Indian Journal of Chemistry Vol. 38A, Apri l 1999, pp. 361 - 365

Kinetics and mechanism of oxidative degradation of DL-serine by chromium(VI)

Zaheer Khan *

Chemistry Department, lamia Millia Islamia, l amia Nagar, New Delh i I IO 025, India

and

Kabir-ud-Din Chemistry Department, Aligarh Muslim Univers ity,

Aligarh 202 002, India

Received 10 Febroiary 1998, revised 30 October 1998

The oxidation kinetics of DL-serine by Cr(V I) has been studied spectrophotometrically in aqueous sulphuric acid. The reaction occurs through the path :

slow HO- CH2-CH(NH)-COOH --~) OHC- CH(NH)-COOH

+ +

An intermediate, 2-amino-3-oxo propanoic acid, has been identified as its hydrazone derivative. The net rate law, as measured by the formation of Cr(IlI), is given by,

kl K [H2S0 4]2[DL-serinel k = es T

obs I +K [DL-serine]T es

The values of kl and Kes have been evaluated at different ac idities. Mechanism with the associated reaction kinetics is ass igned and discussed.

The kinetics and mechanism of electron transfer reactions (chromic acid oxidations) of sulphur and oxygen containing orlf.anic substrates have been investigated extensively -5. The phenomenon of ligand replacement as a prerequisite to redox processes has been discussed6 as has the correlation between the rates of oxidations by oxoanions and those of their ligand-exchange reactions. Coordination of the reductant to the metal centre is a step preced ing oxidat ion has been observed in several instances7

.

Most commonly, mechanistic deductions have been drawn from the observations of changes in kinetic data that occurred when the structure of the organic substrate was altered in somewal,9.

It is thought that breakdown of biologically important amino acids w~ing Cr(VI) as oxidant would

be worth studying. This note dea ls with the kinetics of

oxidation of DL-serine by Cr(VI) in ac id medium. Serine is a special amino acid which has primary -OH group in' addition to the ammonium group on the carbon chain. In effect, carbon skeleton of serine is more oxidized than that of other amino acids (except sulphur containing). The main objective of the present investigations is to e lucidate suitable mechan ism and to put forward a rate law consistent with experimental data.

Experimental DL-Serine (chromatograph ically homogeneous,

BDH), potassium dichromate (S igma), sulphuric acid (~98% Merck), potassium su lphate (Fluka), manganese sulphate (Fluka) and acrylonitrile (Sigma) were used without further purifi cation . Solutions of all the reagents were prepared in doubly distilled water.

The method has been detailed elsewhere le. The

progress of the reaction was fo llowed by monitoring formation of one of the products, Cr(IIl ), at 575 nm . Pseudo-first order conditions were maintained in all runs with excess DL-serine (~I OX). The pseudo-first order rate constants (kobs) were determined from the linear part of plots of log (A a-Ao)/(Aa- At) versus time where Ao=absorbance at zero time, Aa=absorbance at infinite time and At=absorbance at time, t. Such plots were linear fo r at least 80% completion of the reaction and duplicate measurements agreed to ±2%.

A solut ion of potassium dichromate (0.01 mol dm-3

, 2 cm3) in 0.5 mol dm-3 H2S04 was added to a

mixture of the DL-serine (0 .1 mol dm-3, 4 cm' ) and

23% (v/v) acrylonitrile (1.0 cm3) at 50°C. After 40

min a white precipitate of polymer appeared slowly. Experiments conducted at high [H2S04] (~6.0 mol dm-' ) as well as blank experiments (without added DL-serine) did not show formation of such a white precipitate. The inability to detect free radicals in high ac idic solutions is in agreement with previollsly reported results 'o.

