Indian Journal of Chemistry Vol. 40A, July 2001, pp. 747-751

Oxidative cleavage of guanidine hydrochloride by alkaline Mn(VII) + S --~~~ Mn(V) + Products

C P Kathari, A L Harihar & S T Nandibewoor* Mn(VII) + Mn(V) --~~ 2 Mn(VI) P.G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India where S = substrate, k4 > > k3 E-mail: [email protected] Received 17 January 2001; revised 9 April 2001 Scheme 2

The oxidation of guanidine hydrochloride (GH) by alkaline Guanidine (GH) finds its applications in analytical permanganate has 1:2 stoichiometry and exhibits unit order in chemistry, pharmaceuticals and polymer industries. [Mn04-j, less than unit order in [GH] and zero order in [OH-j. Although a variety of inorganic substrates8 and amino The reaction proceeds via formation of an oxidant-substrate 9 acids are oxidised by permanganate in aqueous complex and its subsequent decomposition to yield a free radical of guanidine hydrochloride which is followed by further fast steps alkaline medium, no work has been reported on the to give the products. The reaction constants involved in the oxidation of guanidine hydrochloride. Hence the title mechanism and the activation parameters have heen computed. reaction is investigated in order to arrive a plausible There is a good agreement between the observed and calculated mechanism and to understand the redox chemistry of rate constants under different experimental conditions. permanganate in alkaline medium. Extensive studies has been made on permanganate Experimental oxidation of organic and inorganic substrates in both Stock solutions of guanidine hydrochloride acidic and alkaline medium I. Furthermore, in aqueous (s.d.fine-chem) and permanganate (BDH) alkaline medium, permanganate ion oxidises a were prepared by dissolving appropriate amounts of number of organic compounds which are not or only respective samples in doubly distilled water. The very slowly attacked in acidic or neutral medium. The solution of permanganate was standardised against process Mn(VII) to Mn(IV) can be divided into a oxalic acid. Potassium solutions were number of partial steps and examined separately. If 3 prepared as per literature method 10 and standardised [OH-] > 0.1 mol dm- , permanganate will be reduced spectrophotometrically employing 1. <:m quartz cell at only to manganate in the first step and owing to its 608 nm (£ = 1530 ± 20 dm3 morl cm-"t). All other much lower reactivity, further reaction to manganate 4 reagents were of analytical grade. NaOH and NaCI04 will be much slower2- . The Mn02 appears only after a were used to maintain required alkalinity and ionic long time i.e. after the complete consumption of strength respectively. Mn04- from the system. In strongly alkaline medium s 6 the stable reduction product . of permanganate ion is Kinetic studies manganate, MnO/-. Mechanistic information to All kinetic measurements were carried out under distinguish between a direct one-electron reduction to pseudo-first order conditions where [guanidine Mn(VI) (Scheme 1) and a reaction in which hydrochloride] was at least in ten fold excess over hypomanganate is formed in a two electron step [permanganate] at a constant ionic strength of 2.0 mol 7 3 followed by a rapid oxidation of hypomanganate ion dm- . The reaction was initiated by mixing thermally (Scheme 2) is not available. equilibrated solutions of Mn04- and guanidine kJ hydrochloride which also contained required Mn(VII) + S ---l~. Mn(VI) + S· quantities of NaOH and NaCI04 to maintain the required alkalinity and ionic strength respectively. Mn(VII) + S· ----t~~ Mn(VI) + Products The temperature was uniformly maintained at 26 ± 0.1°C. The course of reaction was followed by where S substrate, k] » kJ = monitoring the decrease in the absorbance of Mn04- Scheme 1 in 1 cm quartz cell of Hitachi 150-20 748 INDIAN J CHEM, SEC A, JULY 200 I

