NOTES

Chlorination of Phenyl Acetate & Its NuclearSubstituted Derivatives by Chloramine-T

P. S. RADHAKRISHNAMURTI* & B. M. SASM.-\.L

Department of Chemistry, Berhampur UniversityBerhampur 760007 (Orissa)

Received 27 December 1977revised and accepted 14 August 1978

Kinetics of chlorination of phenyl acetate, p-cresylacetate, o-cresyl acetate, p-chlorophenyl acetate ando-chlorophenyl acetate by chloramine-T (CAT) in20% aqueous acetic acid medium in the presence ofHCIO. are reported. Fractional dependence on sub-strate and inverse fractional dependence on acidityhave been observed; both in 20% aq. acetic acid andpure aqueous rnedium. Solvent variation and ionicstrength variation indicate the reactions to be dipole-dipole type. Tr e effect of temperature has beenstudied and thermodynamic parameters evaluated.A total pH profile for phenyl acetate in pure aqueousmedium has been atrernpte d to unravel the nature ofactive CAT species involved in chlorination. Thecomposite rate law consistent with the results obtainedhas been derived.

IN continuation of our earlier work", we report inthis communication the. chlorination of phenyl

acetate and its nuclear substituted derivatives bychloramine-T (CAT) in aqueous acetic acid as wellas in pure aqueous medium in the presence of per-chloric acid.

Phenyl acetate and its derivatives were preparedby the standard procedure". Freshly prepared pro-ducts were dried over anhydrous CaCl2 or MgS04 anddistilled. The purity of the compounds was checkedbefore use. The disappearance of CAT (May &Baker, AR) was followed by standard iodometricmethod. The first order rate constants have beencomputed from the usual tangential method.

In aqueous acetic acid and in pure aqueous medium,the order of the reaction at constant acidity is frac-tional with respect to the substrate in the concentra-tion range O·OOlM to 0·007M (Table 1). The orderwith respect to CAT is always unity as indicated bythe good linear plots of log (a-x) versus time. In boththe media, increase in [HCI04J retards the rate. Theorder with respect to [H+J is inverse fractional in theconcentration range 0·00625M to 0·05M of HCI04(Table 2). The inverse fractional dependence onacidity may be traced to an equilibrium arisingbetween the protonated and unprotonated substratemolecules.

The reaction rate decreases with the increase in thepercentage of acetic acid, from 20 to 50% (v/v) in thepresence of sodium acetate. The plots of log kl vsliD are linear indicating the dipole-dipole nature ofthe reaction. The change in the ionic strength byadded NaCl04 had marginal effect on the reactionrate.

Reactions have also been carried out in aqueousacetic acid-sodium acetate buffer to compare withthe reactivity in aqueous acetic acid-Ht.Ki, mixtures.The availability of the free phenyl acetate molecule

(

TABLE 1 - EFFECT OF VARYING [SUBSTRATE] ONREACTION RATEt

{[CAT] = 0·0005M; [HCIO.] = 0'0125M; temp.= 35°}

20% HOAc medium Pure aqueous medium

103 [substrate]M

103 [substrate]M

PHE:>YL ACETATE* PHENYL ACETATE

1·58 3·97 (0·52) 1·04 10·40 (1'35}2·49 5·01 (0·65) 2·02 12·74 (1'66}3·15 6·84 (0'89) 3·34 17·24 (2'24)5·10 7·84 (1-02) 5·02 18·02 (2'34)7·50 10-69 (1,39)

P-CRESYL ACETATE p-CRESYL ACETATE

1·00 2·76 1·07 8·462·17 3·79 2·10 10·613·12 4-91 3·13 13·094·90 5·40 5·27 19·577·72 6·97

O-CRESYL .\CETATE O-CRESYL ACETATE

1-19 1-57 1·29 10·841·91 1·78 2·25 11·943'13 1·93 3·31 13·364·97 2·49 5'18 16·257·65 2·85

P-CHLOROPHENYL ACETATE

0·98 0·182·34 0·253·01 0·424·97 0·697·77 0·82

O-CHLOROPHENYL ACETATE

1·18 0·292·07 0·413·08 0·635·03 0·747·79 1·08

*Values in parentheses represent the partial rates ofphenyl acetate (ortho-isomer).

tP-Chlorophenyl- and o-chlc rophenyl-acetates are insolublein water.

