N-Chlorination of secondary amides. I. Kinetics of N-chlorination of N-methyl acet amide

Transcription

1 NChlorination of secondary amides. I. Kinetics of Nchlorination of Nmethyl acet amide M. WAYMAN AND E. W. C. W. THOMM Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto 5, Ontario Received November 1, 1968 The rate of Nchlorination of Nmethyl acetamide has been studied in acetate buffered hypochlorous acid solutions. The reaction rate is dependent upon ph, chloride ion concentration, and acetic acid concentration. The results fit the following rate expression d [CH3CO.NCl.CH3l/dt = kobs[ch3c0.nh.ch3] [HOCI] The observed rate constant, kobs, has three components The results suggest that the active chlorinating species in the ph range 3.G6.0 are HOCI, Clz, and CH,COOCI, in order of increasing reactivity. The values of the corresponding rate constants at 25 "C are k, E 0.1, kc, E 5.0 x lo3, and k,coc, 5.8 x lo5 1 mole' min', the last two rate constants being related to k, and k3 through the respective equilibrium constants. At temperatures near room temperature, the Nchlorination reaction has an apparent activation energy of 14 kcal/mole when Clz is the active species, and 5 kcal/mole when CH3COOC1 is the chlorinating agent. Canadian Journal of Chemistry, 47, 2561 (1969) Introduction The kinetics of Nchlorination of Nmethyl acetamide in hypochlorous acid solutions has been previously studied over a narrow range of conditions. Soper and coworkers (1, 2) investigated the Nchlorination of the amide by hypochlorous acid in aqueous solutions, buffered with acetate solutions containing acetic acid up to 0.12 M at 25 "C, ph 5 and 6. Mauger and Soper (1) proposed that the active chlorinating agents under their conditions were hypochlorite ion and acetyl hypochlorite. The specsc rate constant for acetyl hypochlorite, however, was not obtained. The role of chlorine in the chlorination of Nmethyl acetarnide was not determined. Recently, Wayman et al. (3) have reported the development of chlorine exchange resins which. contain arnide functional groups, of which the nitrogen atoms take up positive chlorine reversibly. In order to shed more light on this Nchlorination process, it was decided to reinvestigate the Nchlorination of Nmethyl acetamide in the ph range 35, in which the resins are most useful. Accordingly, the results extended those of Soper and coworkers. N Methyl acetamide was chlorinated in hypochlorous acid solutions buffered with acetic acid sodium acetate solutions. The effects of ph, chloride ion concentration, acetic acid concentration, and temperature were investigated. Experimental (a) Reagents NMethyl acetamide was purchased from Eastman Kodak Co. It was distilled under reduced pressure in a nitrogen atmosphere before use. The undistilled amide showed no significant difference from the distilled product in the kinetic runs, and later experiments were carried out using the amide as purchased. Sodium hypochlorite was purchased from Fisher Scientific Co., as a 5% solution. The chloride and hydroxide contents of the solutions made up from this concentrated solution were determined before use. Hypochlorous acid was prepared by acidifying the hypochlorite solutions. Other reagents were British Drug Houses "AnalaR grade chemicals. (6) Experimental Procedure Kinetic runs were carried out according to Mauger and Soper (1). A brief outline is presented below. Hypochlorous acid and Nmethyl acetamide solutions of twice the required concentration were prepared in acetic acid sodium acetate buffers, to which sodium chloride had been added. The acetic acid concentration was corrected for the hydroxide content of the sodium hypochlorite stock solution. The chloride concentration was determined by a Mohr titration. When both solutions had attained the temperature of the thermostat, which was held to k0.02 "C, 100 ml of the hypochlorous acid solution was added to an equal volume of the amide sblution. The ph of the reacting solution was frequently measured during the course of the reaction. The NchloroNmethyl acetamide content of the reaction mixture was determined at suitable intervals by pipetting 5 ml portions into 5 ml of saturated, freshly prepared phenol solutions and stirring for 30 s. This was followed by the addition of 5 ml of 3 N acetic acid, and then 1 g of potassium iodide. The liberated iodine was

2 2562 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 titrated with standardized sodium thiosulfate solutions. The combined hypochlorous acid plus chloroamide content was determined by the same procedure but omitting the addition of the phenol solution. It was verified by determination that the chloroamide did not react with phenol to any noticeable extent in the duration of the analysis. The decomposition of the hypochlorous acid was found to be negligible during the time of the reaction. Results of a typical run Titre of Total Time (min) NCI (ml) titre (ml) PH NOTE: Reactionconditions: 25% [CH,COOH] = 0.25 M, [ClI = M, [CH,C0.NH.CH3], = M, INa2S203] = N. (c) Treatment of Data All results fit the following rate expression Thus, kabs was calculated from the best straight line of the plot of log [CH,C0.NH.CH3]/[HOCI] vs. time. Results and Discussions The observed rate constants, k,,,, at different ph, acetic acid concentration, and temperature: are presented in Figs. 1 5 as a function of chloride concentration. It was found that k,,, is a linear function of [Cl1. Furthermore by comparing the slopes Akobs/AIC1] in Figs. 1,2, and 3, it can be readily seen that k,,, is also a linear function of the hydrogen ion concentration, as shown in Table I. By extrapolating the lines in Figs. 15 to zero chloride concentration, rate constants, k,, are TABLE I Variation of "observed" rate constants, kobs, with chloride concentration at 25 "C ph MobslA[c1 1 (1' mole' mini) [GI] (mole /I obtained, and have been presented in Fig. 6 as a function of the acetic acid concentration of the buffer. As can be seen, ko is a linear function of the acetic acid concentration, and is independent of hydrogen ion concentration. The extrapolated ko values at ph 3, 4, and 5 agree very well with the observations at ph 5 and 6 reported by Mauger and Soper (1). The rate constant ko is primarily the contribution to the total rate constant, kobs, by the active species formed by reaction between hypochlorous acid and acetic acid, and is independent of [Cl] and ph, plus a small residual value k,. As can be seen in Fig. 6, the ko vs. [CH,COOH] lines do not go through the origin, but result in this residual value. The agreement between the ko values at 25" and the data of Mauger and Soper obtained in the absence of chloride lends support to the validity of the extrapolation. The total rate constant k, can be broken up into three components: (i) the residual part that is independent of hydrogen ion and chloride ion

