Indian Journal of ChemistryVol. 22A, July 1983, pp. 631-633

Kinetics & Mechanism of Chloramine- TOxidation of Arginine in

Perchloric & Sulphuric Acid Media

DS MAHADEVAPPA, KS RANGAPPA, N M MADE GOWDA& B THIMME GOWDA*t

Department of Chemistry, Manasagangotri. University of MysoreMysore 570006

Receiued Ts August 1982: revised 1 February 1983: accepted 10 March1983

Kinetics of oxidation of arginine by chloramine-T (CA T) has beenstudied in HCI04 and H2S04 media at 30 C. The reactions followidentical kinetics in both the acids: first order each in [CAT] and[Arginine] and inverse first order in [H +]'.Addition of the reactionproduct p-toluene-sulphonamide or CI04 and SO~ - and variationof ionic strength of the medium have no effect on the rate. Decreaseof dielectric constant of the medium decreases the rate. Activationparameters have been computed. The mechanism has been found tobe the same as that proposed for CA T oxidation of arginine in HCImedium [Indian J Chem, :!OA (1981) 263].

As a part of our mechanistic studies on haloarnino-metric reactions in general t 4 and amino acidreactions in particular3.4, we report herein the resultsof kinetics of oxidation of arginine by chloramine- T(CA T) in perchloric acid and sulphuric acid media at30cC. Earlier we have reported CAT oxidation ofarginine in hydrochloric acid medium".

Purification and standardisation of chlorarnine-T(E. Merck) and t.-Arginine (S.D.'s Lab Chern Industry)were carried out as described earlier". All otherreagents were of analar grade. Triply distilled waterwas used throughout this investigation and ionic

3 .strength of the system was kept at 1.0 mol dm - usingsodium perchlorate.

The reaction was initiated, in a glass stoppered pyrexboiling tube whose outer surface was coated black toeliminate photochemical effects, by mixing thermallyequilibrated solutions of amino acid, sodiumperchlorate and water (to keep the total volumeconstant for all runs) with a measured volume of CA T.The reaction was followed for two half-lives byiodometric estimation of unreacted CAT at regulartime intervals. The pseudo-first order rate constantswere reproducible within ± 3%.

Stoichiometric runs with varying ratios of CAT toarginine in the presence of 0.05-0.4 mol dm -3 HCl04

and H2S04 at 30°C revealed that 2 mol of CAT areconsumed per mol of arginine in accord with Eq. (1).

t Present address: Department of Chemistry, Mangala Gangotri,Mangalore University, Mangalore 574152

R'CH(NH2)COOH + 2RNCINa~

2RNH2 + R'CN +C02 +2Na + +2CI-

where R=p-CH3C6H4S02-=(H2N)C(NH)NH(CH2h

p-Toluenesulphonamide among the reactionproducts was detected by paper chromatography? andthe nitrile formed was identified by its colour reactionwith hydroxylamine and ferric chloride".

The results can be summarized as follows: (i) Fordifferent initial concentrations of the reagents 'inperchloric acid and sulphuric acid media, and atconstant acid concentration with excess [substrate],the plots oflog[CA T]o/[CA T] versus time and log kObs

versus logl Arginine], were linear indicating a firstorder dependence each in [CAT] and [substrate]respectively. (ii) Added CI04- and SO~ - had no effecton the rate in the [acid] range of 0.04 to 0.4 mol dm -3.

(iii) The rate decreased with increase in [H +] (Table I)and plots of log kobs versus 10g[H +] were linear in b~ththe acids, with slopes of -I. (IV) At constant [H J,addition of chloride ion increased the rate in both theacid media. Plots of log kobs versus log [Cl -] werelinear with slopes 0.75 and 0.31 in HCI04 and H2S04

respectively thus showing a fractional dependence ofrate on [CI-l (v) Addition of the reaction product,p-toluenesulphonamide and variation of ionic strengthof the medium had no significant effect on the rate ofreaction in both the acids. (vi) The reaction was alsostudied in aqueous methanol solutions of varyingdielectric constants (D), in both HCI04 and H2S04

media. It was observed that a decrease in dielectricconstant of the medium retarded the rate of reaction.The plots of log kObS versus I/O were linear withnegative slopes.

The rates of reaction were studied at differenttemperatures and the activation parameters computedare given in Table 2.

In strongly acidic or alkaline media amino acidsexist in the following equilibria 7:

and

(1)

R'

H+R'CH(NH2)COO - ¢

OH-S-

Anion

R'CH(NHt)COO -

SODipolar zwitterion

H+¢ R'CH(NHt)COOHOR

SH+Cation '" (2)

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INDIAN 1. CHEM., VOL. 22A, JULY 1983

Table I-Effect of Varying [Reactants] and pH in Perchloric and Sulphuric Acid Media at 30°ca

103[CAT]o 102[Arg]o IO-LH+] HClO4 H2SO4

(moldm -3) (moldm -3) (moldm -3)104kobS 104k'b 104kobs 104k"b(s -I) (s -I)