(unprotonated) which is the reactive species in theformer case, enhances the reaction rate.

The isomer distributions in the case of phenylacetate was found to be 87% para and 13% ortho.This has been utilised for the computation of partialrates with respect to para and ortho substitutions.Taking into account the partial rates for phenylacetate and the usual rates of the other substitutedcompounds, the reactivity of substrates is in the orderof: p-cresyl acetate > o-cresyl acetate > phenylacetate (partial-rate) > o-chlorophenyl acetate >p-chlorophenyl acetate.

The plot of log kl versus a values for differentsubstituents, by taking the partial rate for phenylacetate, is linear with a P value of -2.

To evaluate the thermodynamic parameters, thereactions have been carried out at three differenttemperatures, 35°, 45° and 55°. The plot of log klvs liT is linear. The various thermodynamic para-meters have been calculated at 35°; and recorded inTable 3. Plots of log kl vs -6.9 and log kl VS loglo Aare also found to be linear.

181

rINDIAN J. CHEM., VOL. 17A, FEBRUARY 1979

TABLE 2 - EFFECT OF VARYING [HClO.] ON THE TABLE 3 - THERMODYNAMICPARAMETERSAT 35°REACTION RATE

Substrate ~Et· sin lOglOA ~st ~Ft{[CAT] = 0'0005M; [substrate] = 0'005M; temp.= 35°} kcal/ kcal/ e.u, kcal/

mol mol molMedium 103 kl (min-I) at [HClO.]

Phenyl acetate 15'24 14'62 6·92 -28,88 23,520'00625M 0'0125M 0'025M 0'0375M 0'05M p-Cresyl acetate 15·24 14·62 6,76 -29,62 23,75

o-Cresyl acetate 15·24 14·62 6·42 -31·16 24,22PHENYL ACETATE*

20% HOAc

Pure aqueous

20% HOAcPure aqueous

20% HOAcPure aqueous

20% HOAcPure aqueous

20% HOAcPure aqueous

9·66 7·84 7·23 6·11 4·41 TABLE 4- EFFECT OF VARYING [SUBSTRATE]AT(1,26) (1-02) (0,94) (0,79) (0,57) DIFFERENT ACIDITIES ON REACTION RATE IN26·56 18·02 12·79 10·70 8·54 PURE AQUEOUSMEDIUM(3-45) (2,34) (1-66) (1·39) (1-11)

P-CRESYL ACETATE {[CAT] = 0·0005M; temp.= 35°}

7·83 5·40 4-15 3·94 3·1610s [Phenyl acetate]= 10skI (min-I) at [HClO.]

22,65 19'57 11-31 8·05 5,75 0'0125M 0'025M 0·05MO-CRESYLACETATE 1,036 10·40 (1'35) 5,81 (0,76) 4-81 (0'62)

1·63 1-19 2·014 12,74 (1'66) 7·38 (0'96) 5,85 (0,76)4'01 2·49 2·21 3·072 17,24 (2'24) 10,42 (1·36) 7,39 (0'96)19-66 13·29 7,41 5'51 3-52 5·049 18·02 \2,34) 12·70 (1-65) 8,75 (1'14)7·171 22·01 (2'86) 16·90 (2'19) 10·37 (1'35)P-CHLOROPHENYLACETATE

0·82 0·69 0·64 0·54 0·37=Values in parentheses refer to partial rates of phenyl

(Insoluble)acetate (orthoisomer).

O-CHLOROPHENYLACETATEIn the present system, the total substrate concen-

trations is the sum of free substrate concentrationand the concentration of protonated substrate mole-cule, i.e. [Sh = [S] + [S.H+]

In accordance with Eq. (1) [S.H+] = s, [S] [H+].Therefore [Sh = [S] + tc, [S] [H+]

[Sh

0'87 0·74 0·67 0·60(Insoluble)

0·55

*Values in parentheses refer to partial rates of phenylacetate (ortho-isomer).

