Kinetics and mechanism of oxidation of hydroxylamine by tetrachloroaurate(III) ion

Vimal Soni and Raj N. Mehrotra*Department of Chemistry, JNV University, Jodhpur 342 005, India

Received 11 March 2003; accepted 14 April 2003

Abstract

The oxidation of H2NOH is first-order both in [NH3OH+] and [AuCl�4 ]. The rate is increased by the increase in[Cl)] and decreased with increase in [H+]. The stoichiometry ratio, D[NH3OH+]/D[AuCl�4 ], is �1. The mechanismconsists of the following reactions.

NH3OHþ +(Ka

NH2OHþHþ ðiÞ

AuCl�4 þNH2OH ��!k AuCl�2 þHNOþ 2Cl� þ 2Hþ ðiiÞ

AuCl�4 þNH3OHþ þ Cl� ��!k1 AuCl�2 þHNOþ 3Cl� þ 3Hþ ðiiiÞ

2HNO ��!fast N2OþH2O ðivÞ

The rate law deduced from the reactions (i)–(iv) is given by Equation (v) considering that [H+] � Ka.

kobs=½H2NOH�o ¼ k2 ¼ ðkKa½Hþ��1 þ k1½Cl��Þ ðvÞ

The reaction (iii) is a combination of the following reactions:

NH3OHþ þAuCl�4 ��!fast ½AuCl�4 ::NH3OHþ�zprecursor ðaÞ

½AuCl�4 ::NH3OHþ�zprecursor þ Clþ ��!k1 ½Cl� . . .AuCl�4 ::NH3OHþ�zsuccessor ðbÞ

½Cl� . . .AuCl�4 ::NH3OHþ�zsuccessor ��!fast

AuCl�2 þHNO þ 3Cl� þ 3Hþ ðcÞ

The activation parameters for the reactions (ii) and (iii) are consistent with an outer-sphere electron transfermechanism.

Introduction

Oxidation of the NH3OH+ ion in (0.1–1.0 mol dm)3)HCl is first-order both in [AuCl�4 ] and [NH3OH+]. Thestoichiometry ratio, D[NH3OH+]/D[AuCl�4 ], is �2 and alinear correlation between kobs and [H+])1 is reported[1]. The reaction is stated to proceed initially betweenAuCl�4 and NH3OH+ ions and subsequently throughfree radicals. Although Cl) ion is known to have theinhibiting effect on the rate of several reactions1, thereaction is reported to have no effect by Cl) ion on therate [1]. The reaction was, therefore, investigated inHClO4 (0.01 mol dm)3) in order to avoid the unneces-

sary presence of Cl), at constant ionic strength(l ¼ 0.5 mol dm)3, LiClO4). The effect of Cl) wasstudied using LiCl. The significantly different results inthe reinvestigated reaction, kobs increase with the in-crease in [Cl)] and D[NH3OH+]/D[AuCl�4 ] � 1, arereported in this paper.

Experimental

Solutions of AuCl�4 (Johnson and Matthey) wereprepared daily for a series of experiments with thedesired concentration of HClO4 by weighing, since suchsolutions are stable for 24 h. The purity of the sampleand the concentration of the prepared solution arechecked spectroscopically using a HP 8452A diode array

* Author for correspondence1 For specific reactions, see ref [10–15].

Transition Metal Chemistry 28: 893–898, 2003. 893� 2003 Kluwer Academic Publishers. Printed in the Netherlands.

spectrophotometer. The solutions of NH2OH ÆHCl (E.Merck, puriss), prepared daily, were standardised bro-mometrically [2]. The stock solution of LiClO4 wasprepared by neutralizing LiOH by a standard HClO4

[3] ÆHClO4 was standardised against a standard alkalisolution. All the solutions are prepared in twice distilledH2O purged with N2.

