Indian Journal of Chemistry Vol. 40A, November 2001, pp. 1191-1195 Kinetics and mechanism of oxidation of hydroxylaminehydrochloride by vanadium (V) in the presence of sodium lauryl sulphate Rajendra Swain & G P Panigrahi* Department of Chemistry, Berhampur University, Berhampur 760 007, India Received 30 March 2001.. revised 28 June 2001 Kinetics of oxidation of hydroxylaminehydrochloride in aqueous HCI0 4 medium has been studied at different temperatures both in the presence and absence of SLS. The kinetics shows first order dependence on both [NH 2 0H] and [V(V)) but change in ionic strength and [H+] have no effect on reaction rate. Decomposition of a precursor complex has been suggested as the slow step. The reaction is moderately catalysed by SLS and the catalysis has been rationalised by Menger and Portnoy model. Moderate binding constant value suggests that the precursor complex is bound to the micellar surface through hydrophilic binding. The kinetic data also fit into Piskziewicz model and conclusions drawn there from have limited significance. Hydroxylamine is a well known reductant and its oxidation products are dependent on the nature of the oxidane. Oxidation kinetics of NH 2 0H by oxidant V(Vi 3 has been reported. We have been interested in the investigation of these oxidation processes in surfactant media to find out if the hydrophobic and hydrophilic forces influence the redox processes 7 8 Prompted by reports of electron transfer reactions in micellar media 4-6 oxidation study of hydroxylamine by V(V) in anionic micelles of sodium lauryl sulphate (SLS) was undertaken. Materials and Methods Analytical grade reagents were used. All the solutions were prepared using doubly distilled water. The stock solution of the ammonium vanadate was prepared in aq-hci0 4 medium and the [H+] of the stock solution was fixed volumetrically by titrating with standard Na 2 CO) solution using methyl orange as indicator. The surfactant SLS was used after recrystallisation and the solutions were prepared just before the experiment to avoid the ageing of the micelle. Hydroxylaminehydrochloride (S-Merck) was purified by recrystallisation and its aqueous solution was prepared. Reaction solutions of both hydroxylamine and vanadium(v) were prepared in 50 ml flasks by drawing requisite amount of the stock solution. Calculated amount of SLS was added to the oxidant flask and then made up. Both the solutions were thermostatted for 30 min to attain equilibrium. 25 ml of each were mixed in a clean dry bottle maintained at the same temp. Progress of the reaction was followed by the estimation of V(V) at regular intervals of time by iodometry in 5 ml aliquots of reaction mixture. The reaction mixture is homogeneous and the oxidant is stable during the kinetic study. Rate constants were reproducible within 5%. Results and Discussion Variation of rate with [NH 2 0H} and [H+}- The rate data (Table,l) suggest first order dependence in V(V) and complex dependence in [NH 2 0H). Plot of k ~~s against [NH 2 0Hr l is linear showing formation of a complex between the reactants preceeding the rate limiting step. Further the data suggest that the rate is independent of [H+]. This observation is at variance with that of Nazer and Wells 3 who reported retardation by the H+. Although Bengtsson 2 reported a second order component in [V(V)] dependence at high V(V) no such observation was made by us. However, at low [V(V)] Bengttson reported unit dependence in [V(V)]. The oxidation rate was also not affected by variation in ionic strength of tl\e medium. The reaction was carried out over the temperature range 35-50 C. Energy of activation and entropy of activation values are 60±6.2 kj mol- I and -58.3±6.7 JK- 'mol- I which are comparable to values reported by Nazer and Wells 3. When the reaction mixture was allowed to stand in the presence of acrylamide neither any precipitate nor turbidity was observed even after 24h. This observation shows that no free radial is formed during the oxidation of hydroxylamine by vanadium(v).
