Indian Journal of ChemistryVol. 32A, May 1993, pp. 3R7-394

Kinetics and mechanism of the aqueous cleavage ofphthalic anhydride (PAn)

Mohammad Niyaz Khan I

Department of Chemistry, Bayero University, P.M.B. 3011, Kano, Nigeria

Received 18 June 1992; revised and accepted 13 January 1993

The rate of hydrolysis of phthalic anhydride (PAn) in mixed aqueous-organic solvents shows adecrease of ca. 265-, 190- and 8-fold with increase in the contents of acetonitrile from 2 to80%, v/v, 1,4-dioxan from 10 to 80%, v/v ; and ethanol from 8 to 88%, vlv, respectively. Hydrolysisof PAn in mixed water-acetonitrile and water-I, 4-dioxan solvents has been found to involve the for-mation of complex between PAn and water molecules which then reacts with water molecules toform phthalic acid. In water-ethanol mixed solvents, the complex formed between PAn and watermolecules collapses to phthalic acid without involving any water molecule. The stepwise mechanismfor hydrolysis of PAn has been proposed

Hawkins studied the effect of temperature, acidconcentration, and added electrolytes or dioxanon the rate of hydrolysis of phthalic and 3,6-dimethylphthalic anhydride I as part of an inves-tigation on hydrolysis of trifluoroethyl hydrogen3, o-dimethylphrhalate- and of a series of substi-tuted phthalanilic and 3, 6-dimethylphthalanilicacids'. An extensive study on the effect of [diox-an] on the rate of hydrolysis of phthalic anhydride(PAn) was carried out by Fagley and Oglukirr'.These investigators showed that the hydrolysis ofPAn followed a general reaction mechanism asshown by Eq. (1).

fast

PAn + (H20), ,= Complex (C) ...•Phthalic acid

As part of an investigation on hydrolysis ofN,N-dimethylphthalamic acid we report hereinthe results of the effect of different organic sol-vents on the rate of hyrolysis of PAn and theprobable mechanistic conclusions of the reaction[Eq. (1)].

Materials and MethodsAll commercially available chemicals were ob-

tained from Aldrich, BDH and Fluka and were ofreagent grade. Doubly distilled water was usedthroughout.

"Present address: Jabatan Kirnia, Fakuhi Sains dan PengajianSekitai , Universiti Pcrtanian Malaysia. ·+34()() UPM Serdang,Selangor Darul Ehsan. Malaysia.

Kinetic measurements and data analysisThe rate of hydrolysis of PAn was studied spec-

trophotometrically by monitoring the disappear-ance of PAn at 310 nrn, The details of the kineticprocedure were same as described elsewhere".The reaction was allowed to proceed for morethan 7 half-lives in most of the kinetic runs. In atypical kinetic run, the observed absorbance, Aobs'

at any time t during the course of the reaction,fitted well to Eq. (2)

... (2)

(1)

where [X], is the initial concentration of PAn,capp' koos and A", represent the apparent molarabsorptivity, observed pseudo-first order rate con-stant and absorbance at 1= 00, respectively. Thenonlinear least squares technique was used to cal-culate the unknown parameters, koos' copp and A",from Eq. (2). \

ResultsThe effects of acetonitrile, ethanol and 1, -l-di-

oxan on the rates of hydrolysis of PAn were stud-ied in mixed aqueous-organic solvents. The ob-served pseudo-first order rate constants, k"bs' aresummarized in Tables 1-3. The values of kobs dec-reased sharply with increase in the contents of or-ganic cosolvents.

The rate constants, kobs' were also determinedat various [HCl] ranging from 0.0 to 1.0 M andtemperature range of 30-37°C. The observed dataare shown in Table 4. The rate constants, k"bs'

388 INDIAN J CHEM, SEe. A, MAY 1993

Table I-Effect of acetonitrile on the cleavage of phthalic anhydride[Phthalic anhydride], = 0.002 M, [HCl]= 0.005 M,..t = 310 nm

CH)CN 104 k~b' Ea~p A:, 104 k:b,(%,v/v) (s-I) (M-I em-I) (s -I)

2 119 ± l' 689.4 ± 4.8< - 0.012 ± 0.007< 128 ± 3<10 95.8 ± l.l 688.3 ± 7.0 - 0.010 ± 0.009 103±220 54.5 ± 0.7 605.7 ± 5.3 -0.026±O.o11 63.7 ± 1.7

