J. Org. Chem. 1984,49,863-866
863
Kinetics and Mechanism of the Addition of Amines to Carbon Disulfide in Ethanol Enrique A. Castro,* Sergio A. Pefia, Jos6 G. Santos,* a n d J u a n C. Vega Facultad de Qdmica, Pontificia Universidad Catdlica de Chile, Casilla 114-0,Santiago, Chile
Received July 6, 1983 The reactions of morpholine, benzylamine, and pyrrolidine with carbon disulfide are studied in ethanol, at 25 "C, ionic strength 0.0024 M (maintained with lithium chloride). The reactions are found to proceed through the dithiocarbamic acid intermediqte 1, which yields the dithiocarbamate anion 2 by proton loss to the amine. The reactions of the substrate with morpholine and benzylamine exhibit reversibility, due to protonation of 2 (to yield 1) by the corresponding conjugate acid of the amine. Also, a parallel reversible path between reactants and 2 is found for the latter reactions. This is interpreted as ethoxide anion catalysis of amine attack in the forward step and general-acid catalysis by ethanol in the reverse one. In the reaction with pyrrolidine, the formation of 1 is the rate-determining step, and 1 gives 2 irreversibly. No catalysis by ethoxide anion is found for this reaction. Pyrrolidine is ca. 200-fold more nucleophilic than piperidine toward carbon disulfide. Comparison of this value with the ones toward other substrates is made. Tetramethylenedithiocarbamateis found to be much more stable than the piperidino analogue toward decomposition. The stabilities of both are analyzed and compared with those shown by other compounds possessing the two amino moieties.
Introduction
All reactions were found to follow pseudo-first-order kinetics. The total concentration of the amine (free base plus cationic forms) was at least 20-fold in excess over that of the substrate. The pseudo-first-order rate constants observed (kobsd) were determined by means of "infinity" plots, except in the slowest reactions where Guggenheim plots were employed. The plots remained linear for at least three half-lives, and those resulting with correlation coefficients worse than 0.999 were discarded. In order to be sure that equilibrium was reached and carbon disulfide neither escaped nor was oxidized, tightly stoppered cells (Hellma 110-QS)were used, and some of the slow reactions were followed for 10 half-lives. No noticeable absorbance changes were found after 6 half-lives. The experimental conditions of the reactions and the kobd obtained are shown in Table I. Product Studies. The ammonium salts of oxydiethylene-, benzyl-, and tetramethylenedithiocarbamates were found to be the stable products of the kinetic reactions. This was achieved by comparison of the UV spectra of the kinetic solutions at infiiity time with those of authentic samples (prepared as in Materials) in the same experimental conditions of the kinetic reactions.
Although t h e kinetics of decoinposition of dithiocarbamates in aqueous solution has been subject t o much s t ~ d y l (eq - ~ 1, where R' or R2 can be a hydrogen atom),
R1R2NCS2-
H+
R1R2NH+ CS2
little is known about t h e reverse reaction, i.e., addition reactions of amines to carbon disulfide. We are aware of only a few works on the latter reactions,2q6 with two of them involving amino acids as nucleophiles.6 T h e influence of t h e solvent o n t h e mechanism of both steps has received m u c h less attention, t h e decomposition of dithiocarbamates being studied only in methanol-water3v4 a n d dioxane-water3 mixtures. Following previous work in our laboratory on t h e reaction of carbon disulfide with piperidine in ethan01,~we report in this paper t h e reactions of t h e same substrate with pyrrolidine, morpholine, and benzylamine in ethanol in a n a t t e m p t to shed more light on t h e mechanism of amine addition t o carbon disulfide in this solvent.
Results
Experimental Section Materials. Pyrrolidine, morpholine, and benzylamine were distilled over potassium hydroxide. All other reagents were purified as previously de~cribed.~ The ammonium salts of tetramethylene-, oxydiethylene-,and benzyldithiocarbamateswere prepared by a standard proced~re.8*~ The IR and 'H NMR spectra of these compounds were in good agreement with their structures. Kinetic Measurements. The kinetics were followed by recording the dithiocarbamate increase at 290-294 nm by means of a Pye Unicam SP 1800 spectrophotometer, as previously described.' (1) Zahradnik, R.; Jensovsky, L. Chem. Listy 1954,48,11. (2) Miller, D. M.; Latimer, R. A. Can. J. Chem. 1962,40, 246. (3)Joris, S.J.; Aspila, K. I.; Chakrabarti, C. L. J.Phys. Chem. 1970, 74, 860. (4)De Filippo, D.; Deplano, P.; Devillanova, F.; Trogu, E. F.; Verani, G. J. Org. Chem. 1973,38, 560. (5)Ewing, S.P.;Lockshon, D.; Jencks, W. P. J.Am. Chem. SOC.1980, 102,3072. (6)Zahradnik, R.Chem. L k t y 1956,50,1885.Leonis, J.; C. R. Trau. Lab. Carlsberg,Ser. Chim. 1948,26,316;Chem. Abstr. l949,43,2247g. (7)Castro, E.A.; Cortgs, R.; Santos, J. G.; Vega, J. C. J. Org. Chem. 1982,47, 3774. (8)Cook, A. H.;Levy, A. L. J. Chem. SOC.1950,642. (9)Holland, D. 0. J. Chem. SOC.1950,2134.
