Agrawal, M. C.

Rawat, B. S.

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ladlBll Journal of ChemistryVol. 14A, July 1976, pp. 481-484Mechanism of Oxidation of Ascorbic Acid by N-ChlorobenzamideB. S. RAWAT & M. C. AGRAWALDepartment of Chemistry, Harcourt Butler Technological Institute, Kanpur 208002Received 19 April 1975; accepted 21 August 1975The oxidation of ascorbic acid by N-chlorobenzamlde (NCB) In 30% methanol follows firstand zero order dependences in NCB and ascorbic acid respectively. The order In hydrochloricacid Is ,...,2being unity in both H+ and CI- ion concentrations. Ionic strength and solvent effectsare ne~li~ible. A mechanism Involving the formation of free chlorine from the interaction ofNCB and hydrochloric acid is proposed in the rate determtntng step. The activation eneray ofthe oxidation process is 18·4±O·2 kcal mole=,THE use of N-chlorobenzamide (NCB) as avolumetric reagent for the oxidimetric deter-mination of several inorganic and organicsubstances, in acidic medium is described by Singhet al.), No work has been reported so far on themechanism of NCB oxidative action on organicsubstrates. Possibility of oxidation of ascorbic acid(AH2) by NCB is mentioned in literature but nodetails are given. In the present paper the kineticsof oxidation of ascorbic acid by NCB in acid mediumare reported and a suitable mechanism has beenproposed.Materials and MethodsStandard solutions of NCB (m.p. 116°)2,3 werealways prepared fresh in methanol (AR, BDH) andstored in dark bottles. Aq. solution of ascorbic acid(GR. S. Merck) was also prepared fresh. Conducti-vity water was used throughout the course of in-vestigation. All other reagents used were of ARgrade. Reactions were carried out in 150 ml genaglass flasks coated black from outside to avoid anyphotochemical decomposition.Kinetic measurements - The progress of reactionwas followed by estimating ascorbic acid at definitetime intervals against the standardized eerie sulphateusing variamine blue as indicator'. It was ascert-tained that no reaction took place between Ce(III)and NCB in such media.For determining the stoichiometry, the reactionmixtures containing known excess of ascorbic acidover NCB were kept at 40° for 4 hr in the presence of0'2M HCI. The amount of ascorbic acid remainingunreacted revealed that equivalent amounts ofreagents were consumed giving dehydroascorbic acidand benzamido as the products (Eq. 1).CeHs06+C6H6CONHCl=C6H60e+CeHsCONH2+HCI... (1)ResultsThe kinetics of oxidation of ascorbic acid by excessNCB was studied at several initial concentrations ofthe reactants (Table 1). The plots of [ascorbic acid]versus time were always linear (Fig. 1) thereby in-dicating the pseudo zero order rate constants (ko)482TABLE 1 - EFFECT OF VARYING [REACTANTS]AT300 IN 30% METHANOLExp. [AHJ [NCB) [HCI) kex 108No. Mxl0' Mx10' Mxl0' moleslitre-1see'?1 6 10 10 11·22 8 10 10 11·23 10 10 10 10·84 14 10 10 11·55 20 10 10 11·66 10 6 10 6·567 10 8 10 8'898 10 14 10 16·09 10 20 10 23·010· 10 10 6 6·4011· 10 10 8 9·7912- 10 10 10 14-113- 10 10 14 24'014- 10 10 20 47·0-In the presence of 1·2M NaCIO,.5.01'------------_-,-g 4.0sE 1.020 40Time in rmnutesFig. 1 - Zero order rate plots in ascorbic acid at 30°([AH.] = 10-aM, [HCI] = 0·1M, [MeOH) = 30%. and[NCB] = 6·0, 8·0. 10. 14 and 20 x 10-1M for A, B, C. Dand E plots respectively]RAW AT & AGRAWAL: N-CHLOROBENZAMIDE OXIDATION OF VITAMINCTABLE 2 - EFFECT OF ADDED NaCIO. AND NaCI AT 30°{[NCB] = 0·01M. [AHa] =lO-sM. [HCI] = O·lM. [MeOH]= 30%}[NaClO.] ko X108 [NaCl] ko X108 [Cl-]* », X 108M moles M moles M moleslitre'? Iitre'? litre"!50<:-1 50<:-1 50<:-1None 10'8 None 10·8 0·1 14·1added added0·2 10·9 0·1 18·0 0·2 24-80·4 11·3 0·2 24-6 0·3 33·90·6 11·7 0·3 31'3 0·4 47·10·8 13·3 0·4 40·6 0·5 56·51'2 14·1 0·6 55·6*In the pre3ence of 1·2M NaCIO,.in ascorbic acid (Table 1). An increase in [NCBJ in-creased the rate (ko) proportionately and the averagevalue of ko[NCBJ was found to be 11·1 ± 0·4 X 10-6seel at 30° in O'IM HCI. The order in hydrochloricacid was calculated from the slope of the linearplot of log ko versus log [HCIJ and it was found tobe 1·75.Eifecl of added salts - The effect of adding neutralsalts, viz. NaCIO, and NaCl, was investigated. Theresults show that the oxidation of ascorbic acid byNCB has specific chloride ion effect. A large varia-tion in [NaCIO.l (1-'-=0'1 to 1·3) increased the rateonly slightly while addition of NaCl strongly catalysedthe reaction (Table 2). Further, the effect of Cl- atconstant ionic strength (in the presence of 1·2MNaCIO,) was also investigated and the order in [Cl-Jwas calculated to be 0·8 from the slope of the linearplots of log ko versus log [Cl-J. As the reactions wereinvestigated in methanolic solutions, the effect ofmethanol on the rate of oxidation was also studied.A variation in [methanolJ from 20 to 50% changedthe rate constant from 10·2 