1.0 2.0 5.0 2.41 15.022.0 2.0 5.0 2.44 15.102.5 2.0 5.0 2.45 15.123.0 2.0 5.0 2.41 15.083.5 2".0 5.0 2.39 15.214.0 2.0 5.0 2.48 15.092.0 0.5 5.0 3.51 702.02.0 0.8 5.0 5.60 700.02.0 1.0 5.0 1.42 142.0 6.80 680.02.0 1.5 5.0 1.91 127.3 10.50 700.02.0 2.0 5.0 2.44 122.0 15.10 755.02.0 2.5 5.0 3.20 128.0 17.80 712.02.0 3.0 5.0 3.82 127.32.0 4.0 5.0 5.10 127.52.0 5.0 5.0 6.40 128.02.0 2.0 4.0 2.82 0.11 18.70 0.742.0 2.0 5.0 2.44 0.12 15.10 0.762.0 2.0 6.0 1.91 0.11 12.60 0.762.0 2.0 8.0 1.42 0.11 9.10 0.732.0 2.0 10.0 1.11 0.11 7.95 0.802.0 2.0 20.0 4.40 0.882.0 2.0 30 2.70 0.81

a/l=0.5 moldm -3 with HCI04 and 1.0 mol dm -) with H2SO4

"k' =ko.J[Arg] or k" = kobJ[H +]; k' and k" have units of drn ' mol-I s -I.

Chloramine-T (RNCINa, where R p-CH3C6H4SO:J behaves like a strong electrolyte" inaqueous solutions and dissociates as,

RNCINa ¢(RNCi) - + Na + ... (3)

Depending on the pH ofthe medium, CAT furnishesdifferent types of reactive species in solution such asmonochlorarnine- T (RNHCl), dichloramine- T(RNCl2), HOCI and possibly H20CI + in acidsolutions" -10.

Under the present experimental conditions, RNCI2and HOCI cannot be considered as the reactive speciesas their concentrations are very low. It is probable thatRNHCI is the oxidising species which reacts with thesubstrate to form a reaction intermediate which in turnundergoes further reactions to give the products+" asgiven in Scheme I.

Table 2-Activation Parameters of Chloramine- TOxidation of Arginine

E. sn; tJ.Gt tJ.stkJ mol :" log A kJ mol -I kJ mol -I JK -I mol -I

47.342.7

6.26.2

44.840.2

99.792.0

-166.5-167.4

In Scheme 1, S' is the N-chloro derivative of thesubstrate. Many investigators have observed orproposed the formation of N-chloro derivatives asintermediates11 -13. A detailed mode of oxidation ofarginine by CAT was reported earlier in HCI mediumsand the mechanism in the present investigation is verymuch similar. The rate law is given by Eq. (7).

d[CAT]dt

klk2 [CAT] [SH+]k-l [H+] (7)

SH+ ~ S+H+k _I

RNHCl+S~12 S'+RNH2Sow

The catalytic effect of Cl - ion can be rationalised... (4) in terms of the mechanism outlined in Scheme 2.

klSH + ¢ S + H + ... (8)

(5) k -1

S' + RNHCl_Fk3 Products

ast ... (6)

Scheme I

632

RNHCI+Cl-/H

R-N"CI. .. Cl -

(X)

... (9)

X+S~I'S S'+RNH2+Cl-Sow

S' X k6 d+ ~F pro uctsast

Scheme 2The following rate expression may be derived from

Scheme 2:

d[CAT]dt

k1k4ks[CATh[SH+] [Cl-]k -I {k -4 + k4[CI ]}[H +]

... (I 2)

For the limiting case of zero angle of approachbetween two dipoles or an ion-dipole system, Arnis!"has shown that a plot of log kobS versus liD is linearwith a negative slope for a reaction between a negativeion and a dipole or between two dipoles, while apositive slope results for a positive ion-dipoleinteraction. The negative dielectric effect observed inthe present studies clearly supports the dipole-dipoleinteraction reported.

ReferencesI (a) Mahadevappa D S & Gowda B T, Indian J Chern, 17A (1979)

484.

NOTES

(10) (b) Mahadevappa D S, Gowda B T & Gowda N M M, ZNaturforsch, 348 (1979) 52.

(c) Ahmed M S, Gowda B T & Mahadevappa D S, Indian JChern, 19A (1980) 650.

2 (a) Mahadevappa D S, Jadhav M B & Naidu H M K, lnt J chernKinet, 11 (1979) 261.

(b) Katgeri S N, Naidu H M K & Mahadevappa D S, Indian JChern, 19A (1980) 876.

3 Mahadevappa D S, Rangappa K S, Gowda N M M & Gowda BT, J phys Chern, 85 (198\) 3651 and references therein.

4 Gowda B T & Mahadevappa D S, J chem Sac, Perkin-Il, in press.5 Mahadevappa D S, Rangappa K S & Gowda N M M, Indian J

Chern,20A (1981) 263.6 Soloway S & Lipschitz A, Anal Chern, 24 (1952) 898.7 Jakubke H D & Jeschkeit H, Amino acids, peptides and proteins

(John Wiley, New York) 1977.8 Bishop E & Jennings V J, Talanta, 1 (1958) 197 and references

therein.9 Higuchi T, Ikeda K & Hussain A, J chern Sac (B), (1967) 546;

(1968) 1031 and references therein.10 Campbell M M & Johnson G, Chern Rell, 78 (1978) 65.11 Kantouch A & Abdel-Fattah S H, Chern Zvesti, 25 (1971) 222.12 Haberfield P & Paul D, J Am chem s«, 87 (1965) 5502.13 Gassman P G & Campbell G A, J Am chem Sac, 93 (I 971) 2567.14 Amis E S, Solvent effects on reaction rates and mechanisms

(Academic Press, New York), 1966.

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