摘要:

1978 23 Kinetics and Mechanisms of the Reaction between Thioacetamide and Lead(ii), Cadmium(ii), and Cobalt(1i) Ions in Acetate Buffered Solution By Oswald M. Peeters, Norbert M. Blaton, and Carnie1J. De Ranter," Pharmacy Department, Katholieke Universiteit Leuven, 3000 Leuven, Belgium The interaction between thioacetamide and lead(ii), cadmium(ii), and cobalt(ii) ions in acetate buffered solution leads to acetonitrile and metal sulphide. A mechanism is suggested which involves the rate-determining decompo- sition of the deprotonated form of 1 :1 metal-thioacetamide complexes. The mechanism accounts for the rate dependence on thioacetamide, hydrogen ion, and metal ion concentration. The influence of the nature of the metal ions on the reaction rate, characterised by the sequence Pb > Cd > Co, is discussed in the light of the hard-soft acid-base principle.The difference in mechanism between the hydrogen ion and metal ion catalysed reaction of thioacetamide is also rationalised. THIOACETAMIDE(TAA) is used as an alternative to hydrogen sulphide for the precipitation of metal ions in qualitative and quantitative analysis. Several kinetic studies 1-8 have shown that the precipitation of metal sulphides by TAA can occur by two distinct reaction paths, depending upon the circumstances. At pH < 3 it is usually found that TAA is first hydrolysed to hydrogen sulphide, whereupon metal sulphide precipi- tates. At pH > 3 a direct interaction between metal ion and TAA leads to the metal sulphide. In most previous work the precipitation rate of the metal sulphide was indirectly determined from the concentration of the metal ion remaining in solution after timed intervals.In none of these studies has a plausible reaction mechanism been formulated. In the present study the disappearance of TAA has been measured directly. This allowed us to examine the kinetic effect of the metal ion on the hydrolysis rate of TAA. The reactions were studied in the pH range 5-6 at 80 "C where they occurred at a measurable rate. The use of a pH stat method, as was done in our previous studies of the hydrolysis of TAA,g*10 was impossible because of local metal hydroxide precipitation. There-fore an acetate buffer was selected to keep the pH constant. EXPERIMENTAL Reagents.-All reagents were AnalaK grade.The stock solutions were prepared by weight without further purifi- cation. They were: 0.012 5~-TAA; O.lM-Pb(ClO,),; 1M-CdC1,; lM-CoC1,; lw-NaOAc; and 1M-NaC10,. A 2.5~-acetic acid solution was used to adjust the pH to the desired value. The glass electrode was calibrated with a 0.05~-potassium hydrogen phthalate solution. Other solu- tions were 0.05hl-EDTA and 0.05~-zinc acetate. Afiparatm-The reaction vessel with the accessories was the same as that used previou~ly.~~~~ The measure- ment of the absorbance of TAA at 261.5 nm was performed continually by circulating the reaction solution by means of l E. H. Swift and E. A. Butler, Analyt. Chem., 1956, 28, 147. D.F. Bowersox and E. H. Swift, Analyt. Chem., 1958, 30, 1288. D. F. Bowersox, D. M. Smith, and E. H. Swift, Talanta, 1959, 2, 142. D. F. Bowersox, D. M. Smith, and E. H. Swift, Talanta, 1960, 3, 282. 5 E. A. Butler and E. H. Swift, Analyt. Chem., 1957, 29, 419. D. V. Owens, E. H. Swift, and D. M. Smith, Talanta, 1964,11, 1521. a peristaltic pump (L. K. B. Varioperpex). The solution was successively pumped through a sintered glass filter of porosity 2, a spiral glass tube immersed in a thermostatted bath at 20 "C, and a flow-through cell. The transmission of the solution was registered on a Tacussel Graphispot recorder from which the absorbance at each moment was calculated. Procedure .