to 11·3.Eifect of temperature - The effect of temperaturewas quite marked. The values of rate constants (ko)were obtained as 7'50, 10'8, 19'4, 32·4 and 51·7 X 10-8moles Iitre'< sec-1 at 25°, 30°, 35°, 40° and 45° res-pectively. The activation energy (L\EtJ of theoxidation of ascorbic acid by NCB was found to be18·4 ± 0·2 kcal mole'",DiscussionKinetics of the oxidation of ascorbic acid byhydrogen peroxides, Os(VIII)6, hexacyanofer-ratefl H)", hexachloroplatinate'', peroxydisulphate"and Ag(I)1° have been reported. Some metal ionand metal chelate catalysed oxidations-! of ascorbicacid have also been examined. In general, theseoxidations show a negative influence of hydrogenions. However, the oxidation of ascorbic acid byNCB is strongly catalysed by HCl and the order isfound to be --2 (1·75). H" and Cl- ions indepen-dently catalyst the rate of oxidation and the order in[Cl-J calculated separately is nearly unity (0,8).The order in hydrogen ions is, therefore, unity bydifference. Apart from the above, the oxidation ofascorbic acid by NCB follows zero and first orderdependences in ascorbic acid and NCB respectively.This clearly reveals that ascorbic acid is involved inthe fast step (ii) while the oxidant and acid constitutethe slow reaction [step (i)J.H h,C6H6CON( +HCl ~ C6H5CONH2+CI2 ... slow (i)Cl s.,CI2+AH2! A+2HCl .. .fast (ii)During the Orton rearrangement of N-haloacet-anilides, such specific halogen acid catalysis has beenreported by Hughes and Ingold12. The postula~edmechanism involves the production of free chlorineas suggested in step (i) above.Taking k2x [AH2J ~ ILl [benzamide] as a suitableapproximation and [H'Cl] ex: [H+J[CI-], the rate-lawEq. (2) is obtained.- ~ [AH2J =k[NCB] [H+J [CI-J ... (2)The rate-law Eq. (2) satisfies the experimental ob-servation that the oxidation of ascorbic acid involveszero and first order dependences in ascorbic acid andNCB respectively, and the order with respect toboth H+ and Cl- ions is unity or two in HCI. Underthe conditions it is not possible to distinguish kineti-cally between the participation of undissociated HClon one hand and of H+ and Cl ions on the otherhand. Since the acid is almost completely disso-ciated, the result is consistent with the assu.mptionthat undissociated Hf.l molecules are the effectivecatalysts for the oxidation of ascorbic acid by NCBrSimilar conclusions have been drawn to account fotthe halogen acid catalysis during the rea.rrangcmen-of Nvchloroacetani lidcsw. The catalysis ofhydrobromic acid in the decomposition of hydrogenperoxide has also been explained-t on the basis ofrate-determining reaction between H202 and HBr.Hughes and Ingold-s have suggested the po~sibilityof a rate-determining reaction between Cl-. H?~ an.dthe protonatcd N -haloamide. Such a possibilit.y ISless likely for the oxidation of ascorbic acid becau~ethis would involve negative ionic strength and di-electric constant effects. The observed kinetic datashow an insignificant effect of ionic strength ([N aCl04Jfrom 0 to 1·2M) and dielectric constant ([methanol]from 20 to 50%) and thus indicate the involvement ofa neutral molecule in the rate-determining step. Thehigh energy of activation (18·4 kcal mole-') also doesnot favour a reaction between the oppositely chargedions. .A slight decrease in experimental value of t~eorder in Cl: ion (0·8) can be explained on the ?~SlSof the fact that at high [Cl-J, thermal decompositionof NCB becomes significant thus resulting in adecrease in the rate of oxidation of ascorbic acid.AcknowledgementThe authors thank Dr S. D. Shukla, Director,HBTI, Kanpur, for his encouragement and facilities.References1. SINGH. B.• VERMA. B. S. & SAPRA. L. R.. Z. analyt.Chern .• 196 (1963). 323.2. DATTA. R. L. & GHOSH. J., J. Am. chem, Soc .• 35 (1913).1044.483INDIAN J. CHEM., VOL. l+A, JULY 19763. ELLIOT, G. R, J. chem, SOC., ill (1922), 203.4. ERDEY, L. & BODOR, E., Z. analyt, Chem., 137 (1953),410.5. GRINSTEAD, R R, J. Am. chem. soc., 82 (1960),3464.6. MEHROTRA, U. S. & MUSHI~AN, S. P., Can. J. Chem.,48 (1970), 1148.7. MEHROTRA, U. S., AGRAWAL, M. C. & MUSHRAN, S. P.,J. Phys. Chem., 73 (1969) 1996.8. MEHROTRA, U. S., AGRAWAL, M. C.·& MUSHRAN, S. P.,J. inorg . nucl, Chem., 32 (1970), 2325.4849. MEHROTRA, U. S. & MUSHRAN, S. P., J. Indian chem.Soc., 47 \1970), 41.10. MUSHRAN, S. P., AGRAWAL, M. C., MEHROTRA, R. M. &SENEHI, R., J. chem. Soc. Dalton Transactions, (1974),1460.11. KHAN, M. M. T. & MARTELL, A. E., J. Am. chem. Soc.,89 (1967), 4176.12. HUGHES. E. D. & INGOLD. C. K.. Q. Rev. (Lond.). 6(1952). 34.13. BELL. R. P .• Acid base catalysis. (Oxford). 1941. 49.14. BRAY, W. C., Chem. tu«, 10 (1932). 161.

Mechanism of Oxidation of Ascorbic Acid by N-Chlorobenzamide

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Mechanism of Oxidation of Ascorbic Acid by N-Chlorobenzamide

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