Spectral studies of gold(III) solution

The spectrum of the aqueous AuCl�4 solution (3 ·10)4 mol dm)3), was recorded ca. 30 min after its pre-paration since this much time is likely to lapse before theruns for the rate measurements are made, has themaximum optical density 0.94 at 287 nm, resulting in ae value of 3.1 · 103 dm3 mol)1 cm)1. The wavelengthof maximum absorbance of a similarly prepared1 · 10)4 mol dm)3 AuCl�4 solution in 0.01 mol dm)3

HClO4 is red-shifted to 310 nm, and the e value changedto 4.82 · 103 dm3 mol)1 cm)1 in agreement with theliterature (4.86 · 103 dm3 mol)1 cm)1) value [4–6]. Theconcentration of the gold(III) solutions in HClO4 wascalculated using the observed molar absorptivity.The stability of the aqueous AuCl�4 solution and

solutions in HClO4 (0.004–0.1 mol dm)3) was studiedby measuring their optical density at room temperatureover 24 h. The optical density of the acidic solutionsremained constant within limits of measurable errors,whereas that of the aqueous solution increased after ca.2 h. possibly due to the formation of other gold(III)species. Hence, AuCl�4 solutions were prepared inHClO4.

Optical density variation with alkali metal ions

The optical density of the AuCl�4 solution (2 · 10)4

mol dm)3) increased in the presence of MCl (Mþ ¼Liþ;Naþ;Kþ and Csþ)([AuCl�4 � ¼ 1:12� 10�4 mol dm)3

in the presence of NaCl). An isosbestic point at 296 wasnoted in each case [5, 7, 8]. An interesting observationwas that the optical density corresponding to a given[MCl] was almost the same within the experimentalerror. It is, therefore, concluded that the optical densityincreased due to the interaction of Cl) ions with theAuCl�4 ion and not due to the formation of aAuCl�4 ..M

+ ion-pair.

Rate measurement

The rate was studied in HClO4 (0.01–0.35 mo1 dm)3)under pseudo first-order conditions ([NH3OH+] �[AuCl�4 ]) at constant ionic strength (l ¼ 0.5 mol dm)3,LiClO4) by measuring the optical density of the reactionmixtures at 360 nm using a Photoelectric InstrumentsSFA 11 microprocessor controlled colorimeter inter-faced with a printer [9]. The validity of Beers law wasverified at this wavelength. The reaction mixture wascolourless after completion of the reaction. The plots oflog At versus time ‘t’ were linear for more than two half-

lives (@80%) (At is the optical density at time ‘t’). Alinear regression program was used to evaluate thepseudo first-order rate kobs from the slopes of the linearplots. The reproducibility of the kobs from two to threeruns under similar conditions was within ±5%. Instudying the effect of the variation of NH3OH+ on thekobs, [Cl)] was kept constant by LiCl at a level thatwould be present at the highest [NH3OH+] used sinceNH2OH ÆHCl is used as the source of NH3OH+ ion.

Stoichiometry

The stoichiometry of the reaction was studied usingdifferent ratios of [NH3OH+] and [AuCl�4 ] at different[H+] though [AuCl�4 ] was always present in excess over[NH3OH+]. The unreacted AuCl�4 was estimated at360 nm until completion of the reaction. The experi-mental results indicated that the stoichiometry ratio,D[AuCl�4 ]/D[NH3OH+] ¼ 1.06 ± 0.07, is independentof initial [H+]. Therefore, the stoichiometry equationof the reaction is expressed by Equation (1).

2NH3OHþþ2AuCl�4 ��!2AuCl3�4 þ6HþþN2OþH2O

ð1Þ

Test for free radicals

Solutions of AuCl�4 and NH3OH+, taken separately inglass stoppered conical flasks, were degassed with N2.Then 2 cm3 of redistilled MeCN was added to eachflask. The reagents remained clear for sometime beforemixing. There was no polymerisation of the monomer inthe reaction mixture even after ca. 30 min, indicatingthat free radicals are not generated during the reactionalthough a granular black precipitate appeared onprolonged standing.

Reduction product of gold(III)

The presence of the granular precipitate indicated thatgold(III) was not reduced to metallic gold. It is assumedthat gold(III) is reduced to gold(I) because gold(II) doesnot exists in solution. Hence, AuCl�4 is acting as a two-electron oxidant and therefore free radicals are notformed during the reaction.