30 26.9 ± 0.6 513.7 ± 5.7 - 0.012 ± 0.011 32.8 ± 1.040 13.2 ± 0.2 379.0 ± 5.2 - 0.029 ± 0.011 16.2 ± 0.650 6.03 ± 0.13 308.4 ± 3.8 0.024 ± 0.007 9.05 ± 0.3160 3.08 ± 0.05 253.7 ± 1.3 - 0.011 ± 0.002 3.88 ± 0.05

(171.5 ±0.8)d

70 1.48 ± 0.02 216.5 ± 1.2 - 0.013 ± 0.003 1.63 ± 0.03(177.1 ±0.6)

80 0.45 ± (W6 170.9 ± 3.6 - 0.006 ± 0.007 0.61 ± 0.03(177.7 ± 1.1)

90 133.5 (156.7 ± 0.3)lOO 115'

"Temp. = 25°C; "temp. = 30°C; 'error limits are standard deviations; "parenthesized values were calculated from equation:Auh, =[R]" Eapp[l- exp( - koh,t)] + ", where initial reactant was N,N-dimethylphthalamic acid; <the reaction became extremelyslow; 'obtained from the extrapolation of the plot of E"ppversus %, vlv, CH,CN.

Table 2-Effect of ethanol on the cleavage of phthalic anhydride

[Phthalic anhydride]" = 0.002 M, [HCI]= 0.005 M,..t = 310 nm, and reaction mixture contained 2%, v/v ;CH,CN

C)H,OH lO4 k,;h, E:pp A:, 104 k:b,

%,v/v (S-I) (M-Icm-I) (S-I)

8 168 ± 5' 638.4 ± 4.4' - 0.006 ± 0.003< 194.± 2'18 164 ± 3 655.3 ± 7.5 - 0.014 ± 0.005 183 ± 428 158±2 589.4±5.2 -0.012±0.004 174±3

38 123±3 529.7·±3.5 -0.017±0.004 137±248 95.0 ± 2.1 439.1 ± 2.2 - 0.017 ± 0.004 104 ± 158 73.9 ± 2.1 364.3 ± 2.7 - 0.007 ± 0.003 80.5 ± 1.568 43.7 ± 1.8 294.6 ± 2.2 - 0.008 ± 0.004 59.4 ± 1.378 37.6 ± 0.9 230.7 ± 1.4 - 0.001 ± 0.003 42.5 ± 0.688 20.1±0.4 167.6±1.2 0.000±0.002 21.9±0.4

100 100d

"Temp. = 3SOC;"temp. = 3rC; 'error limits are standard deviations; "obtained from the extrapolation of the plot of Eappversus%, v/v ; C2H,OH.

appeared to be independent of [HCl]. The changein ionic strength from 0.005 to 1.0 M did notshow any significant effect on kobs (Tables 1-4).The value of kobs obtained at 98%, v/v, H20 maybe compared with the reported values of12.6xlO-3 S-l at 25°C (ref. 6), 10.5 x 10-3 S-l

at 25°C (ref. 7) and 7.8xlO-3 S-1 at 24.6°C(ref. 3).

DiscussionThe calculated values of A; summarized in

Tables 1-3 indicate that the hydrolysis product(phthalic acid) of PAn does not absorb at 310 nmto a detectable level. The values of fapp shown inTables 1-3 therefore represent the molar absorp-tivity of PAn at 310 nm. The increase in the con-tents of organic cosolvents in mixed aqueous sol-vents reveal a marked decrease in the values offapp (Tables 1-3). Spectral studies on hydrolysis ofPAn in 1, 4-dioxan-water mixtures led Fagley andOglukian" to propose the complex formation bet-ween water and PAn. The effect of mixed orga-

KHAN: KINETICS OF CLEAVAGE OF PHTHAUC ANHYDRIDE 389

Table 3-Effect of 1, 4-dioxan on the cleavage of phthalicanhydride

[Phthalic anhydride], =0.002 M, [HCll = 0.005 M, A. = 310 nm,Temp. =30°C, and reaction mixture contained 2%, v/v, CH3CN

1,4-Dioxan 104 kObS Eapp A""(%,v/v) (S-I) (M-Icm-I)