1192 INDIAN J CHEM, SEC A, NOVEMBER 2001 Table I-Pseudo-first order rate constants for the oxidation of [NH 2 0H] by [V(V)]at 35 C in aqueous medium 10[HCI0 4 ] 10 2 [V(lV)] IO J [V(V)] 10 2 [NH 2 OH] 102[NaCI0 4 ] 10 3 [SLS] 10 1 k~ mol.dm- 3 mol.dm- 3 mol.dm- 3 mol.dm- 3 mol.dm- 3 mol.dm- 3 min. - I 5.0 15.0 3.0 5.0 7.0 9.0 1l.0 5.0 5.0 7.5 10.0 12.5 15.0 20.0 5.0 5.0 7.0 10.0 15.0 20.0 Equilibration of hydroxylamine and V(V) in different ratios were carried out. Analysis of the reaction mixture showed that one mole NH 2 0H was consumed by 2 moles of V(V). Thus the stoichiometry is observed to be NH 2 0H:V(V)=1:2. Earlier workers 2. 3 too reported a stoichiometry of more than 1: 1.1. This was attributed to formation of N 2 0 in addition to a mole of N 2. Oxidation of hydroxylamine by V(V) in the presence of fixed sodium lau l"yl sulphate concentration has similar kinetic features as those in the absence of SLS. Data are given in Table. I. Hence 5.0 3.65 3.78 3.64 3.77 3.70 5.0 3.29 5.0 2.99 10.0 2.90 15.0 3.18 20.0 2.93 30.0 3.01 5.0 8.0 4.76 5.0 4.85 10.0 4.80 15.0 4.79 20.0 4.82 30.0 4. 89 2.5 8.0 2.68 5.0 4.75 7.5 5.21 10.0 5.40 12.5 5.89 5.0 3. 14 3.09 3.24 3.38 3.28 3.18 5.0 8.0 4.76 4.58 4.58 4.34 4.72 the mechanism of oxidation of hydroxylamine by V(V) in the presence of SLS can be explained by Scheme- I. Since the oxidation was catalysed by the presence of SLS, pseudo-first order rate constants obtained at different [SLS) in various experimental conditions are given in Table 1 and 2 and plots of rate constants kljl against [SLS) at different temperatures are given in Fig. I. The data in Table 2 and asymptotic plot (Fig. I) definitely prove moderate catalysis of the oxidation process with increase in [SLS).
SW AIN et al.: KINETICS OF V(V) OXIDATION OF HYDROXYLAMINEHYDROCHLORIDE 1193-0-3 r-----------------------, <>35 C; 0 40 C; I!. 45 C; o 50 C 015 0 2 0 05 OL-_~ _L L ~ ~_~ o 0 005 0 01 0 015 0 02 0 025 003 [SLSJ.M Fig. I-Plot of k'll min- I versus [SLS]10 2 [NH 2 0H] = 5.0 mol.dm- 3 ; 10' [V(V)] = 5.0 mol.dm- 3 ; 10 [H+] = 3.0 mol.dm- 3. Menger and Portnoy9 and subsequently Bu'nton 10 used the Scheme-l to explain the micellar inhibition which is as follows: km L Ks S + On ~ ProdUCts..J kw Scheme 1 The Scheme-l leads to rate law ( 1) k = _k_w _+_k-,,-m-,k,..-"s_{_[ O_J_-_c_m,,-c_} \ji 1 + K s {[ 0 J - cmc }... (1) where k\jl is the observed first order rate constant and k m is the rate constant in the micellar phase. Equation (1) can be re-arranged to Eq. (2) ----=----... (2) Although Eq.(2) holds good for continuous inhibition, it can be used for catalysis if changed to Eq.(3). ----=----... (3) Table 2-Rate constants in the oxidation of hydroxylamine by V(V) at. different temperatures in the presence of SLS 10 2 [NH 2 0 H]=5.0 mol dm- 3, 10 3 [V(V)]=5.0 mol.dm- 3, IO[H+]=3.0 mol dm- 3 10 3 [SLS] 10 2 k!l! min- I mol. dm- 3 35 40 45 50 C 3.49 4.88 7.76 12.10 2.0 3.63 5.08 9.05 13.78 4.0 4.34 5.28 9.53 14.70 6.0 4.53 5.60 10.70 15.75 8.0 4.75 5.62 11.67 16.51 10.0 4.83 6.16 12.46 18.10 12.0 5.20 7.37 13.04 19.70 14.0 5.97 7.94 16.0 5.86 8.13 13.49 20.98 20.0 5.98 8.71 14.44 22.80 24.0 6.56 9.39 16. 10 24.56 28.0 7.79 10.10 17.78 25.84 Validity of the Scheme-l and Eq.(3) has been established from the linearity of llk.r k w against 1/{ [OJ-cmc} plots (Fig.2). From the slope and intercept Ks and k m values were computed and recorded in Table.3. Further support to the model comes from the observation that hydroxylamine V 5 + complex, which is positively charged, is favourably bound by the negatively charged micelles both by hydrophilic as well as electrostatic forces as demonstrated by moderate Ks values. Increase in Ks values with temperature demonstrates that the binding process is an endothermic process. Interestingly the rates in the micellar phase ( k m ) also increase with the temperature. The k m values at different temperatures were used to compute Arrhenius parameters from the plot of k m versus 10 3 1T which is linear. Energy of activation and entropy of activation are 50±6.7 kj.mor l and -82.4±6.3 Jmol- IK- I which are much smaller than that in the absence of the micelles. It appears that the potential energy barrier is lowered in the presence of SLS and the fairly large negative entropy of activation shows loss of degrees of freedom in the transition state in the presence of SLS. This is not surprising in view of the binding of the vanadium-substrate complex to the micell ar surface which is responsible for the rigidity of the reactive species. This in turn lowers the energy barrier and facilitates decomposition of the complex leading to formation of the product.