Results

The reaction is first order in [NH3OH+] as indicated bythe proportionate increase in kobs with the increase in[NH3OH+] at constant ionic strength and [HClO4], andby the linearity of the plot of kobs versus [NH3OH+],Figure 1, that passes through the origin. The kobs valuesdecreased with the increase in [H+] at constant ionicstrength. The kobs values at five temperatures are listedin Table 1. The plots of kobs versus 1/[H

+] (Figure 2) are

894

linear, having perceptibly real intercepts on the rateordinate, indicating two paths for the reaction. Sincekobs is independent of ionic strength (Table 2) thereforethe rate-determining step either involves a neutralmolecule and an ion, or neutral molecules. The resultof the addition of M+ (Li+, Na+, and Cs+) on the rate,measured at constant ionic strength (Table 3) indicatesthat the size of M+ has no effect on the rate, ruling outthe possibility of ion-pair formation between M+ andAuCl�4 ions. This conclusion is in agreement with thatbased on a study of the effect of different MCl on theoptical density of the reaction mixture described in theearlier part of this paper. The results (Table 4) indicatethat kobs is independent of the charge and size of thecations and is similar to the results in Table 3. The effectwas investigated using chloride salts of Li+, Ba++ andAl+++. The concentrations were chosen so as to give aparticular ionic strength without the addition of LiClO4.

Effect of Cl) on the rate

The Cl) ion has a different effect on the rate viz. the Cl)

retarded the rates of oxidation of nitrous acid [4], oxalicacid [7], mono- and dicarboxylic acids [10], methylma-lonic acid and dimethylmalonic acid [11], formic acid[12] and diethylenetriamine [13]. The rate, however,increased in the hydrolysis of AuCl�4 ion [5] and theoxidation of the FeII ion [14]. The result of adding LiClat constant ionic strength (LiClO4), (Table 5) indicatedthat Cl) ions increased the rate. The plot of k2 (=kobs/

[NH3OH+]) versus [Cl)] (Figure 3) is linear with theintercept indicating two paths, one of which is indepen-dent of [Cl)].

Discussion and mechanism

Some of the results of the previous study [1], first orderin [AuCl�4 ] and [NH3OH+] and an inverse dependenceof the rate on [H+], are confirmed. However, the presentplots of kobs versus [H

+])1 have an intercept on the rateordinate indicating two paths, one of which is indepen-dent of [H+]. The significantly different result is theincrease in the rate with the increase in [Cl)], theprevious study [1] carried out in presence of HClreported the rate independent of [Cl)].The mechanisms proposed for the hydrolysis of

AuCl�4 have equilibrium steps involving formation ofAuCl3(H2O), AuCl3(OH)) and AuCl2(H2O)(OH)) asintermediate species [5, 8, 15]. The value of the equili-brium constant K2, for the overall equilibrium (2),inferred from the Robb’s data [15] is 2.36 · 10)6

mol dm)3 respectively. Further, the inclusion of theequilibrium (2) or the other equilibrium steps [5, 8, 15] inthe mechanism does not lead to a rate law that wouldaccount for the increasing effect of [Cl)] on the ratethough the retarding effect of [H+] is correctly repre-sented. Thus in view of the constraint on the deducible

Fig. 1. The linear plot of the kobs versus [NH3OH+] passing through

the origin. 103[AuCl�4 ] ¼ 1.0, [HClO4] ¼ 0.1 and l ¼ 0.5 mol dm)3

and 30 �C.

Table 1. The dependence of kobs on [H+] at different temperatures 103[AuCl�4 ] = 1.0 , [NH3OH+] = 0.1, and l = 0.5 mol dm)3

[H+](mol dm)3) 0.01 0.025 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Temp. (�C) 102kobs(s)1)

25 1.82 0.80 0.468 0.210 0.139 0.105 0.078 0.0617 0.0510

30 2.79 1.21 0.705 0.342 0.221 0.193 0.136 0.110 0.960

35 4.08 1.91 1.15 0.558 0.430 0.292 0.246 0.200 0.155

40 6.21 2.88 1.60 0.890 0.640 0.480 0.370 0.330 0.320

45 9.43 4.02 2.25 1.30 1.01 0.854 0.763 0.700 0.660

50 13.9 6.09 3.48 2.15 1.72 1.56 1.37 1.29 1.24

Fig. 2. The linear plot of kobs/[NH3OH+] versus [H+])1 at tempera-

tures 30 �C (s), 35 �C (h), 40 �C (n), 45 �C (d) and 50 �C (j).