-An aqueous solution (250 ml) containing known concentrations of sodium acetate and sodium per- chlorate was thermostatted in the reaction vessel.The pH was adjusted to 6.5 with the pH stat mode of the auto- titrator. The stock or diluted solution (25 ml) of the metal ion was added, except in experiments to study the effect of buffer components; the pH was adjusted to the pre- determined value. The peristaltic pump was started and the transmission set to lOOyo. 0.012 ~M-TAA solution (1 ml) was added, the recorder started, and the change in transmission followed as a function of time. At the end of the experiment, after centrifugation and filtration of the precipitate formed, the metal ion concen- tration was determined titrimetrically with EDTA at pH 5 and 70 "C, using Xylenol Orange as indicator.RESULTS AND DISCUSSION Efect of Bufer Components on the Hydrolysis Rate of TALI.-The increase of the hydrolysis rate of TAA in ammonia and carbonate bufiers l1 shows that ignoring the effects of the buffer components may lead to un- expected results. It was therefore necessary to study the influence of the acetate buffer employed. The observed rate constants at different buffer concen-trations and pH are given in Table 1. In the presence of an acetate buffer the observed rate constant can be written as (1). By expressing [HOAc] kobs. = + kH,O+aH,O+ + kOH-aOH-+ ~HOAC[~O~~]~HOAC+ kOAc-[OAC-]fOAc-(1) as a function of [OAc-] equation (1) becomes (2). A plot kobs. == + kH,O+aH,O+ + kOH-aOI3-+ (koAC!-+ kROAcaH,O + )[OAc-I .foac-(2)K, 7 C.B. Moore, Thesis, California Institute of Technology, 1958. 8 D. C. Taylor, D. M. Smith, and E. H. Swift, Analyt. Chem., 1964, 36, 1924. 9 0.M. Peeters and C. J. De Ranter, J.C.S. Perkin TI, 1974, 1832. 10 0. M. Peeters and C. J. De Ranter, J.C.S. Perkin 11, 1976, 1062. 11 D. G. Peters and E. H. Swift, Talanta, 1958, 1,30. of the rate constants, at two different pH values, as a function of OAc- concentration gives straight lines. At pH 5.50 the slope and intercept obtained were 2.26 x (standard deviation s 0.311 x and 8.10 x (s 0.85 x at pH 6.00 2.37 x (s 0.24 x lo-*) and 8.15 x (s 0.65 x respectively. From these results, and the known rate constants of the TABLEI Effect of buffer concentration on the first-order rate constant of TAA hydrolysis at 80 "C and 1 0.4 pH 5.50 pH 6.00 [OAC-]/M lO*k,b,./rninP 104h,b,./min-l 0.1 1.09 1.08 0.2 1.25 1.18 0.3 1.49 1.52 0.4 1.80 1.72 hydrogen ion 9 and hydroxide ion catalysed hydrolyses, the rate constant of the spontaneous reaction with water, KO, and the catalytic constants KaOhc and kohc-were calculated.The activity coefficient fOAc-was ignored. The constants obtained were: k, 8.0 x min-l, kHoAc6.71 x and Roac-2.21 x 1 mol-l min-l. Since the difference between the slopes of the straight lines is smaller than the standard deviations the value of kHOAcis not significant. The values of KO and KoBc-reveal that the spontaneous reaction is very slow and that OAc- has a positive catalytic effect.The latter probably results from a general base catalysis although a nucleophilic reaction is also possible. To obtain a constant catalytic effect by the acetate species a O.OSM-OAC- concentration was used in all subsequent work. The Metal-Thioacetamide Interaction.