spectrophotometer at its absorption maximum 526 nm 0 .600 r------as a function of time. Preliminarily it was verified that there was negligible interference from other reagents at this wavelength. The application of Beer's law for permanganate at 526 nm had earlier been verified 3 I giving E = 2083 ± 50 dm mol-I cm- (literature value 2200). The first order rate constant, kobs was evaluated from plots of log [AI-A»] versus time, where AI refers oJ to absorbance at any time t and A X) at infinite time. ...c: The first order plots in almost all the cases were linear .8 ..~ kobs A up to 80% completion of the reaction and were <{ reproducible within ± 5%. During the course of measurements, the solution changed from violet to blue and then to green. The spectrum of the green solution was identical to that of MnO/-. It is probable that the blue colour originated from the violet of permanganate and the green from the manganate, excluding the accumulation of hypomanganate. The formation of Mn(VI) was also 0 ·000 1--_'----'_--I-_-L.._...l 450 500 550 600 650700 evidenced by Fig. 1 where the absorbance of Mn(VII) A,nm decreased at 526 nm and that of Mn(VI) increased at 608 nm during the course of the reaction. However, Fig. I-UV- vis. spectral changes during the oxidation of guanidine hydrochloride by alkaline permanganate at [Mn04-] = on long standing Mn(VI) slowly reduced to Mn(IV) 3 3 2.3xlO-4, [GH]2.0xlO- , [OW] = 1.0, 1=2.0/ mol dm- , scanning under our experimental conditions. time interval 2.0 min, at 26°C. The effect of dissolved oxygen on the rate of reaction was checked by preparing the reaction mixture and following the reaction in an atmosphere that of an authentic sample. Carbon dioxide was of nitrogen. No significant difference was observed qualitatively detected by bubbling N2 gas through the between the results obtained under the nitrogen and in acidified reaction mixture and passing the liberated presence of air. Added carbonate had no effect on the gas through a tube containing lime water. reaction rate. Fresh solutions were used while The reaction orders were determined from the conducting the experiments. slopes of log kobs versus log concentration plots by varying the concentration of reductant and alkali in Results and discussion turn while keeping concentrations of others constant. The oxidant, [] was varied in The reaction mixture contammg excess 5 3 permanganate over guanidine hydrochloride was the range of 3.0xl0- to 3.0x1O-4 mol dm- and the mixed in the presence of 1.0 mol dm-3 NaOH adjusted linearity of plots of log[Mn04 -] versus time indicates to constant ionic strength of 2.0 mol dm-3. After the the order in [Mn04 -] as unity. This was also elapse of reaction time, solid KI was added following confirmed by varying [Mn04-] which did not show acidification by H2S04 (10%), then remaining any change in pseudo-first order constants, kobs (Table permanganate was titrated against standard sodium 1). The substrate, guanidine hydrochloride was varied 3 thiosulphate. The results indicated that two moles of in the concentration range 5.0x1O-4 to 5.0x1O- mol 3 Mn04 - consumed one mole of guanidine dm- at 26°C keeping all other concentrations hydrochloride as given by Eq. (1) constant. The order in [GH] was found to be less than unity (Table 1). H ~ N-C(NH)-NH2+2Mn04 -+20H-+H20~ The effect of alkali on the reaction was studied in 3NH3+2Mn04 2-+C032- ... (1) the concentration range 0.2 to 2.0 mol dm-3 at constant [GH] and [Mn04 -] keeping constant ionic The main reaction products were identified as strength of 2.0 mol dm-3 at 26°C. The rate constant ammonia by Nessler's reagent and manganate was found to be independent of [OH-] indicating the spectrophotometrically comparing its spectrum with order with respect to [OH-] as zero (Table 1). r NOTES