1194 INDIAN J CHEM, SEC A, NOVEMBER 2001 100,.--------- ----r- - -----,24 80 o o - ~.--_t_- t---;---,r --' 0 o 50 100 150 200 250 300 [SLS CMCr'. M Fig. 2-Plot of (k';1 - kwr' versus [SLS-CMCr l 10 2 [NH 2 0H] = 5.0 mol.dm- J ; 10J [V(V)] = 5.0 mol.dm- 3 ; 10 [WI = 3.0 mol.dm- 3. 18 1.0,..------------------;---, 0.8 ~ 0.6 ~ -':' 0.4.>t;E 0.2 ~ 0.0.>t; '~ -0.2 ~ -0.4 ~ -0.6-0.8-1.0 L--~~~--+-~~--+---.--~--+~~-I -2.7-2.5-2.3-2. 1-1.9-1.7-1.5 [og[sls] Fig. 3-Plot of log [(k';1 - kw)/(k m - k';1 )] versus log [SLS] 10 2 [NH 2 0H] = 5.0 mol.dm- 3 ; 10 3 [V (V)] = 5.0 mol.dm- 3 ; 10 [W] = 3.0 mol.dm- 3 Table 3-Binding constants and k m values in the oxidation of hydroxylamine by V(V) at different temperatures IO[H+]=3.0 mol.dm- 3. Temp, oc Ks k m 35 34.38 0.077 40 29.78 0.120 45 68.18 0.244 50 90.56 0.329 Piszkiewicz model A number of micellar catalysed reactions has been fi tted to a model developed by Piszkiewicz [ [ on analogy with the model used to explain enzyme catalysis by Hill [2. For a sigmoid type of catalysis observed in the presence of micelles Piszkiewicz proposed the following steps: kw S ----. products Scheme 2 where KD is the dissociation constant of the micelle back to its free components, k m is the rate of the reaction within the micelle, kw is the rate of the reaction in the absence of the surfactant and n is known as the co-operativity for the reaction Scheme 2. The observed rate constant (k, ) is expressed as a function of the concentration of the detergent "0" by k \II = km[o)" + KDk w K D +[O]n. " (4) Table 4-Derived constants in oxidation of hydroxylamine by V(V) at different temperatures IO[WI=3.0 mol dm- 3. Temp, oc n KD 35 0.86 3.42x lo- 2 40 3.07 2.02xlO-6 45 1.53 1.12xlO- 3 50 1.8 3.09x lo-4 The equation can be re-arranged to log k ' - k ) w = n log[ 0]- log K D [ km-k,... (5) According to this equation a plot of log(k, -kwl km-k, ) versus 10g[O] for a micelle catalysed reaction should be linear with a slope of "n" and at log {(k'l' kwlkm-k, ) }=O, nlogo is equal to log KD. To test the applicability of the Piszkiewicz model the experimental data of the present reaction were used to calculate log {(k'l'-kwlkm-k, )} which were then plotted against 10g[SLS]. The plots were found to be linear (Fig.3). From the slopes and intercepts n and log KD values were derived and given in Table-4. Piszkiewicz observed n values ranging between I to 6 for a large number of micelle catalysed reactions which is also found to be the case for the reaction under investigation in different experimental conditions. The n values do confirm the model proposed by Piszkiewicz [ [. Further KD values are very small suggesting that di ssociation of the substrate bound to the micelle is negligible. The log Koin is equal to iog[o]so at which half the maximal velocity is obtained could not be realised. Of
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