10 103 ± 2' 658.2 ± 4.8'20 70.0 ± 1.1 589.9 ± 3.4

30 44.6 ± 0.5 519.3 ± 2.640 24.6 ± 0.3 452.1 ± 2.450 11.9 ± 0.2 383.7 ± 2.560 4.54 ± 0.10 320.1 ± 3.670 2.15±0.07 222.3±2.180 0.54 ± 0.04 183.3 ± 6.890 131.5

93 113.0100 95<

- 0.002 ± 0.007a

- 0.008 ± 0.0060.006 ± 0.003

- 0.001 ± 0.004- 0.020 ± 0.005- 0.029 ± 0.008

0.Q25 ± 0.004

0.008 ± 0.007

'Error limits are standard deviations; "no change in Aobscould be seen within 7 to 20 hr; Cobtained from extrapolationofthe plot of E,pp versus %, v/v, 1, 4-dioxan.

Table 4-Effect of [HCl] on the cleavage of phthalicanhydride

[Phthalic anhydride], =0.002 M, ionic strength 1.0 M,A = 310 nm, and aqueous reaction mixture contained 2%, v/v,

CH3CN

[HCll Temp. 104 kobs E,pp A""(M) (0C) (S-I) (M-I cm-I)

0.000 30 111 ± 1" 745.7 ± 4.6' 0.005 ± 0.004'0.010 30 113±1 756.5 ± 4.1 - 0.003 ± 0.004

0.005 35 136±4 756.4 ±4.7 - 0.007 ± 0.010

0.100 35 140 ± 5 748.5 ± 5.1 - 0.009 ± 0.012

0.500 35 119±3 698.2 ±4.2 - 0.009 ± 0.0091.000 35 118±5 754.3 ± 5.2 - 0.Q18± 0.Q15

0.100 37 160 ± 3 778.5 ±6.6 - 0.005 ;I:0.005

0.500 37 162 ± 3 727.8 ± 5.7 - 0.004 ± 0.0050.980 37 153 ± 2 729.9 ± 5.4 - 0.004 ± 0.006

'Error limits are standard deviations.

nic-water solvents on Eappmay be easily explainedin terms of associated complex (C) formation bet-ween water and PAn [Eq. (3)].

. .. (3)

The observed absorbance (~bslat any time t = 0may be given as

~bs = Eo[PAn]O+ EdClo ... (4)

G

12.' o

l.a 2.6

'09 •

·1.0 1.8

1.~L.,-----'----,-JO.L9 -,-------:-'1.-:-, --

Fig. I-Plots showing the dependence of log E (whereE = E,pp - Eo) upon log [H20l for the solution of phthalic an-hydride in mixed aqueous solvents, H20-CH3CN (0), H20-

C2H50H (fl.) and H20-I, 4-dioxan (El).

where Eo and Ec represent the molar absorptivityof PAn and C, respectively, and [PAn]o and [C]oare the respective concentrations of PAn and C att= O. Equation (5) may be derived from Eqs (3)and (4) where Eapp= ~b/[X]O and[X], = [PAn]o+ [C]o i.e. initial concentration ofPAn.

EO + lOcK [H20rE =app 1 + K[H20r

Equation (5) may be reduced to Eq. (6) pro-vided K [H2O]' < 1 as discussed in the Appendix.

Eapp= Eo + (Ec - Eo) K [H20]n ... (6)

Equation (6) predicts that the .plot of log(Eapp- Eo) versus log [H20] should be linear. Suchplots as shown in Fig. 1 are linear. The interceptsand slopes of these plots gave the respective va-lues of K and n as 1.99 x 10-3 M:» and1.55 ± 0.06 for water-acetonitrile, 18.6 x 10 - 3

M:» and 1.00 ± 0.03 for water-ethanol and8.62x1O-3 M:» and 1.19±0.02 for water-I,4-dioxan solvent systems. The value of Ec of748.3 M':' cm "! was considered in the calcula-tion of K from the intercepts .

Although Fagley and Oglukian" presented evid-ence for the associated complex (C) formation[Eq. (1 )], one might argue that the decrease in E. h appWIt decrease in water contents in mixed aque-ous-organic solvents may be due to merely theblue shift of n -+ Jr* transition. Such blue shift is

... (5)

390 INDIAN J CHEM, SEe. A, MAY 1993

believed to occur due to energetically favourableinteraction between polarized ground state, ca.o+C-oo-, and solvent molecules. The associatedcomplex (C), however, represents similar inter-action between carbonyl group of PAn and thewater molecules.

