]

(7)

The plots of k obs versus [ArO-] and k obs versus lI[OH- ] were linear with zero intercepts on the ordi­nates, in accordance with the rate law (7). Further, K, for the equi librium,

K, OCl- + H20 E :> HOCI + OH- . .. (8)

was calculated from the dissociation constant, Ka (2.95 X 10-8

)24 of HOCI and the ionic product of wa­ter, Kw (10- '4) as

4 .0

3 . 0

2. 0 III .D 0

.:>t:

rn 0 1.0 + <.J)

0 .0 \4~NO' -1 . 0~. ______ L-____ -L ____ ~ __ L-____ ~

- 0 .2 0.2 0.6 1.0

Fig. 1-Plot of kobs VS. Op 103 [NaOCllo = 50 [ XC6H40H)o = 12.5 [OHl = 3.33 1= 1.0 mol. 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 medi um

1 03[NaOCllo = 12.5 [OHlerr = 3.33 1= 1.0 mol dm-3

102 [ArOHl 104 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

Kw = [HOCl][OH - ][H 20] = K [H 0] Ka [OCl][H

20] I 2

Hence

= 1.0xlO-14

= 3.39xlO-7

2.95 x lO -8

K =3.39xlO-7

=3.39xlO -7

I [H20] 1000118

=6.1xI0 -9

... (9)

3.39xl0-7

= 55.56

... (10) The values of k2 for all the phenols were calculated from the slopes of either kobs versus [ArOH] or kobs versus lI[OH-] plots.

K1k , [H , O] Slope = - - orslope=K 1k, [H 20][ArO- ],

[OH - ] -

respectively ... (II)

or k, = slope x [OH - ] or k = slope - K 1[H 2

0] 2 K1[H

20]{ArO - ]

... (12) where [OH-] and [ArO-] are the standard run concen­trations. Two sets of k2 values were calculated for all the phenols from the slopes of the plots, kobs versus [ArO-] and kobs versus II[OH-] and the values agree very well. The k2 values calculated from the former

01

°

5.0

4.0

2 .0

,.0

I.-Cl

I.- Br

OL-____ -L ______ ~ ____ -L ____ ~

-0.4 0.0 0·4

GP

Fig. 2 - Plot of kz vs. a

0·8

plots are (103 k2 , dm3 mort s-') = 3.8, 16.0, 0.65,58 .5, 1.9, 2.2, 8.7,7.8,0.85,0.14,0.65,0.57 and 0.004 for the parent and 2-CH3.2-Cl, 3-CH3 , 3-Cl, 3-COOH, 4-CH3. 4-C2HS. 4-COOH, 4-COCH3. 4-Cl, 4-Br and 4-N02 substituted phenols, respectively.

Validity of Hammett equation for the chlorination of phenol by NaOCI has been tested (Figs 1 and 2). Equations (13 and 14) were found to be valid for the chlorination of para substituted phenols

log kobs = 2.88 - 3.4 (J (r = 0.988) (13)

log k2 = 3.5 - 3.25 (J (r= 0.985) (14)

The larger values of 3.4 and 3.25 for the reaction con­stant support ionic type of reactions.

The enthalpies and the free energies of activation for reactions of all the substituted phenols have been correlated. The correlation was reasonably linear with an isokinetic temperature of 300 K. Further, the ener­gies of activation of all the phenols were optimised corresponding to the log A of the parent phenol using the equation, Ea = 2.303 RT (log A -log kobs). Simi­larly log A values of all the phenols were optimised corresponding to Ea of PhOH through the equation, log A = log kobs + Ea/2.303 RT.

\Il .0

' ,8

1·6

;,co 1·2

g' 3 -CI + .... 1'0

o

06V 4-CI

o 5 10 15

0'0 oft-BuOH (v/v)

/'

20

Fig. 3-Plol of log k obs VS . % i-BuOH

1202 INDIAN 1. CHEM., SEC A, NOVEMBER 2001

Scheme 2

As may be seen from Tables 3 and 4, Ea increased with the introduction of electron-withdrawing groups into the benzene ring, while the introduction of the electron-releasing groups lowered Ea. Similarly log A decreased with the introduction of electron­withdrawing groups into the benzene ring, while log A increased on substitution with the electron-releasing groups into the benzene ring.

The free energies of activation remain almost the same in both the optimisations indicating the opera­tion of similar mechanisms in all the cases .

The observed increase in rate with decrease in di­electric constant of the medium (Fig. 3) may be ex­plained by the Laider and Eyring25 equation.

In kD = In k~ + Z~e2 (~-~) 2k B TD rB r~

... (15)

where kD and k~ are the rate constants in the media of dielectric constants D and infinity respectively, rB and r" refer to the radii of the reactant species and the ac­tivated complex respectively. It is expected that the rate should be greater in a medium of lower dielectric constant when r" > rB. It is probable that the radii of the activated complexes in the present cases are greater than the reactant molecules, as the reactions involve the interaction between the n,egative ions and the dipolar molecule.

A detailed mechanism of chlorination is shown in Scheme 2. As the phenoxide ion is highly resonance

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

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