The main product, glycine, was characterized as

fo llows: DL-serine (0.01 mol dm-3, 5 cm3

) in 0.5 mol dm-3 H2S04 was added to a solution of potassium

d ichromate (0.1 mol dm-3, 5 cm3

) at 30°C. The reaction mixture was sti rred for 12 h. Sodium

362 INDIAN J CHEM. SEC A. APRIL 1999

1.()

~_-:-L~ __ ..1I __ ...J! __ --'-- I

0-04 ().oe ()'1'Z 0·16 ().20

[DL-SCrlnc)T(mo! c:lrfil)

Fig. 1-: Ett:ect of [oL-se~ine] on the observed rate constant (koo.) of the oXidatIOn ofoL-senne with Cr(VI). [(H2S04)=1 , 7.0; 2, 6.0;

3, 5.0 mol dm')]

hydroxide was added until the pH of reaction mixture became 5.0, and glycine was then estimated with standard ninhyhdrilll method. Formationof glycine was also confirmed by the chromotropic acid. Reduction of chromium(VI) was not observed after prolonged incubation in the presence of glycine and H2S04

which indicated that oxidation of glycine by chromium(VI) is not possible under our experimental conditions. This is an additional support for the oxidation product of DL-serine.

The intermediate, 2-amino-3-oxo propanoic acid was detected by the following method: An ice cold solution ofDL-serine (0.5 gm) in H2S04 (I 8 mol dm-3 )) ,

6 cm) and 5 cm saturated aqueous potassium dichromate were added to a 2,4-dinitrophenyl hydrazine acid solution (0.3%, 25 cm\ After I h, the reaction mixture was filtered out and the yellow residue was washed with ethanol and recrystallized by acetic acid. The compound was analysed by IR spectra and was identified as the 2 4-dinitrophenylhydrazone of 2-amino-3 -oxo propan~ic acid . (vNH2=3 200-2800, vCOO (as)=1600 and vC=N=1650 cm- I

). The melting point of the hydrazone was fou Jl1d to be 170°C.

The formation of CO2 was estimated qualitatively as described elsewhere}}.

Results and discussion The stoichiometry of the reaction between K2Cr20 7

and oL-serine in the presence of H2S04 was determined using spectrophotometric titrations. The

'·0

0"

0 ·2

W!S~--:-'::----:L---_J to 4.0 600 8·0

[~SO,]moI drft3

Fig. 2 - Effect of [H2S04] on the koo• at different [oL-serine] [(serine}rl , 0.18; 2, 0.10; 3, 0.06 mol dm')] .

spectra of reaction mixture containing various concentrations of chromium(VI) and fixed COL-serine] were measured after the completion of the reaction. The absorbance of remaining chromium(VI) was monit~red at 475 nm (at this wavelength, chromlUm(III) has negligible absorbance). The stoichiometry was found to be 1:4 i.e , one mole of oxidant was copsumed by 4 mol of reductant.

At different [Cr(VI)h i.e., 1.0, 1.5 and 2.0x I 0-) mol dm-) but fixed [oL-serineh (0.1 mol dm-3) the values of pseudo-first order rate constants were found ~o ?e ~onstant with increase in [Cr(VI)h (Table 1) mdlcatmg that the total [Cr(VI)] must be used in the rate law with first order dependence on [Cr(VI)h. The rate law is, therefore, as in Eq. (I):

d [Cr(lll)J/dt=-d [Cr(V l)]/dt=kob,[Cr(VI)h ... (I)

The effect of varying [oL-serine] (0.02-0.20 mol dm-)) on the reaction rate was studied at a fixed [Cr(VI)h(=1.0x I0-:) mol dm-)) as a function of [H2S04], The results are presented in Table I. It is observed that as [oL-serineh increases, k obs increases but non-linearly (Fig. 1). A marked curvature is observed in all the cases indicating outer sphere complexation between the reactive species . oL-serine participates in acid-base equilibria

K.I

HO-CH2-CH(NI-tJ)-COOH""'HO-CH2-CH(NH )-COO-+H+ + + f. J

K .2

HO-CHz-CH~H3)-COOH""'~O-CH2-CH(NH3)_COO-+H +

NOTES 363

' ·0

0-8

315 425 475 Wove'~th(nm)

Fig. 3 - Absorption spectra of [Cr(VI)] in different [H2SOjl]; [Cr(VJ»)=O.OO28 mol dm-l

; [(H2S04)=1, 0.035; 2, 2.0; 3,4.0; 4, 6.04 and 5, 8.0 5 mol din- l

]

From the pKa values (2.2 and 9.4) one can ascertain that, under the present experimental conditions of acidity, [H2S04]=2.0 to 8.0 mol dm-3

, the mono­positive species exists in significant concentration.

