Outer-sphere mechanism of oxidation of hydrazine and ... - NOPR

Indian Journal of Chemistry Vol. 38A, July 1999, pp.680-685

Outer-sphere mechanism of oxidation of hydrazine and hydrazinium ion by 12-tungstocobaltate(III) ion in acetate-acetic acid butTer: Marcus treatment Rupa Shah, Aditya Prakash* & Raj N Mehrotra* Department of Chemistry, JNV University, Jodhpur 342 005, India

Received 1 September 1998; revised 7 April 1999

Oxidation of hydrazine by 12-tungstocobaltate(III) in acetate buffer is expressed by the following equation in view of the stoichiometric ratio i1[Co(III)W 5-]Ii1[hydrazine]= 4.05 ± 0.09.

The spectral studies of the reaction mixture at pH ~ 2 indicate that the absorbancy of the reaction mixture is more than that of [COIIIWP- ion and less than the [CoIIIWP- ion at pH> 2. This is attributed to the equilibrium between N,_H 4 and N,H _ 5+and that both of these species are reactive. Oxidation of N2H4 is at least 104 times faster than the oxidation of N2H/ ion . This is attributed to the availability of two lone pair of electrons on nitrogen atoms in N 2H 4 . In N2H/ one of the lone pair is polarised towards the nitrogen bearing the proton and is thus not easily available for coordination. The de pendence of rate on [W] is given by the expression k2 = (k + k,KJH+]-') where k2 = k"h/4[hydrazine]and k and k, are the composite rate constants The reaction is considered outer-sphere in nature and the same is supported by the application of Marcus theory.

Oxidation of hydrazine has attracted attention, perhaps, because its oxidation products differ with one and two electron equivalent oxidants l.2. The oxidation of nitrogen compounds by 12-tungstocobaltate(III) ion , [CO"10/ WO,)IY-' hereafter represented as [COIIIWP-, has received little attention. We recently in vestigated the ox idation of hydroxylamine' and found that NHpH and not NHpH+ formed an intermediate complex with [Co"1WF- which disproportionate into the products . It was surprising because the positive charge on NH,OH+ made it the most appropriate species to react with the negatively charged [CoIIIWP- ion. The inactivity of NHpH+ is traced to the lack of availability of a lone pair of electron on nitrogen atom needed for coordination . It was, therefore, considered appropriate to investigate another similar compound to confirm whether the availability of the pair of electrons on nitrogen atom is essential.

Materials and Methods 12-Tungstocobaltate(III) was prepared as described earlier 4 .The sample had maximum absorbancy at 388

nm against the value 390 nm reported in the literature' . Buffer solutions of desired pH were prepared from sodium acetate and acetic acid as described e1sewhereo. Hydrazine perchlorate was prepared by mixing equivalent amounts of hydrazine hydrate(Loba-Chemie, LR) and perchloric acid(E Merck, GR) and analysed by bromate titration 7 • Other chemicals were either AnalaR or GR grade. The solutions, prepared in doubly distilled water, were standardised by standard titrimetric method s. Doubly distilled water was used throughout. Rate measurements The preliminary investigations indicated that the rate was unaffected when atmospheric oxygen was replaced with nitrogen gas . Hence, no efforts were made to exclude air from the reaction mixture. The kinetics was studied in acetate-acetic acid buffer (PH 4.27) at constant ionic strength (Il = 0.4 mol dm-\ NaCI04 ) under pseudo-first order conditions ([COIIIW]5- » [hydrazine]) by monitoring the optical density of [Co(III)WP- ion at 390 nm using a Photoelectric Instruments SFA-II colorimeter based on the stopped-flow method . The optical density at infinite time, A~, was always measured after 20-24 h after the

SHAH et at.: OXIDATION OF HYDRAZINE BY 12-TUNGSTOCOBALTATE(II1)

d

0'28600

b

0 :29000

681

0·28160

0-28280 UJ

u

~

lj 0·27560 z

a:

cr

III CD

.. + N2H:

... (7)

tog K12

Fig. 4 - The linear plot between 210gk z against 10gK'l" The slope of the linear plot is 0 .84 ± 0 .08 which is in fair agreement wi th the espected value of unity.

fast NH ' +W 2 )

.. . (8) -d[Co(III)Wj5-

fast N 2H)' + [COIIIWP- ... ~ NZH) + [Co"Wjf>.. + W

...(9)

dt

N)H) + 2[Co"IW r -

~

[Wj + K, + (KJWj + KK) [N zH5 j"l ... ( 12)

fast N) + 2[Co"Wjo. + 2W

...( I 0)