103[AuCl�4 ] ¼ 1.0, [NH3OH+] ¼ 0.1 and l ¼ 0.5 mol dm)3.

895

rate law and the inferred values of K2, the equilibrium(2) is overlooked. In fact such an equilibrium has notbeen considered in the oxidation of methionine [16]studied in perchloric acid solution.

AuCl�4 þH2O )*K2

AuCl3ðOHÞ� þHþ þ Cl� ð2Þ

The retarding effect of [H+] on the rate can also becaused either by the equilibrium (3) or equilibrium (4)

related to the deprotonation of HAuCl4 or theNH3OH+ ion.

HAuCl4 )*KAua

AuCl�4 þHþfKAua ¼ 1:35 ðref: ½17�Þ

or 1:11mol dm�3 ½14�g ð3Þ

NH3OHþ )*Ka

NH2OH

þHþfKa ¼ 5:9� 10�6moldm�3 ½18�gð4Þ

It is obvious from consideration of the respective valuesof the equilibrium constants KAu

a and Ka that theequilibrium (4) can be neglected in relation to theequilibrium (3) which is included in the followingmechanism. The proposed mechanism does not considerthat the formation of the intermediate AuCl�4 ANH3

OH+ complex is probable, first because NH3OH+ hasno lone pair of electron and AuCl�4 has no vacantorbital and, secondly, because the spectral study of thereaction mixtures with varying [NH3OH+], described inthe earlier part of the paper, did not indicate theformation of any intermediate.The electron transfer through an axially coordinated

transition state, similar to one considered in the oxida-tion of hypophosphite [19], iodide [20] and mandelateion [21] by square planar AgðOHÞ�4 which is isoelec-

Table 2. The effect of the ionic strength (LiClO4) on the rate of the reaction at 40 �C 103[AuCl�4 ] = 1.0, [NH3OH+] = 0.025,

[H+] = 0.01 mol dm)3

[LiClO4] (mol dm)3) 0.014 0.064 0.164 0.264 0.364 0.464 0.664

Total l 0.05 0.10 0.20 0.30 0.40 0.50 0.70

103kobs (s)1) 2.18 2.23 2.24 2.12 2.22 2.16 2.19

Table 3. The effect of alkali metal ions (M+) on the rate of the reaction at 30 �C 103[AuCl�4 ] = 1.0, [NH3OH+] = 0.1, [H+] = 0.02 mol dm)3

S.no [MCl] Total l 103kobs(Li+, s)1) 103kobs(Na+, s)1) 103kobs(Cs

+, s)1)

1 0.05 0.171 1.64 1.63 1.65

2 0.15 0.271 1.71 1.68 1.79

3 0.30 0.421 1.77 1.76 1.75

Table 4. The effect of increasing charge on the cation on the rate at 30 �C 103[AuCl�4 ] = 1.0, [NH3OH+] = 0.1, [H+] = 0.02 mol dm)3

S.no Total l [LiCl] (mol dm)3) 103kobs (s)1) [BaCl2] (mol dm)3) 103kobs (s)1) [AlCl3] (mol dm)3) 103kobs (s

)1)

1 0.171 0.05 1.64 0.0167 1.72 0.00835 1.70

2 0.271 0.15 1.77 0.0501 1.65 0.02505 1.79

3 0.421 0.30 1.71 0.1002 1.66 0.0501 1.75

Table 5. The effect of anions on rate of the reaction at 30 �C 103[AuCl�4 ] = 1.0, [NH3OH+] = 0.1, [H+] = 0.02 mol dm)3

[Cl)](added) 0.0 0.089 0.139 0.189 0.289 0.389

[Cl)]total 0.1 0.189 0.239 0.289 0.389 0.489

103kobs 3.12 3.45 3.53 3.76 4.08 4.42

102kobs/[NH3OH] 3.12 3.45 3.53 3.76 4.08 4.42

Fig. 3. The linear plot of kobs/[NH3OH+] versus [Cl)] at 30 �C.103[AuCl�4 ] ¼ 1.0, [NH3OH+] ¼ 0.1 and [HClO4] ¼ 0.02 mol dm)3.