Table I-Effect of [Mn04-]. [GH] and [OW] on oxidation of guanidine hydrochloride by perrnanganate in aqueous alkaline ,medium at 26°C and 1= 2.0 mol dm -3. 4 3 3 [Mn04-] x 10 ' [GH] X 10 [OW] kQQS X 10 S-I mol dm-3 mol dm-3 mol dm-3 Expt. Calc. * 0.3 2.0 1.0 1.75 1.73 .:l.7 0.9 2.0 1.0 1.77 1.73 1.5 2.0 1.0 1.76 1.73 j 2.3 2.0 1.0 1.73 1.73 .J .:l.B 3.0 2.0 1.0 1.77 1.73 2.3 0.5 1.0 0.60 0.59 2.3 1.0 1.0 1.l0 1.05 .:l.1I 2.3 2.0 1.0 1.73 1.73 2.3 3.0 1.0 2.31 2.22 2.3 5.0 1.0 3.12 3.85 0.0134 0.0133 0.0132 0.0131 0.013 0.0121 0.0128 2.3 2.0 0.2 1.57 lID 2.3 2.0 0.6 1.53 2.3 2.0 1.0 1.73 Fig. 2-Plot of logkobs versus 1'12 and plot of logkobs versus lID 2.3 2.0 1.4 1.83 2.3 2.0 2.0 1.97 used to calculate the activation parameters. The *Experimental and calculated. 3 3 values of k (S-I) were 5.0 ± 0.2xlO- , 7.1 ± 0.3xlO- , 3 3 The effect of ionic strength was studied by varying 9.1 ± 0.4xl0- and 12.5 ± 0.6xlO- at 26, 31, 36 and [sodium perchlorate] in the reaction mixture. The 41°C respectively. The activation parameters corresponding to these constants were evaluated to be ionic strength of the reaction medium was varied from l 3 Ea = 48±2 k J mor , 10gA = 17.2±O.6, Mt = 43±2 k J 1.0 to 4.0 mol dm- at constant [permanganate], [GH] I and [alkali]. It was found that the rate constant mol-I, flIt = -32.3±1.5 J K- mol-I and fld = 34.1±1.6 k J mol-I. enhanced with increase in [NaCI04] and the plots of log kobs versus [112 was linear with positive slope The permanganate ion, Mn04-, is a powerful (Fig. 2). oxidant in aqueous alkaline medium. As it exhibits The relative permittivity effect was studied by multitude oxidation states, the stoichiometric results varying the t-butanol content keeping the others and pH of the reaction media play a significant role. conditions fixed. Attempts to measure the relative The Diode Array Rapid scan spectrophotometric permittivity were not successful. However, they were (DARSS) studies have shown that at pH > 12 the computed from the values of pure liquids as in an product of the reaction of Mn(VII) is Mn(VI) and no earlier workll. There was no reaction of the solvent further reduction was observed as reported by earlier 8 with oxidant. The rate constants, kobs, increase with workers • decrease in dielectric constant of the medium. The The reaction between guanidine and permanganate plots of log kobs versus liD was linear (Fig. 2). in alkaline medium has a stoichiometry of 1:2 with a The initially added products such as manganate and unit order in [permanganate] and less than unit order ammonia did not show any significant effect on the in [GH] and zero order in [alkali]. No product effect rate of the reaction. was observed. The results suggest the formation of an The reaction mixture was kept for an hour in the oxidant-substrate complex and its subsequent acrylonitrile scavenger in an inert atmosphere. decomposition to yield a free radical of guanidine in a Diluting the reaction mixture with methanol, gave slow step. This radical reacts with another mole of precipitate indicating the presence of free radical in Mn04- in a fast step to give manganate and urea. The the reaction. urea formed undergoes alkali hydrolysis to give NH3 The rate of reaction was measured at different and CO/- in another fast step (Scheme 3). The temperatures by varying [guanidine hydrochloride]. evidence for complex formation was obtained from The rate constant, k of the slow step was obtained the UV-visible spectra of both Mn04- and Mn04-­ from the intercept of lIkobs versus lI[GH] and k was guanidine hydrochloride mixtures, in which a 750 INDIAN J CHEM, SEC A, JULY 200 I

bathochromic shift of Mn04- from 270 to 275 nm and 1 1 --= +- ... (4) hyperchromicity at 212 nm was observed. This was kobs k K [GH] k also evidenced from the Michaelis Menton plot, and such complex formation between substrate and l2 According to Eq. (4), the plot of llkobs versus lI[GH] oxidant have also been observed in other studies . should be linear and is found to be so. The slope and K intercept of this plot lead to the values of k and K at 3 3 H2N-C(NH2)=NH+Mn04" .. Complex (C) 26°C as 5.0 ± 0.025xlO- S-I and 266 ± 14 dm mOrl k respectively. Using these values, the rate constants Complex (C) 1 ~ H N-C(NH )=N·+MnO/-+H+ sow 2 2 over different experimental conditions were calculated and compared with experimental data as given in Table 1. There is a good agreement between them. The effect of increase in ionic strength on the rate qualitatively explains the reaction between two negatively charged ions as seen in Scheme 3. However, increasing the content of t-butyl alcohol in Scheme 3 the reaction medium leads to increase in the rate of reaction, contrary to the expected slower reaction The structure of the complex (C) might be between like ions in the media of lower relative permIttIvIty. Perhaps this effect is countered NH2 0]- substantially by the formation of active reaction H N- t -O-Mn-O species to a greater extent in a low relative [ 2 I I NH 0 permittivity media leading to the net increase in reaction rate 14. The values of /)J-/' and !.lS' were both Since Scheme 3 is in accordance with the generally favourable for electron transfer processes. Negative well accepted principle of non-complimentary value of !.lS' within the range for radical reactions oxidations taking place in sequences of one electron have been ascribed 15 to the nature of electron pairing step, the reaction between the substrate and oxidant and electron unpairing processes and to the loss of would afford a radical intermediate. A free radical degree of freedom, formerly available to the reactions scavenging experiment revealed such a possibility. on the formation of rigid transition state. The This type of radical intermediate has also been observed modest enthalpy of activation and relatively observed in earlier work l3 low value of entropy of activation and higher rate Scheme 3 leads to rate law (2) constant indicate that the oxidation presumably occurs by an inner-sphere mechanism. This conclusion is l6 d[MnO~] k K [GH][MnO~] supported by earlier work . ----'- =------'--- ... (2) It is also interesting to note that the oxidant species d t (1 + K[GH])(l + K [MnO~]) [Mn04-] required a pH > 12 and below which system get disturbed and the reaction will proceed further to In view of low concentration of Mn04- used, the term give reduced product of oxidant as Mn(lV) which ( 1+ K [Mn04- ]) is approximated to unity. develops tobacco coloured turbidity slowly. Thus it Hence, becomes apparent that in carrying out this reaction the role of pH in the reaction medium is crucial. It is also Rate = _ d[MnO~] = k K [GHh[MnO~h noteworthy that under the conditions studied, the dt 1 + K[GH] reaction occurs in two successi ve one-electron reductions (Scheme 3) rather than two-electron reduction in a single step (Scheme 2). Rate _ k _ kK[GHh ... (3) [MnO~ h - obs - 1 + K[GH] References I Szammer J Jaky M & Germasimov 0 Y, Int J chern Kinet, 24 (1992) 145 and references therein. Equation (3) can be rearranged to Eq. (4) which is 2 Stewart R, Oxidation in organic chemistry Part A, edited by suitable for verification. K B Wiberg, (Academic Press, New York) (1965). NOTES 751