Hydrazinium radical cation, N 2H/ , has been identified and characterised in aqueous solutions by pulse radiolysis 9 . Its presence was qualitatively detected in the present case by the induced polymeri sation of acrylonitrile . It appear s that the radical cation disproportionates immediately into the hydrazyl radical(N 2 H) and H +. The hydrazyl radical can either dimerise or undergo di smutation to give diazine 'o .. . ( II )

Further, diazine suffers rapid oxidation to N2as shown in Eq. (10). The above sequence of reactions are considered to account for the stoichiometry of the reaction requiring 4 moles of [Co(III)WP- per mole of hydraz ine. The rate law based on reaction s (3) to (10) is expressed by equation ( 12). Since the reaction has a first order dependence in [N 2H,l." the total [hydrazine] , it is apparent that ([H+] + K) » (KJH+] + Kka)[ N 2H),,. In view of the value of K" it is assumed that [H+] » K"" Eq . ( 12) is reduced to Eq . (13) which is identical with the empirical rate law in Eq. (2) .

or, . .. ( 13)

Equation (13) is consistent with the linearity of the plots of k2 versus [H+]- ' (Fig. 3). The values of k and k at various temperatures, calculated from the intercepts" and slopes of the plots are given in Table 2. It is interesting to note that the k, value is higher than that of k by an order of four indicating that the oxidation of N2H4 is 104 times swifter than that ofN2H/ io n. The hi gh reacti vity of N2H4 can be ascribed to its mol ecular structure. The hydrazine molecule H2N- NH2 is composed of two tetrahedrons in staggard configuration and one co rner of each is occupied by a lone pair of e lectron s " . It can, therefore, act as a dibasic mol ecul e giving ri se to both N2H/ and N2H62+ ions. The N2H/ ion exists in N 2HnC 20 4 which behaves like hydrazinium monooxalate '2 . The N 2H(i2+ ion is reported to be present in solution of hydrazine sulphate 13 • In th e present ex perimental conditions use of hydraz ine perchlorate and acetic acid 111 buffer strongly indicate the existence of N2H/ ion.

684

INDIAN J CHEM. SEC A, JULY 1999

Table 3 No.

oxidant

E"

Kil

I. 2. 3. 4. 5. 6. 7.

IrCl/'

0.89 0.84 1.07 1.20 0.84 1.00 0.75

3.21 2.37 6.26 8.46 2.37 5.08 0.84

IrBr /Ir(OH2)CI~'

Ir(OH 2)lCi 4 Mn(Cydta ) [Co ( III)wl'Mo (CN) .3-

Data for Marcus Cross Relation IO*k '2

log

6.00 1.84 54.0 387 852 40.5 0.18

2.71 1.86 5.75 7.95 1.86 4.57 0.33

Ki l

21ogk

'2

-6.44' -7.47' -4.53 ' -2.82' 6.93 1 -4.78 -9.46'

*Rcf. 22 ]' 20H,o the central ColJI ion is deeply buried within a shell of W0f> octahedra ( represented by W in the assumed formula) and is, therefore, completely shielded from contact with external species. Therefore, the central ColJI ion is unavailable for direct bonding with the ligand. Hence the formation of an inner-sphere complex in which the ligand is coordinated to the central COlli atom is highly improbable. The formation of the coordinated complex

may therefore involve the ligand and one of the tungstates present in the outer framework. Thus the electron-transfer has to be necessarily through an outer-sphere pathway. The electron transfer to 12-tungstocobaltate(IlI) ion during the oxidation process can occur in two way s. One possibility is that the central COlli atom remains intact and the tungstate framework is reduced as in the oxidation of oxygen and nitrogen containing organic compounds l6 . In the other possibility the outer framework remains intact and the central COlli atom is reduced as in the oxidation of alkyl aromatic hydrocarbons 17. The spectrum of the reaction mixture at the end of the reaction was identical with an authentic solution of [COIIW]6- ion IX and the same is true presently as well as in the oxidation of thiolactic acid 5 and hydroxylamine 4 • This suggested that in these oxidations the central tetrahedral COlli is reduced to COli . Since the direct electron-transfer between COlli and the ligand is unlikely in view of the di fficult formation of the inner-sphere complex, the electron tran sferred to tungstate frame work seems to be simultaneously transferred to the central COlli atom. Although an inner-sphere electron transfer has been suggested in the oxidation ofthiolactic acidS, the actual mode of electron transfer is not properly understood. The above argument in favour of an outer-sphere mechanism is sought to be supported by the application of the Marcus cross relation l9 given by Eqs. (14) and (15) where symbols used have their usual meaning. ... ( 14)

SHAH et al.: OXIDATION OF HYDRAZINE BY 12-TUNGSTOCOBALTATE(III)

... ( 15)

685

ence and Technology (DST) for the grant to purchase the stopped-flow instrument.