-Products. Experiments similar to the kinetic runs, except that the TAA concentration was 0.01~ wereinstead of 5 x 10-5~, performed to identify the products of the metal-TAA interaction. Analysis by g.1.c. on a Porapak Q column showed that the organic reaction product was acetoni- trile. Quantitative experiments with an internal stan- dard (CH,OH) gave acetonitrile concentrations which agree within 10-20~o with the amounts of TAA used up. The precipitated inorganic products were identified, after filtration and drying, by X-ray powder diffraction as PbS, CdS, and p-CoS.Kinetics. Plots of the logarithm of the absorbance, due to the TAA concentration, as a function of reaction time are linear over more than one half-live. The dis- appearance of TAA is consequently a first-order reaction. The values of kobsare reproducible to within 3%. (a) Effect of metal ion concentration. The observed rate constants at pH 6 and at different initial metal ion concentrations are given in Table 2. The order with respect to the initial metal ion concentration was obtained by linear regression and amounts to 0.84 (s 0.058) for the Pb-TAA, 0.84 (s 0.031) for the Cd-TAA, and 1.14 (s 0.088) for the Co-TAA interaction.(b) Effect of pH. The rate constants obtained at different pH and at constant metal ion concentration are J.C.S. Perkin I1 summarized in Table 2. The order with respect to the hydrogen ion activity is -1.02 (s0.047) for the Pb-TAA, -1.05 (s 0.042) for the Cd-TAA, and -1.07 (s 0.039) TABLE2 Effect of the metal ion concentration and pH on the reaction rate of the Pb-TAA, Cd-TAA, and Co-TAA interaction at 80 "C and 10.36 103k,b, ./min-'1O3k,~,./min-l [Cd2+] = [C02+] = System 102[M2+]/~ pH 6.00 pH 10[Pb2+J= 0.09~ Pb-TAA 0.896 12.0 5.00 1.23 0.717 9.2 5.25 1.8% 0.538 8.58 5.50 3.68 0.358 5.6 5.75 6.87 0.179 3.02 6.00 12.0 Cd-TAR 9.08 26.2 5.00 2.26 7.21 20.7 5.32 16.4 5.50 7.1 3.59 11.2 5.75 12.5 1.82 6.8 6.00 26.2 Co-'I'A '1 8.95 8.3 5.00 0.729 7.22 6.4 5.25 1.41 5.51 4.9 5.50 2.51 3.48 3.4 5.75 5.27 1.86 1.3 6.00 8.3 for the Co-TAA interaction. The reaction is con-sequently inversely first order with respect to hydrogen ion activity.(c) Effect of ionic strength. A change of the ionic strength from 0.36 to 0.14 revealed no particular effect on the reaction rate. (d) Effect of temperature. The reactions were carried out in the temperature range 60-90" and the results are collected in Table 3. The activation energies were TABLE3 Effect of temperature on the reaction rate of the I'b-TAA, Cd-TAA, and Co-TAA interaction at pH 6.00, 10.36 and [Cd2+] = [Co2'-] = 10[Pb2+]= 0.09~ Pb-TAA Cd-TAA CO-TAA t/"C 103ka,,,./min-' 103k,b,./min-1 1O3k,b,,/min-' 60 1.98 3.7 1.04 70 5.14 8.0 2.9 80 12.0 26.2 8.3 90 29.7 60 20.8 obtained from a plot of log kobsagainst T1.They are 21.5 (s 0.33) for the Pb-TAA, 23 (s 7 .S) for the Cd-TAA, and 24.1 kcal mol-l (s 0.31 kcal mol-l) for the Co-TAA interaction . Mechanism. The experimental results show that the reaction rate is defined by the concentrations of TAA, metal ion, and hydronium ion. The rate contribution for loss of TAA via OAc- catalysed hydrolysis is small compared with the observed rate so that it can be ignored. The reaction products identified are acetonitrile and metal sulphide. The overall reaction scheme compatible with these results is given by equation (3). CH,CSNH, + M2+ + 2 H20 CH,CN -/-MS + 2H30+ (3) In order to understand the observed reaction orders and the effect of ionic strength a multiple step reaction mechanism has to be considered (Scheme). The proper K1M2+ + C1-4MC1+ K,M2+ + OAc-=+=MOAc+ KS M2+ + TAA =+= MTAA2+ (1) K,MClf + TAA ---L MCITAA+ (7)(11) K,MOAc+ + TAA +MOAcTAAt (IW K8 MTAA2+ + H,O MTAAf $-H,O+ (9)(IV) K?MCITAA+ + H,O +MCITAA + H,0+ (10) (V) MOAcTAAt + H,O === MOAcTAA + H,O+ (11)(VI) MTAA+ + H,O k, MS + CH,CN + H,O+ (12) MClTAA + H,O -% MS + CH,CN + H,O+ + C1-(13) kl,MOAcTAA + H,O --+ MS + CH,CN + H,O+ + OAc-(14) SCHEMEReaction mechanism of the metal thioacetamide inter- action.