The observed values of kobs (Tables 1-3) showa nonlinear decrease with increase in the contentsof organic cosolvents. This decrease cannot be at-tributed to the decrease in the dielectric constantof the reaction medium due to decrease in watercontents. The hydrolysis apparently involves neu-tral reactants and such a reaction should show anincrease in the rate with decrease in the dielectricconstant of the reaction medium",

The general reaction scheme for hydrolysis ofPAn in mixed aqueous solvents containing aproticorganic cosolvent may be shown by Eq. (7).

... (7)

Equation (7) and observed rate law (rate = kobs

[PAnJr, where [PAnJr is the total concentration ofPAn) may lead to Eq. (8) where K, = K[H20]n.

k K [H O]n+n,k = I 2

obs 1 + K. ... (8)

The protic solvent such as ethanol is expectedto react with PAn. The extrapolation of the plotof kohs versus ethanol (%, vIv) gave an approxim-ate value of kobs of ~ 1 x 10-4 s-I at 100%, vlv,ethanol and 35°C which is nearly 170-fold smallerthan kobs at 100%, vlv, H20 and 35°C. This andthe observed value of kobs at 88%, vlv, ethanol(Table 2) show that the rate of ethanolysis of PAncan be neglected compared to the rate of hydro-lysis even at 10%, vlv, H20 in water-ethanol sol-vents. Nearly similar estimated values of Eo asshown in Tables 1-3 reveal that ethanol moleculesdo not involve in the formation of complex, C.

The plots of log lkobs(1 + Ks)} versus log [H20]as shown in Fig. 2, are essentially linear withslight curvature in the case of water-acetonitrilesolvent. These results thus affirm the validity ofEq. (8). The slopes (= n + n I) and intercepts(= log k, + log K) of these plots turned out to be3.6 ± 0.2 and - 8.25 ± 0.27 for water-acetonitrile,3.4 ± 0.1 and -7.69 ± 0.12 for water-I, 4-dioxanand 1.05 ± 0.06 and - 3.52 ± 0.08 for water-etha-nol, respectively. Hawkins I observed the gradientof log kohs against log [H20] as 3.7 ± 0.2 for hy-drolysis of PAn in aqueous-dioxan at 37°C whichis comparable with the observed value of n + n I

log y

ooI1

1. 7 -

2.2

2.71.0 1.3 1.6

Fig. 2-Dependence of log Y (where Y= k2(1 + K,)) on log[H20] for the cleavage of phthalic anhydride in mixed aque-ous solvents, H20-CH3CN (0), H20-C2H50H (L!,.) and

H20-1, 4-dioxan (El ).

at 25°C. These values were used to calculate nl

and k, using the calculated values of n and Kfrom Eq. (6). The respective values of n , and klwere found to be 2.1 and 2.83x1O-6 M-nl S-l

in water-acetonitrile, 2.2 and 2.37 x 10-6 Mr»:s - I in water-I, 4-dioxan and 0.1 and 1.62 x 10- 2

M - nl S- I in water-ethanol solvents. A fractionalorder (nl) such as 2.1 and 0.1 can not be ration-alized if k, represents a single elementary step ofthe reaction. Therefore it seems that n , = 2 in wa-ter-acetonitrile and in water-I,4-dioxan andn , = 0 in water-ethanol mixed solvents.

The hydrolysis of PAn shows a large solventdeuterium isotope effect7 which indicates the sig-nificant amount of proton transfer in and beforethe transition state of the reaction. An order ofparticipation of water of 3 has been suggested inthe hydrolysis of PAn in water-I, 4-dioxan mixedsolvents", The results described in this papershow the order (n + n 1) of participation of waterof 3 in water-acetonitrile and water-I, 4-dioxanand of 1 in water-ethanol mixed solvents.

The order of reaction in water-acetonitrile andwater-I, 4-dioxane is significantly different fromthat in water-ethanol mixed solvents. The mixedaqueous mixture containing ethanol as organic co-solvent is termed as typically aqueous (TA) whilethat containing acetonitrile as organic cosolventsis termed as typically nonaqueous (TNA) sol-vents 9. Although the water-I, 4-dioxan solution is

KHAN: KINETICS OF CLEAVAGE OF PHTHAUC ANHYDRIDE 391

characterized as TA solution, the properties ofthis mixture sometimes conflict with hID, Thesesolvents (TA and TNA) are known to have sever-al characteristically different solution properties 10.