3 Panari R G, Harihar A L & Nandibewoor S T, J phys org II Hugar G H & Nandibewoor S T, Trans met Chem, 19 (1994) Chem 12 (1999) 340. 215; Radhakrishnamurthy P S & Pati S C, Indian J Chem, 7 4 Panari R G, Chou gale R B & Nandibewoor S T, J phys org (1969) 687. Chem II (1999) 448. 12 Hugar G H & Nandibewoor S T, Indian J Chem, 32A (1993) 5 Simandi L I, Jacky M & Schelly Z A, J Am chem Soc, 107 1056; Tuwar S M, Nandibewoor S T & Raju J R, J Indian (1985) 4220. Chem Soc, 69, (1992) 651; Tuwar S M, Nandibewoor S T & 6 Jacky M Szevereni Z & Simandi L I, Inorg chem Acta, 186 Raju J R, Trans met Chem, 16 (1991) 196; Devi J, Kothari S (1991) 33; Timmanagoudar P L, Hiremth G A & & Banerjee K K, Indian J Chem, 34A (1995) 116. Nandibewoor S T, Trans met Chem, 22 (1997) 193; 13 Harihar A L, Kembhavi M R & Nandibewoor S T, Indian J Timmanagoudar P L, Hiremth G A & Nandibewoor S T, Chem, 39A (2000) 769; Balreddy K, Sethuram B & Polish J Chem, 70 (1996) 1459; Nadimpally S, Rallabandi R Navaneeth Rao T, Indian J Chem, 20A (1981) 395; Hogle M & Dikshitulu L S A, Trans met Chem, 18 (1993) 510. P & Pawar P K, Acta Indica Chem, 12A (1986) 228; Laloo D 7 Panari R G, Chougale R B & Nandibewoor S T, Polish J & Mahanti M K, Polish J Chem, 60 (1986) 589. Chem 72 (1999) 448. 14 Nandibewoor S T & Morab V A, J chem Soc, Dalton Trans, 8 Nadimpally S, Rallabandi R & Dikshitulu L S A, Trans met (1995) 483. Chem, 18 (1993) 510; Timmanagoudar P L, Hiremath G A & Nandibewoor S T, Trans met Chem, 22 (1997) 193; 15 Walling C, Free radicals in Solutions (Academic Press, New Timmanagoudar P L, Hiremath G A & Nandibewoor S T, York), 1957 p 38. Polish J Chem, 70 (1996) 1459. 16 Sutin N, Ann Rev Phys Chem, 17 (l966)1l9; Lancaster M & 9 Halligudi N N, Desai S M & Nandibewoor S T, Trans met Murray R S, J chem Soc, (1971) 2755; Martinez M, Pitarque Chem 26 (2001) 28 and references therein. M & Eldik R V, J chem Soc, Dalton Trans, (1996) 2665; 10 10 CalTington A, & Symons M C R, J chem Soc,(l956) Hiremath G A, Timmanagoudar P L & Nandibewoor S T, 3373. React kinet catal Lett, 63 (1998) 403.