[In the Pb-TAA reaction equations (4), (7), (lo), and (13) have not to be considered, due to the absence of C1-ions] rate-determining steps (12)-( 14) are preceded by a number of fast equilibria (4)-(11).In all reactions solvating water molecules were omitted for simplicity. Equilibria like (15)are ignored because under the given lM(H20),12’ + H,O * [M(H,O)n-, OH1 + H3O I-(15) circumstances the concentration of hydroxo-complexes is <1yo of the initial metal ion concentration. On the other hand nionochloro [reaction (4)] and monoacetato [reaction (5)] complexes are formed in a significant proportion ( >50y0).12 Although in this study no spectrophotometric evidence was found for the rapid formation of detectable quanti- ties of metal-TAA complexes [reactions (6)-(S)], they have been prepared under well defined conditions.From X-ray diffraction data on single crystals could be concluded that the metal is co-ordinatively bound to the sulphur atom.13-15 Reactions (9)-( 11) follow from the fact that in thioamides co-ordination through the sulphur atom always decreases the CS stretching fre- quency and generally increases the CN stretching frequency of the free ligand.l6-18 These shifts indicate that mesomeric structure B is favoured, peImitting an easier release of an amino-proton. CH,C(NH,):S --+ M ++CH,C(:NH,)S-M A B l2 L. E. Sillen and A. E. Martell, Chem. SOC.Special Public- ation, No. 17, 1961. l3 M. Rolies and C. J. De Ranter, Cryst. Struct. Comm., 1977, 6, 157. l4 M. Rolies and C. J. De Ranter, Cvyst. Struct. Comm., 1977, 6, 275.The proposed mechanism leads to the rate equation (16) which is compatible with the orders observed for -k,,’K,K,[MCl+] + kll’K8K,[&~GAc (17) TAA and H30+. It also accounts for the obtained fractional order with. respect to the initial metal ion concentration since (i) it may be expected that the metal ion complexes involved will react with TAA at a different rate and (ii) the concentration ratios of these complexes depend on the initial metal ion concentration. The proposed slow steps (12)-( 14) are bimolecular reactions involving at least one neutral molecule. The rate of such steps would not be expected to be very susceptible to changes in ionic strength. Although variation of the ionic strength has some effect on the equilibria preceding the rate-determining steps, this effect will be small, in the region 0.1-0.4 I,as can be calculated using activity coefficients obtained by the Davies equation.lg To discuss the rate differences between the metal-TAA interactions, Pearson’s hard-soft acid-base (HSAB) principle 2o can be applied.Due to the delocalisation of the x system in the thioamide function, TAA is a soft base with sulphur softer than nitrogen. Con-sequently, borderline and soft acids will co-ordinate through sulphur, as confirmed in the crystallized c~mplexes.l~-~~The metal-sulphur bond strength is expected to decrease from Cd through Pb to Co. Measurements on the stability constants of Cd and Pb thiourea complexes,12 a ligand which is isoelectronic with TAA, show this order.In the present study, however, the hard-soft inter-action of TAA with cadmium and lead ions seems to be inverted. Arguments for this inversion can be found in the different ionic species and concentrations or in the different decomposition rate of the deprotonated inter- mediates. As in the Pb-TAA reaction the initial metal concen- tration is only one-tenth that in the other solutions and as no chloride ions are present, the results of the Cd- TAA and Co-TAA interactions were extrapolated to an l5 C. J. De Ranter and M. Rolies, Cryst. Struct. Comm., 1977, 6, 399. l6 C. D. Flint and M. Goodgame, J. Chem. SOC.