In order to reveal the nature of the CAT speciesinvolved in chlorination, the effect of pH on thechlorination of phenyl acetate in aqueous mediumwas studied. The reaction rate increases linearlywith increase in pH up to 4·5 and thereafter de--creases to a minimum at PH 6·85. Surprisingly there.is no reaction at and above pH 10. The maxi-mum rate at pH 4·5 is supposed to be due to HOCIgenerated from CAT, as reported earlier by Mushranei ai». The insensitivity of the reaction at PH 10deserves comment. The reaction path might becompletely changed from halogenation to oxidationin alkaline medium" which does not occur with hardsubstrates like esters.

Derivation of composite rate law - Among all thespecies of CAT, p-toluenesulphochloramide (CH3•

C6H4S02N.HCl) produced from CAT in a fast stepin considered to be the active species under theexperimental conditions. Sulphochloramide pro-duced in the fast step (1), forms a complex with thefree phenyl acetate molecule (step 2), which breaksdown in a rate-determining slow step (3) to give theproducts (o-chloro- and p-chloro-isomers).

K,Phenyl acetate + H+ ~ Phenyl acetate.H" ... (1)

fastK,

Phenyl acetate + CH3.C6H4.S02.N.HCI ~fast

Complex ... (2)k,

Complex ~ Products ... (3)slow

or [S] = 1 + K1[H+] ... (4)

Similarly, the total CAT (R.N.HCI) concentrationis the sum of concentratirns of free CAT (R.NHCl)and complexed CAT, i.e. [CATh=[CAT] +[Complex].

In accordance with step (2), [Complex] = K2 [CAT][S]. Therefore [CATh = [CAT] + K2 [CAT) [S]

[CAThor [CAT] = 1 + K

2[S]

The rate law is given byd[CAT]

- -d-t - = k3 [Complex] = k3K2 [S] [CAT] ... (6)

Substituting the values of [S] and [CAT] from (4) and(5) in Eq. (6) the rate law takes the final form (7),which explains all the facts observed by us.

d [CAT] K2k3 [CATh [Shdt 1 + tc, [H+] + K2 [Sh

The kinetics of chlorination of phenyl acetate byCAT has been studied in pure aqueous medium atvarious concentrations of the substrate and at diffe-rent acidities to derive the composite rate law (Table4). It is observed that (i) at constant acidity, theorder with respect to substrate is always fractionalin the concentration range O'OOlM to O'007M and(ii) the dependence on acidity is always inversefractional.

The composite rate law depending on the productformation is obtained by the graphical analysis ofthe data from the experiments with varying [sub-strate] and [H+]. At each acidity, the rate of forma-

... (5)

... (7)

(

NOTES

tion of the product kAnC! is linearly dependent onthe concentration of AnH.

The composite rate law takes the form (8)

Rat = k'[CAT] + k"[CAT] +e [H+]kill [AnH] [CAT]

[H+]

This equation symbolizes all the facts that havebeen observed by us. The value of k" (0·034 min-I)of this equation obtained by graphical analysis agreeswell with the calculated value (0·029 min-I), justi-fying the validity of graphical analysis.

... (8)

References1. RADHAKRISHNA MURTI, P. S. & SASMi\L, B. M., Indian

J. Chem., 16A (1978), 598.2. VOGEL, A. 1., A text book of practical organic chemistry

(English Language Book Society). 1971. 669.3. BERLINER, E., J. Am. chem. Soc., 76 (1954), 6179.4. MUSHRAN, S. P., MEHROTRA, R. M. & SENEHI, R., J.

Indian chem, Soc. (1974). 594.5. RADHAKRISHNA MURTI, P. S. & PRASAD RAO, M. D.,

J. Indian chem, Soc., LIV (1977), 1048.

Kinetics of Reduction of Some Aryl MethylSulphoxides with Ti(III)

V. BALIAH*t & P. V. V. SATYANARAYANA

Department of Chemistry, Annarnalai UniversityAnnamalainagar 608101

Received 14 April 1978; accepted 2 September 1978

The kinetics of reduction of ortho-; meta- and para-substituted phenyl methyl sulphoxides with Ti(III) inaqueous ethanol has been studied. The reaction issecond order - first order each in sulphoxide and tlta-nium(I1I). Hammett plots for meta- and para-substituted phenyl methyl sulphoxides show goodlinear correlation with a (f0 value of -1·07. The re-action seems to be devoid of the steric effect of theortho substituents. Fair correlation is obtained whenlog (k/kH) is plotted against (f0 values. The negativep value indicates that the attack of Ti(III) on sulphinyloxygen is the rate-determining step.