The reaction was studied as a function of [H2S04] between 1.0x 10-6 and 8.0 mol dm -3 at various fixed [DL-serineh and constant [Cr(VI)h (1.0x 10-3 mol dm -3) at 30°C. No oxidation of DL-serine was observable at lower acidity (1.0x I.0-6-1.0x l0-2 mol dm-3

), even after prolonged incubation at 30°C. Since we found no significant reaction at short reaction times at [H2S04]=1.3xI0-2 mol dm-3

, the reaction was carried out III highly acidic medium (2.0~[H2S04]~8 .0) . These results confirm that the reductant (DL-serine) reacts with Cr(VI) at a significant rate only when it is protonated or must supply Cr(VI) with a proton". These results are in close afeement with the observations on oxidation of lactate' , oxalate and formate 13 by the same oxidant at low pH which also failed to reduce Cr(VI) at higher pH.

The plot of rate constant versus [H2S04] was a curve passing though the origin (Fig. 2). Further, a log-log plot was linear with slope 2.0 indicating the order in to [H2S04] to be two. The absorption spectra of mixtures containing the same Cr(VI) and H2S04 in different molar ratios exhibited different absorptions

Table 1 - Effect of [Cr(VI)), [DL-serine), [Mn(U)] and [H2S04] on the observed rate constants (kot..) for the oxidation ofDL-serine with Cr(VI), at temp=30°C. Values of k.:.. are given in the paren-

thesis

101[Cr(VI»)y [DL-serine] 101 [Mn(II)] [H2S04] mol dm-l mol dm-l mol dm-l mol dm-l

. 1.0

1.5 2jl 1.0

1.0

1.0

0.01

0.01

0 .02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.10

0.0

2.0 4.0 8.0

10.0 50.0

0.0

0.0

6.0

6.0

6.0

1.0 2.0 3.0 4.0 5.0 6.0 7.0

31> 3.1 3.1 3.1 3.1 3.0 3.2 3.4 1.2(1.\) 2.2 (2.1) 3.1 (2.1) 4.0 (3 .8) 4.7(4.6) 5.4 (5.2) 5.8 (5 .8) 6.2 (6.3) 6.5 (6.9) 6.7 (7.4) 0.3 0.6 1.5 2.3 3.7 4.6 5.8

at same Amax=450 nm (Fig. 3). The absorbance decreases with increase in [H2S04] indicating that major portion of the existing species, Cr207, is con­verted'4 into HOCr020S03H.

To find out the presence of Cr(IV) involved in the rate-limiting step, reaction rates were measured in the presence of varYing [Mn(II)] (Table 1). The rate constant remained unchanged. Mn(ll) had no effect on the reaction rate, this would suggest that the oxidant in this reaction is not involved after the rate­determining steps. Disproportionation of chromium(IV) is assumed to take place. A similar observation was made by Haight and co-workers '0. However, a different role of Mn(lJ) can be seen as in extremely high-acid solutions no inhibition by Mn(II) would be expected'5.

On the basis of the above results, a mechanism, shown in Scheme 1, has been proposed for the oxidation of DL-serine with Cr(VI).

As soon as one molecule of DL-serine enters into the inner coordination sphere and OS is formed, the coordination of second molecule of DL-serine would make the central chromium atom more positive and thereby increase the tendency of the chromium to

364 INDIAN' J CHEM, SEC A, APRlL 1999

K .. HOCr(0)20S0)H+HOCH2CH(NH)COOH ..