0022-3263/84/1949-0863$01.50/0
T h e reactions of amines with carbon disulfide under the conditions of t h e present work are subject t o t h e rate law given by eq 2, where k,,m and kN are the pseudo-fist-order
and the second-order rate constants observed, respectively, ko is t h e rate constant in t h e absence of amine, a n d N represents t h e amine free-base form. The values of ko and kN were obtained as the intercept and the slope, respectively, of linear plots of kOM vs. molar concentration of amine free base, at constant molar fraction of t h e free base. Table I1 shows the values of K O a n d kN obtained for t h e reactions of t h e three amines under study. I n the reactions of t h e substrate with benzylamine a n d morpholine, ko and kN exhibited a dependence on t h e free-amine fraction. Plots of k0 vs. [NH+]/[N] a n d kN vs. [N]/ [NH+] (where NH+ represents the conjugate acid of the amine) gave straight lines with correlation coefficients 0.996-0.9996. T h e reactions of the substrate with pyrrolidine did not show any dependence of kN on the free-amine fraction and t h e value of k , was too small t o be measured (Table 11). T h e final value of kN was obtained, in this case, as the slope of a plot of kOM vs. [N], including all the runs a t the three
0 1984 American Chemical Society
864 J . Org. Chem., Vol. 49, No. 5, 1984
Castro e t al.
Table I. Experimental Conditions and kob& for the Reactions of Carbon Disulfidea with Amines in Ethanol at 25 '(2, Ionic Strength 0.0024 M (Maintained with Lithium Chloride) 103[~lt0t,b free-basec number amine M fraction i03kobd, s-1 of runs morpholine 5.8-9.6 0.813 3.45-3.88 16 1.9-9.6 0.873 2.44-3.06 17 1.9-9.6 0.935 11 1.55-2.67 benzylamine 2.4-4.8 0.620 5 0.08-0.13 3 2.4-4.8 0.750 0.09-0.15 2.9-4.8 0.875 4 0.11-0.18 pyrrolidine 1.2-4.8 0.490 1.88-9.73 5 1.2-4.3 0.620 4 2.60-10.28 1.2-4.8 0.745 5 2.70-13.70 a The initial substrate concentration was (0.5-1.5) x Range of concentration of total amine (free-base plus M. acidic forms). Molar fraction of amine free base from the total amine.
Table 11. Values of kN and k , a t Various Free-Amine Fractions for the Aminolysis of Carbon Disulfide in Ethanol a t 25 "(2, Ionic Strength 0.0024 M (Maintained with Lithium Chloride)' free-base amine fraction kN, s - l M - l k,, s-' morpholine 0.813 4.7 x 3.4 x 10-3 2.4 x 10-3 0.873 7.4 x 10-2 0.935 14.5 X 10.' 1.2 x 10-3 benzylamine 0.620 3.3 x 3.5 x 10-5 1.9 x 1 0 - 5 0.7 50 3.6 X 0.875 4.3 x 2.8 x 10.6 pyrrolidine 0.490 4.4 C 0.620 4.0 C 0.745 4.1 C 4.0 = 0.4b a The rate constants were obtained as slopes and intercepts of plots of eq 2 at a given free-amine fraction by least-squares analysis. Value obtained as slope of a plot of eq 2, which included all the points (for the three freeamine fractions) by least-squares treatment. The error is the standard deviation. Negligible values under the experimental conditions.
different free-amine fractions (14 points). The final value of kN is also shown in Table 11. The reactions of the substrate with morpholine and benzylamine exhibited reversibility as evidenced by the shapes of the absorbance a t infinity time (A,) vs. [N] curves for these reactions at constant initial substrate concentration. The shapes of these curves are in accord with an equation obtained in the reactions of the substrate with piperidine in the same experimental conditions as in the present reaction^.^ Considering the dependence of the rate constants observed on the free-amine concentration and on the amine fractions and the dependence of A , on these variables, a scheme of reactions can be envisaged for morpholine and benzylamine (Scheme I). This is the same scheme as the one found in the reactions of the substrate with piperidine.'
Scheme I
s=c =s
1
2 k { = kl [HNRlR']
k;
k2 [HNR'R']
k-2
k-2[H;NR1R2]
k j = k3 k13
[EtO-] [HNR'R']
k,[EtOH]
From the application of the steady-state approximation to the dithiocarbamic acid intermediate 1, together with the assumption that k-l