Thus, the different values of nl obtained in water-acetonitrile, water-l , 4-dioxan and water-ethanolcould be the consequence of the different solutionproperties of TA and TNA solvents.

Nearly 170-fold larger reactivity of H20 com-pared to C2HsOH cannot be explained in termsof nucleophilicity of H20 and C2HsOH mole-cules. The intrinsic nucleophilic reactivity ofC2HsOH toward electrophilic carbonyl carbon islarger than H20 (ref. 11) which is conceivable forthe fact that C2HS group is electron-donatingcompared to H as evidenced from oJ values'".Thus, the larger reactivity toward PAn of H20than C2HsOH is the consequence of the complexformation between PAn and H20 while PAnmolecules do not form complex with ethanolmolecules.

The rate of hydrolysis of PAn appears to beslightly affected by the presence of HCl. The rateconstants, kobs' show the decrease of ca. 5% and15% with increase in [HCI] from 0.005 to 1.0 Mat 35°C and 3rC, respectively, and 1.0 M ionicstrength (Table 4). Phthalic anhydride revealedthe flat pH profiles for hydrolysis in aqueous so-lution over the range from 4 M HCI to pH 5.2(ref. 1) and 5 M HCI04 to pH 2.0 (ref. 16).

Mechanistic proposalA plausible stepwise mechanism for hydrolysis

of PAn may be shown in Scheme 1. The rate-de-termining step is considered to be either kd or kjstep which involves the proton transfer in a ther-modynamically favourable direction. The protontransfer in kd step takes place through protonswitch mechanism which involves one or morethan one water molecules. The magnitude of thepseudo-first order rate constant for thermody-namically favourable proton transfer through pro-ton switch mechanism involving water moleculeshas been reported to be 5 108 s- I (ref. 13). But inthese reactions, the mediator for proton transferi.e. water molecule is significantly weaker basethan the conjugate base of proton-donor sites.The acidity of the protonated hydroxyl group of~± is affected by both the acid-strengthening andacid-weakening groups attached to the same car-bon atom. It is therefore very likely that the pKaof TI± may not be very much different from thatof H30+ or MeOH;. T.hus, the value of ki maybe considered as much larger than 108 s- I. Thekj step presumably involves proton transfer in a

o

M0(>c

')'o~O()o

C

~ •..• H+OH)n

c

Scheme 1

nearly thermodynamically favourable directionand therefore the magnitude of kj [H20]5 5 x 1011S - I if the value of the second order rate constantfor. thermodynamically favourable diffusion-con-trolled proton transfer is considered to be 1010M - I S - I (ref. 14). It seems difficult to assess themagnitude of kd and thus to ascertain whetherkd> kj[H20] or kj[H20] > kj. But the formationof TI- appears to be unlikely due to followingreason.

The pK-4 (=k~4/k1) of r; may be estimatedto be nearly 12 (ref. 15). The value of kd[H30+]is ca. 5 x 107 S - 1 at 0.005 M HCI because kdstep involves thermodynamically favourable pro-ton transfer. The reported values of rate constantsfor breakdown of 1 range from 6.4 x 103 s- 1 forthe hydrate to 3.8 x 107 s -I for the trifluoroethylhemiacetal'>, The pKa of the conjugate acid of theleaving group in k/ step is much smaller than thatof trifluoroethanol and therefore the expected va-lue of k71 is much larger than 3.8 x 107 s -1. Thisindicates that k/ ~ kd[H30+]. Under such condi-tions, tl.e reaction should occur through kj step.According to the principle of microscopic rever-sibility, the reverse reaction i.e. cyclization of

392 INDIAN J CHEM, SEe. A, MAY 1993

o

IR-C-ORI

IH

CH,

. I .BH'O-C-?-R

I HH

R = H- CH2CFa

BH '" General de id

ph thalamic acid to phthalic anhydride should alsooccur via k~8' k~7 steps. But this is not correctbecause a large number of related cyclizationreactions occur through nonionized substrate andthe ionized substrates are completely nonreac-tive3.16·IB.These reactions show their occurrencethrough steps like k~6 and kl ; It is thus appar-ent that the hydrolysis of PAn, under present con-ditions, does not involve the formation of T1- andhence the reaction takes place via kl, kd, kJsteps.