THE re~uction of sul~hoxides has been studiedextensively by vanous workersv". Barnard

and Hargrave" found the reduction with tit a-nium(III) to be the most satisfactory method for theestimation of sulphoxides. Recently Ho andWong' reported the reduction of sulphoxides withtitanous chloride on a preparative scale. Theeffect of various substituents on the kinetics ofreduction of sulphoxides with titanium (III) hasnot been reported in the literature. In the presentinvestigation we have prepared some ortho-, m,eta-and para-substituted phenyl methyl sulphoxidesand studied the kinetics of reduction with titanouschloride in aqueous ethanol. The applicability ofthe Hammett equation to the reaction is tested.

+Present address: Vice-Chancellor, Nagarjuna University,Nagarjunanagar 522510, Guntur (AP).

(

Purified ethanol was used. Ferric alum, ammo-nium thiocyanate and potassium dichromate wereof the AR grade.

A stock solution of the titanous chloride wasprepared as follows: titanous chloride solution(15%, AR) was boiled with concentrated hydro-chloric acid (AR) under nitrogen atmosphere for1 hr and cooled. The same solution was used forconducting. the kinetic runs and for back-titratingthe aliquots after diluting appropriately.

Estimation of titanium(III)- Into as ml standardsolution of potassium dichromate was pipetted out10 ml of titanous chloride solution and the excessof titanous chloride titrated with ferric alum using10% aq. ammonium thiocyanate (0·5 ml) as indi-cator. The titration was carried out under Nzatmosphere. The titanous chloride solution wasalso titrated with ferric alum solution withoutadding potassium dichromate solution. From thedifference in titre values, the strength of ferric alumsolution and thence the strength of titanous chloridesolution were calculated.

Kinetic procedure - The apparatus was thoroughlyflushed with nitrogen before preparing for thekinetic run. The stock solution of titanouschloride (20 rnl) was diluted to 100 ml by addingethanol, adjusting the amount of water to 30%in the reaction medium. The sulphoxide andtitanous chloride solutions were thermally equi-librated at 31°. Equal volumes of these solutionswere mixed under N2 atmosphere at 31°. Thereaction was followed by quenching the aliquotsin a standard ferric alum solution and the excessof ferric alum solution was back-titrated with tita-nous chloride from a reservoir burette system underN2 atmosphere. A blank experiment was alsocarried out under the same conditions and itwas found that the decomposition of titanouschloride solution was negligible during the reactiontime.

Product analysis - TLC of the product on kine-seIgel showed identical behaviour of the reductionproduct and an authentic sample of methyl phenylsulphide. Infrared spectrum also showed that theproduct was sulphide.

Stoichiometry - Two moles of titanous chloridewere found to be consumed by one mole of sulph-oxide in accordance with Eq. (1):

RzSO+2Ti3++2H+-+R2S+2Ti4++H20 ...(1}

The second order rate equation is applicable upto about 90% of the reaction, being first order eachin sulphoxide and titanium(III). In Table 1 aregiven the rate constants for the reduction of meta-and para-substituted phenyl methyl sulphoxides.The rate is decreased by the electron-withdrawinggroups and increased by electron-releasing groups.

A plot of the logarithms of the rate constantsfor the reduction of meta- and para-substitutedphenyl methyl sulphoxides against Hammett (1

constants gave satisfactory linear correlation (r =0·959, S.D.= 0·050) with a P value of -1·07.although the correlation was much better whenmeta-substituted compounds alone were taken(r = 0·995, S.D.= 0·025). A plot of log k versus(j+ values for para-substituted compounds did not

183

'\

\\