896

tronic with square-planar AuCl�4 , is likely with NH2OH.In the transition state AuCl�4 , probably, undergoes atwo-electron change because free radicals were notdetected. The one electron reduction of gold(III) isunlikely because of the difficult removal of d10 electronsuggesting that gold(II) is normally nonexistent. Thegold(II) compounds such as AuS and AuCl2 aregenerally considered to contain equal numbers of gold(I)and gold(III) species. The following reactions areconsidered likely for the probable mechanism of thereaction.

Mechanism I

HAuCl4 )*KAua

AuCl�4 þHþ ð5Þ

AuCl�4 þ Cl� )*KCl

AuCl2�5 ð6Þ

AuCl�4 þNH3OHþ��!k

0AuCl�2 þHNOþ 2Cl� þ 3Hþ

ð7Þ

AuCl2�5 þNH3OHþ��!k0 AuCl�2 þHNO þ 3Cl� þ 3Hþ

ð8Þ

2HNO ��!fast N2OþH2O ð9Þ

The dimerisation of the nitroxyl radical is well docu-mented [18]. The rate law, deduced from the reactions(5) and (6)–(9), is given by the Equation (10).

�d½AuCl4�dt

¼ðk0KAua ½Hþ��1þkoKCl½Cl��Þ½HAuCl4�½NH3OHþ�½Hþ�

½Hþ�þKAua þKAu

a KCl½Cl�� ð10Þ

The Equation (10) is reduced to Equation (11) with theassumption that ½Hþ� þ KAu

a � KAua KCl[Cl

)] in viewof the respective values of the equilibrium constants andthe concentrations and [NH3OH]o @ [NH3OH+].

kobs½NH3OH�o

¼ k2 ¼ðk0KAu

a þ koKCl½Hþ�½Cl��Þ½Hþ� þ KAu

a

ð11Þ

Equation (11) requires the plot of k2([H+] + KAu

a )versus [H+] to be linear with an intercept on the rateordinate, but the actual plot (Figure 4) is a curve. Thedeviation could be due to the inclusion of equilibrium(5) because calculations using the dissociation constantsof HAuCl4 show that the concentration of AuCl�4 ion in1 mol dm)3 HClO4 solution is more than half the total[HAuCl4]. Hence in view of the above together with thefact that the reactivity of HAuCl4 compared to AuCl�4 isnegligible [14], the exclusion of the equilibrium (5) fromthe proposed mechanism can be rationalised. Thereforeanother mechanism needs consideration.

Mechanism II

In this mechanism, equilibrium (5) is replaced byequilibrium (12) to account for the retarding effect ofH+ ion on the rate. The ratio Ka/K2 ‡ 2.5 suggests thatthe dissociation of NH3OH+ has precedence over theequilibrium (2). The inclusion of the equilibrium be-tween NH3OH+ and NH2OH has necessitated theconsideration of the reaction (14) needed to explainthe accelerating effect of Cl) ion on the rate in place ofthe reaction (7) considered in the previous mechanism.The following sequence of reactions is most likely toconstitute the alternative mechanism.

NH3OHþ )*Ka

NH2OHþHþ ð12Þ

AuCl�4 þNH2OH ��!k AuCl�2 þHNO þ 2Cl� þ 2Hþ

ð13Þ

AuCl�4 þNH3OHþ þ Cl�

��!k1 AuCl�2 þHNOþ 3Cl� þ 3Hþ ð14Þ

2HNO ��!fast N2OþH2O ð15Þ

The rate law (16) is deduced from reactions (12)–(15)that can be written as Equation (17) considering that[H+]�Ka.

�d½AuCl�4 �dt

¼ ðkKa½Hþ��1 þ k1½Cl��Þ½AuCl�4 �½NH3OHþ�o½Hþ�Ka þ ½Hþ� ð16Þ

or,

kobs=½NH3OHþ�o ¼ k2 ¼ ðkKa½Hþ��1 þ k1½Cl��Þð17Þ

The rate law in Equation (17) is consistent with thelinearity of the plots between (i) k2 and [Cl)], Figure 3,

Fig. 4. The nonlinear plot of kobs/[NH3OH+]([H+] + KAua ) versus

[H+] where k2 (=kobs/[NH3OH+]). 103[AuCl�4 ] ¼ 1.0, [NH3OH+] ¼0.1 and l ¼ 0.5 mol dm)3.