If k21 step is rate limiting then the energeticconsequence requires that k ~ 1 > kl, andkJ > k~2' As concluded earlier in the text that klshould be larger than kj[H20] which meanskl > 10II S- I. The estimated values of the rateconstants for the expulsion of H20 andF3CCH10H from 2 are 1020 S-I and 1023 S-I, re-spectively". It may be also noted that the stabilityof the hydrogen bonded dipolar intermediate (3)is nearly 106-fold larger than that of 2. This couldbe attributed to the decrease in the electron-do-nating ability of oxyanionic group of 2 due to in-termolecular hydrogen bonding between 2 andgeneral acid (BH). These results indicate thatk ~I ~ 1014 S - 1 and consequently it appears thatk~1 > kl·

The occurrence of proton transfer in k: 2 stepis thermodynamically unfavourable. Considering aBronsted slope (fJ) of 1.0 for a reaction involvingproton transfer in thermodynamically unfavour-able direction!", the roughly estimated value ofk~2 is ca. 10-3 S-I (taking [H2b] = 55 M). Signifi-cantly high observed solvent deuterium isotopeeffects 7 rule out the possibility of kd step beingrate-limiting. This shows that kd must be largerthan kl., ("" 10- 3 S-I). The order of the magni-tude of kJ is difficult to ascertain. But Sorensenand Jencksl5 showed that the change from 0-group to OH group in 2 has caused the reductionin the rate constant for the expulsion of ROH(R = H) from 2 by a factor of ca. 1014.This shows

that the value of ki should be larger than 1011 (ifkJ > 10 - 3 S - )) which seems to be reasonable inview of the reported values of rate constants forthe expulsion of OH- (pKa of H20 is 15.74) andF3CCH20- (pKa of F3CCH20H is 12.37) from 1are of the order of 103 s - 1 and 107 s -I, respec-tively",

The concerted conversion of T? to P throughkJ step will be energetically favourable only ifkJ > kl and k~9 > k~6' The pKa of ionizable pro-ton in T? and p± are ca. 12 (ref. 15) and - 6 (ref.20), respectively, and hence the occurrence ofgeneral base (GB) catalysis (H20 acts as GB) inkJ step is expected to be effective only whennearly 75% ["" 1 - (6 -1.7)/(12 + 6)] double bondformation between ° and C of substrate has pro-gressed in the transition state (TS1). This showsthat kJ may not be very much different from kdif one takes the consideration of the entropic bar-rier due to intermolecular nature of the GB cata-lysis in kJ step. As described in Appendix, the es-timated value of k ~6 is ca. 55 s - I. Thus, if kJstep is dominant over kd then k ~9 should be larg-er than 55 s - I. Phthalic acid in water exists al-most 100% as the free carbonyl compound. How-ever, if it is assumed that phthalic acid in waterexists in approximately 0.1% as the hydrate thenthe value of k~9/kJ should be ca. 10-3. Thisshows that the value of kJ is ca. 105 s - I ifk ~9> 55 s -I. The values of rate constants, k, for

. the reaction as shown by Eq. (9) have been shown

RCH( OH)( + OHR I).s.,RCH( = 0) + R IOH

... (9)

to be 104 S-1 for R=Rl =H, 107 S-1 for R=Hand RI =F3CCH2, 106 S-1 for R=CH3 andRI = F3CCH2 and 109 s -) for R = CH3 • andR, = F3CCH2 (ref. 15). These results show thatthe magnitude of k: is highly dependent on boththe pKa of leaving group and the other groupsattached to the carbon atom from which the leav-ing group departs during the course of the reac-tion. In view of these results, the magnitude of kJof 105 s - 1 seems to be unreasonably high andhence k ~9 may not be larger than 55 s - I. Thisreveals that k~6 > k i ; and hence the conversionof P to TI proceeds via p± intermediate. Accord-ing to the principle of microscopic reversibility, ifk~6 and k~5 steps are involved in the formationof Tl from P then the conversion of TI to P mustproceed via kJ and kJ steps.