+

HSO)OCr(OHh(0)OCH2CH(NH)COOH (OS) +

k

:t1+ H2::~ne-'-; Cr(V) + HOCHCH~H)COOH

Cr( HCCH(NH)COOH 2,4-dinitrophenylhydrazine + (A) •

fM! 1 -co,

HOOC-CH2NH]

'\.. Hydrazone derivative

~Cr(VI)

HOOCCH(NH,)COOH

+

fast \ Cr(VI)

NH]CH2COOl-I +

fast

+

Cr(IV) + Serine ~ Cr(IJI) + HOCHCH(NHJ)COOH +

fast 2Cr(IV) ~ Cr(V) + Cr(IlI )

fast HO-CH2CH(NH})C00H + Cr(V) ~ Cr(lJI) + A ,

Scheme I

oxidize the coordinated DL-serine. This argument is very similar to the observations made by Natile e/

al.16 and confirms that the interaction of second DL­serine molecule with chromate-DL-serine ester (OS) is not a rate determining step.

The rate law corresponding to Scheme I can be expressed as Eq. (2)

Rate=d [Cr(III»)/dt=kl[OS][H2S04) .. . (2)

which on comparison with Eq. (I), gives Eq. (3).

k _ k,Ke.,[H2S04 j2[DL - serinejT obs - 1+ Kes[DL _ serineh ... (3)

Accordingly plots of kobs versus [H2SO,j]2 at constant [DL-serineJr should be linear. This has been found to be so. Equation (3) can be rewritten as (4)

... (4)

The values of kl and Kes have been calculated from the intercepts and slopes of the plots of II kobs versus I I[DL-serine Jr at constant [H2S04] and are given in Table 2. The rate constants have been calculated (kcal) in various kinetic runs by substituting the values of kl and Kes in Eq. (3) and compared with the kobs values (Table I). The close agreement between the kob~ and kcal provides the supporting evidence for the proposed mechanism. Oxidation of DL-serine by Cr(VI) occurs in two kinetically distinguishable steps. The first is a rapid ester formation between DL-serine and Cr(VI) and the second, a slower electron transfer step. The Burk Lineweaver-type double reciprocal plots (i.e. I1kobs versus I1[DL-serineJr), indicate association of

the oxidant and substrate in some pre-equilibrium steps before the electron-transfer step and also satisfy the Michaelis-Menton 17 reciprocal relationsh ip (kinetic proof for complex format ion). Hence a complex formation between DL-serine and reactive species of Cr(VI) occurs a priori.

The reaction was studied as a function of temperature between 30 to 45°C to evaluate the activation parameters (Table 2). Large negative val ues of t:.St indicate existence of compact activated state stabi lized by strong hydrogen bonding and large solvation in the electron transfer step. Although

Table 2 - Values of rate constant (k l ) ester formation constant (Kes), acti vation and thermodynamic parameters for the ox idation of DL-serine with Cr(VI)

[H2SO4] 105 kl Kes mol dm -} mol- I dmJ S- I mol- I dm}

5.0 4.4 4.5 6.0 5.5 2.9 7.0 6.8 2.3

[H2SO4] I1H t 115' I1C t M-{o 115 · I1Co mol dm-J kJ mor l JK-I mor l kJ mol- I kJ mor l JK- : mor l kJ mo r l

5.0 19.1 -322.3 11 4.3 -23 .9 -0.07 - 3.8 6.0 20.0 - 320.2 11 4.5 -27.7 -0.08 -2.7 7.0 21.0 -318.3 11 4.9 -25.8 -0.08 -2.1

NOTES 365

negative entropy of activation is a characteristic of rate .limiting formation of an intermediate complex (i.e. ester), decomposition of the chromic ester (in the case of mandelic acid) was still postulated as the rate I· .. 1819 I h h I f Imlttng , . n t e present case, t e va ues 0 ester formation constant (Kes) and rate constant (k l ) indicate that the decomposition of the chromic ester of DL­serine is also a rate-limiting step due to the observed Michaelis-Menton kinetics.

These results suggest that the DL-serine can be converted into glycine after the oxidation of -OH group of DL-serine by chromium(VI) . G lycine does not react witn inorganic oxidants, Mn(H) and Cr(VI) (at least under the conditions of this ork).

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