(A), 1968, 750. 17 A. J. Aarts, H. 0. Desseyn, and M. A. Herman, Bull. SOG. chim. belges, 1976, 85, 854.18 A. J. Aarts, H. 0. Desseyn, and M. A. Herman, Bull. SOC. chim. belges, 1977, 86, 345. 19 C. W. Davies, J. Chem. SOC.,1938, 2093. 2o R. G. Pearson, J. Amer. Chern. SOC.,1963, 85, 3533. initial cadmium and cobalt chloride concentration of 1.8 x 10-3~. In these acetate buffered solutions, with the metal ion almost entirely present as monoacetate complex, the sequence of reactivity with TAA remains PbOAc+ > CdOAc+ > CoOAc+. Assuming that the acetate ion increases the soft character of the metal ions in the same proportion, the observed inversion can not be explained on the basis of stability constants alone and should result from the greater decomposition rate of PbOAcTAA with regard to CdOAcTAA. Since both hydrogen ion and metal ion are acids, it might be expected that the same mechanism applies on the basis of their catalytic action.This is not so because the hydrogen ion catalysed reaction leads initially to acetamide and thioacetic acid while the metal pro- moted reaction produces acetonitrile. The difference in mechanism probably results from the much smaller electron draining effect toward the side of attachment with H,O+ than with the studied metals. Formation of the enol form of TAA from the S-protonated TAA [reaction (IS)] and subsequently formation of acetonitrile [( 19)] must be less favourable than reactions (9)-( 14). CH,C(NH,):~H+ H,O 'T-CH,C(:NH)SH + H,O+ (18) CH,C(:NH)SH + H20+ CH,CN + HS-+ H,Of (19) 21 A. J. Hall and D.P. N. Satchell, J.C.S. Perkin II, 1974, 1077. J.C.S. Perkin I1 By analogy with the proposed mechanism for thio- benzamide 21 the hydrogen ion catalysed hydrolysis of TAA can be understood by a nucleophilic attack of H,O on the thiocarbonyl carbon with formation of a tetrahedral intermediate. logkobs-pH diagrams for the present and previous data indicate that, if both hydrogen ion and Pb2+, Cd2+, or Co2+ are present, the hydrogen ion catalysed mechanism predominates at pH < 3 while at higher pH the metal ion catalysed mechanism is the more important one. Hall and Satchell studied the reaction between thio- benzamide and mercury( II),~~ and silver(1) 24copper(11),~3 ions in perchlorate medium at pH < 3. The organic product was benzonitrile and the obtained rate equation corresponds in part with equation (16). A mechanism quite similar to that proposed in this work was given. The order of reactivity, Hg > Ag > Cu, is in agreement with the HSAB principle, These results and those obtained in this study show that the order of reactivity of metal ions with a thio- amide group, except for the inversion of Cd and Pb, is in general predictable from the hardness-softness character of the metal ions involved. [6/2352 Received, 30th December, 19781 22 A. J. Hall and D. P. N. Satchell, J.C.S. Perkin II, 1975, 778. 23 A. J. Hall and D. P. N. Satchell, J.C.S. Perkin II, 1975, 953. 24 A. J. Hall and D. P. N. Satchell, J.C.S. Ppvkin TT, 1975, 1273.

ISSN:1472-779X    

DOI:10.1039/P29780000023

出版商:RSC    

年代:1978

数据来源: RSC