An alternative stepwise conversion of TI to Pmay be shown by klo and kll steps which involves

KHAN: KINETICS OF CLEAVAGE OF PHTHAUC ANHYDRIDE 393

transient intermediate Tr. The kinetic step kfo in-vo1ves thermodynamically unfavourable protontransfer from proton-donor site of pK. "" 12 toproton-acceptor site of pKa"" -7 (ref. 20). Themagnitude of kfo may be estimated to be ca. 10 - 7

S - 1 (considering an effective molarity of ca. 100M due to intramolecularity of the process). Therate constant k~6 has been estimated to beca. 55 s- I. An intermolecular proton transfersuch as

2 C6H5COOH --+ C6H5COO-+ C6H5C( = OH + )OH

will require a rate constant of magnitude of ca. 1M - 1 S - I (considering a Bronsted slope (fJ) of 1.0for reactions involving proton transfer in ther-modynamically unfavourable direction 19). It isthus apparent that the effective molarity for theintramolecular proton transfer in k~6 step is ca.55 M. The rate constant for OH- attack at carb-onyl carbon of methyl benzoate is 0.125 M-I S-I

(ref. 21). The substituent constants (01) for OHand OCH3 are nearly same'? and pKa of H20 aswell as CH30H are also nearly same. The rateconstant for OH - attack at carbonyl carbon ofphthalic acid may therefore be expected to benearly 0.125 M -I s - I. Considering the effectivemolarity of ca. 100 M, the rate constant for thereaction [Eq. (10)] may be considered to be ca.12.5 s -I. The pKa of the conjugate acid of thenucleopfuIe in k: II step is - 7 which is muchsmaller than the pKa of conjugate acid of OH- oro-HOOCC6H4CH20-. Thus, although the esti-mated value of rate constant of 12.5 s - 1 for thereaction [Eq. (10)] is very approximate, the valueof E 11 must be much smaller than 12.5 s - 1.

These conclusions reveal that k ~6 ~ k ~ II andkJ ~ kio' It is also obvious that kJ > k ~5' Thus,the most likely route for the conversion of C to Pinvolves the k;, k21, kJ and kJ steps.

Although the concerted process for the forma-tion of T? from C involving transition state TS2cannot be ruled out completely, it is consideredto be less likely compared to stepwise mechanism(Scheme 1) due to following reasons. The proba-bility of three molecules to attain the conforma-tional state of TS2 is extremely low even thoughthe two molecules come from the reaction solventwhere the molecules are strongly hydrogen

( 10 I

bonded. Furthermore, generally a concerted me-chanism is operative only if the correspondingstepwise mechanism requires the formation of anunusually highly unstable intermediate (i.e. an in-tennediate of lifetime of > 10 - 13 s) on the reac-tion path.

AcknowledgementThe author is grateful to the Research and

Higher Degree Committee of Bayero Universityfor a research grant to purchase a UV-visiblespectrophotometer. The author also thanks DrNordin H. Lajis for providing facilities in the pre-paration of the manuscript.

APPENDIX

(a) Derivation of Eq. (6).Equation (5) may be rearranged to Eq. (i)

fapp=(fo+fcKJ(1+K,tl ... (i)

where K, = K[H20j". In terms of power series ex-pansion+, if K, < 1, then

(1 + K,)- I = 1 - K, + K,2 - K,l + ... (ii)

which on neglecting the higher power terms re-duced to Eq. (iii).

(1 + K,t 1 = 1 - K, ... (iii)

Equations (i) and (iii) give Eq. (iv).

fapp = (fa + fcK,)(l - K,) ... (iv)

The rearrangement of Eq. (iv) gives Eq. (6) where- fcK} term is neglected compared to otherterms ofEq. (iv),

(b) Estimation 0/ the magnitudes of the rate con-stants, u;

The chemical equilibria shown in Scheme 2 wasused to estimate the approximate magnitude ofthe rate constants kl

_6. It is apparent from Scheme 2that

K/K2=kl/k~1 ... (v)

394 INDIAN 1 CHEM. SEC. A. MAY 1993

a1/

•~C_O"

~COOH+ a- H

Scheme 2

or

... (vi)

The rate constant k : 3 = k~Ij' The values of pK.of phthalic and protonated benzoic acids are 2.8and - 7, respectively". Thu s, by assuming 0-

COOH and o-COOHt as acid-strengtheninggroups, the values of K, and K2 may be consid-ered to he > 107 M and> 10-28 M, respectively.The thermodynamically favourable internal pro-ton transfer in k} step is assumed to be mediatedby water molecule and hence the magnitude of k}is it: . .'i.5xlO11 S-I. Thus, the value of k:3(= k~6) turned out to be 55 S-I.

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