897

and (ii) k2 versus [H+])1 (Figure 2). The values of therate constants k and k1, calculated from the slopes of theplots in Figures 2 and 3, are in Table 6. The Ka valueswere estimated using the reported enthalpy value.2 The[Cl)] ¼ [NH3OH+] is used to obtain the k1 becauseNH2OH � HCl is used. The deduced values of theactivation parameters are also given therein. It is to benoted that the k value is ca. 104 times the value of k1suggesting the high reactivity of NH2OH over NH3OH+

is probably due to the presence of a pair of electrons.The rates are accelerated by Cl) ion in the oxidation

of methylmalonic acid [11] and iron(II) ion [14]. Thecorrelation between the rates and [Cl)] in these oxida-tions is similar to that observed presently. The increasein the rates in these reactions is explained by theincreased reactivity of the products obtained by theinteraction of Cl) with the Me ÆCH moiety and Fe2+ ionthe later forms the FeCl+ complex. The Cl) ion isunlikely to form a complex either with AuCl�4 orNH3OH+ ions. The reaction (14) is a likely combinationof the two transition states and the rate-determiningdecomposition of the second transition state into theproducts. The formation of the two transition states arethought to proceed as shown below:

NH3OHþ þAuCl�4 ��!fast ½AuCl�4 ::NH3OHþ�zprecursorð18Þ

½AuCl�4 ::NH3OHþ�zprecursor þ Cl�

��!k1 ½Cl�. . .AuCl�4 ::NH3OHþ�zsuccessor ð19Þ

½Cl�. . .AuCl�4 ::NH3OHþ�zsuccessor��!fast AuCl�2 þHNO þ 3Cl� þ 3Hþ ð20Þ

The formation of the transition state, ½AuCl�4::NH3OHþ�zprecursor, is like the diffusion-controlled forma-tion of a precursor complex, or the formation of an ionpair by oppositely charged ions. The formation of thetransition state thus avoids a three body collision asshown in reaction (14). The precursor transition state,½AuCl�4 ::NH3OHþ�zprecursor, is then attacked by the Cl)

ion to form another transition state ½Cl�. . .AuCl�4 ::NH3

OHþ�zsuccessor by the coordination of the Cl) with AuCl�4ion. The formation of ½Cl�. . .AuCl�4 ::NH3OHþ�zsuccessoris likely to cross a high energy barrier because of the

repulsion between similarly charged ions and is thusrate-determining. Thus it is obvious that the enthalpyfor the reaction is greater than that for the reaction (13).The fast decomposition of the ½Cl�. . .AuCl�4 ::NH3

OHþ�zsuccessor is visualised to involve the entering Cl)

forcing the coordinated Cl) at the other end to movesimultaneously towards positively charged NH3OH+

helping it to deprotonate to NH2OH. The NH2OHtransfers the lone pair of electron to gold(III) forminggold(I) and nitroxyl radical as in reaction (13). Theactivation parameters for the reaction (13) clearlysuggest that the oxidation of NH2OH by AuCl�4 ionfollows an outer-sphere electron transfer mechanism.The sequence of the likely reactions, described earlier, issubstantiated by the positive entropy value reflective ofa loose ½Cl� . . .AuCl�4 ::NH3 OHþ�zsuccessor transitionstate. Hence, both paths of the reaction follow anouter-sphere electron transfer mechanism.

Acknowledgements

The authors thank the UGC for financial assistance(F.12-147/2001) and Dr K.M. Gangotri, Head of theDepartment, for laboratory facilities.

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TMCH 5633

Table 6. The values of the rate constants k (s)1) and k1K1 and the

activation parameters

Temp. (�C) 30 35 40 45 50

10)2k 2.15 2.37 2.78 3.16 3.49

102k1 0.541 1.43 2.25 3.47 8.65

DHzk ¼ 18� 1 kJ mol)1; DHk1 ¼ 102� 9 kJ mol)1;

DSzk ¼ �124� 3 J K)1 mol)1; DSk1 ¼ 66� 30 J K)1 mol)1.

2 Values of Ka are (1, 1.29, 1.70, 2.19, 2.88 and 3.72) · 10)6

respectively at 25, 30, 35, 40, 45 and 50 �C.

898