References

19

(a) Morris D F C & Ritter T G, J chem Soc Dalton Trans, (1980)216; (b) Sengupta K K & Sen P K, /norg Chem, 18(1979) 979; (e) Sengupta K K, Sen P K & Sengupta S, Inorg Chem , 16 (1977) 1396; (d) Arselli P & Mentasti E J, J chem Soc Dalton Trans, (1983)689. (a) Higginson W C E, Sutton D & Wright P, J chem Soc, (1953)1380; (b) Higginson W C E & Sutton D, J chem Soc, (1953)1402; (e) Higginson W C E & Wright P, J chem Soc, (1955)1551. Goyal B, Sharma K, Prakash· A & Mehrotra Raj N, Indian J Chem, Sect A, 37( 1998)973. Dholiya S, Sharma K, Prakash A & Mehrotra Raj N, Indian J Chem, Sect A, 38(1999)000. Eberson L, JAm chem Soc, 105(1983)3192. Goyal B, Solanki S, Arora S, Prakash A & Mehrotra Raj N, J chem Soc Dalton Trans , (1995) 3109. Kolthoff I M, Sandell E B, Meehan E J & Burckenstein S, Quantitative chemical analysis, 4th ed .(Maemillan , New York)(l969) p. 861. Smith R M & Martell A E, Critical stability constallls. Vol IV. (Plenum, New York) ( 1976) p. 43 . Hayon E & Cimic M 1, JAm chem Soc, 94(1972)42. Cahn 1 W & Powell R E, J Alii chem Soc, 76( 1954)2568. Penney W G & Sutherland G B B M, J chem Phys, 2( 1934)492; Fcrrari A, Braibanti A, Bigliardi G & Lanfredi A M, Acta Cryst, 119( 1965)548. Nilsson A, Liminga R & Olovs so n I, Acta chem Scand, 22( 1968)719. Nitta I, Sakurai K & Tommiie Y, Acta CI),st, 4( 1951 )289. Yamaguchi A, Ichishima I, Shimanouchi T & Mizushima S, J chem Phys, 31 ( 1959)843. Eriks K, Yanonni N F, Agrawa l U C, Simmons V E & Baker L C W, Acta Cryst, 13( 1960) 1139; Keggi n 1 F, Proc Roy Soc, SerA , I44( 1934)75 . Bellman 1 H, Borders D B, Buehler 1 A & Seiling A W, Anal Chem, 37(1965)264. Chester A W, J Org Chem , 35( 1970) 1797 . Leipoldt 1 G, Bok L D C, Van Wyk A 1 & Dennis C R. React Kinet Catal Lett, 6(1977)467. Marcus R A, J phys Chem , 72( 1968)89 1.

20

Stanbury D M, Inorg Chem , 23( 1984)2879.

I

Assuming the adiabatic nature of the self exchange reactions, Eq. (14) is rearranged to Eq. (16) which requires that the plot of21n kl2 against In KI2 is linear with unit slope.

2

...(16)

The EO for N2H4+1N 2H4 couple, indirectly estimated 211 , is 0.73V. Using the data in Table 3 and neglecting the work functions because of the high ionic strength, a linear plot of 21nk' 2 against InK'2' Fig. 4, with a slope value of 0.84 ± 0 .08 is obtained. It is to be noted that the point corresponding to [Mn(cydta)]- is far away from the least squares straight line (r = 0.98). The estimated slope of the linear plot is in fair agreement with the expected slope value of unity in terms of Eq. (16) . Thus it is reasonable to believe the outer-sphere nature of the oxidation . It might be added that the outer-sphere nature of the oxidations by h(IY) was similarly demonstrated '(a) in which a slope of 0.45 was obtained and considered to be in fair agreement with theoretically expected value of 0.5 . Similarly, the oxidation by Mo(CN)/ ion 211 is al so likely to be outer-sphere because of the non-substitution nature of the oxidant. The strong deviation of th e [Mn(cydta)]- point (5) from the linear plot is not unexpected since this oxidation is stated to be inner-sphere '(d). The oxidation by PtCI/ might be inner-sphere for the rate was inhibited by chloride ionsl(c). Acknowledgement Thanks are due to the UGC (F-12-59/97(SR-I) for financial assistance for the present research work in our laboratory. Thanks are also due to Department of Sc i-

3 4. 5 6 7

8 9 10. II

12. 13 14 15

16 17 18.