3 WAYMAN AND THOMM: NCHLORINATION OF SECONDARY AMIDES. I 2563 [CH3COOH].M B 0.10 D 0.25 SLOPE findings. If positive chloride ions were so formed, then k, would be a function of the hydrogen ion concentration. This has not been found over a 1000fold variation of [H'] (ph 36). Similar argument can be presented against the following equilibrium HOCl =+ CIC f OH Thus, under these conditions, we believe that molecular hypochlorous acid is the most likely chlorinating agent HOCl f CH3CO.N / T 70 concentrations; (ii) the contribution from the.? involvement of hydrogen ions and chloride ions in the reaction; and (iii) the contribution due to 60 acetic acid. E Thus 50 and ko = kl t( k3 [CH3COOH] \ [CH~GOOH],M SLOPE In the absence of CI and CH,COOH, the A 0.20 rate of Nchlorination of the amide is slight. The chlorinating species may be hypochlorous 20 B 0.25 acid itself or its dissociation products. At these ph values, the proportion of OCI is very small. I0 It was proposed by de la Mare et al. (4) that the positive chloride ion, C1+, is a possible chlori 0 nating species C~OH + H+ =+ C~OH~+ = CI+ + HZO [CI] x lo3 (rnole/l I However, this is inconsistent with the present FIG. 3. kob, at p~ 3.00, 298 OK.

4 2564 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 with the rate law expression where khocl = kl. The second term in the k,,, expression suggests that, in the presence of hydrogen and chloride ions, molecular chlorine is formed, and that it is a much more powerful Nchlorinating agent than HOCI. In a number of chlorination studies of aromatics, molecular chlorine has been found to be an effective chlorinating agent (510). The chlorination reaction proposed is as follows CI kc1 / C12 + CH3C0.NH.CH3 2.CH3C0.N \ + HCI and the rate law expression is \ CH3 C A [ch~cooh] M SLOPE A B L [Ctl$OOH].M A 0.05 B 0.10 C 0.15 D 0.25 SLOPE The chlorine concentration in the reacting mixture is such that it obeys the following equilibrium, which is quickly established H+ + C1 + HOCI e CI2 + H20 with the equilibrium constant By substitution, the above rate law ex'pression becomes d[ch3c0.ncl.ch3] = dt kc,,kc,2~h+l[c1l x [HOC.I][CH3C0.NH.CH3] where k~l~kc1~ = k2 The formation of chlorine has been suggested by de la Mare, Ketley, and Vernon (6) via the following scheme H+ + HOCI S CIOHz+ CIOH2+ + Cl S Clz + H20

5 WAYMAN AND THOMM: NCHLORINATION OF SECONDARY AMIDES. I TABLE I1 Nchlorination by molecular chlorine T (OK) Slope, kobrltc1 I [H'I (1' minl) (mole 11) lo' 'Data from ref. 11.?kc12 = kab,/~+iic1l Kclz. T K SLOPE A B C DATA OF MAUGER a SOPER FIG. 6. ko vs. [CH3COOHl. In our work we have no evidence bearing on the species ClOH,'. A mass balance on the C1,chlorination process indicates that there is no net change in hydrogen ion concentration. This was found to be so, as the ph of the reacting mixtures remained constant through the course of the reaction, even when the buffer concentration was very low, The slopes Ako,,/AIC1] of the lines in Figs. Kc!2* (mole I') ko2f (mole 1I minl) 3.05~ lo3 2.03~ lo3 2.1 x lo3 4.9 xi x lo3 4.9 xlo3 2.1 x lo3 5.1 x103 Ave. 5.0 x lo3 1.4 x x are equal to k, [H' 1. Thus kc,, can be calculated bv The calculated results are presented in Table 11. In Fig. 7, kc12 is also plotted against the inverse of temperature. From the slope of the line, the activation energy of the Nchlorination process by molecular chlorine was calculated to be 14.1 kcal/mole. FIG. 7. log kc12 and log kacocl vs. 1/T.

6 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 TABLE III Chlorination by chlorine acetate Slope, T MolA [AcqHl [H20 I ~ACOCI'~ CK) (I2 mole2 mine') (mole I') Kacocl (mole I' min ') In a mixture of HOCl and acetic acid, an intermediate is formed which serves to be an effective chlorinating agent. Various authors (1, 2, 7, 9, 10) have proposed acetyl hypochlorite (chlorine acetate) formation [CH3C0.0C1][H20] K ~ c ~ ~ l = [CH3COOH] [HOCI] Stanley and Shorter (7) reported that there is considerable combination of HOCl and acetic acid in 75 % aqueous acetic acid solutions even though de la Mare and coworkers reported a small KAcoc, value of at 25 "C (9). The present investigation lends support to the chlorination process by chlorine acetate with the corresponding rate law expression By substitution, the preceding rate law expression becomes where It can be seen also that k3 is equal to the slope Ak,/A[CH,COOH] of the k, vs. [CH,COOH] lines in Fig. 6, and kacocl can be calculated as At 15 "C, KAcocl was determined to be in acetic acid medium by employing the methods of de la Mare et al. (9). If we can assume that the equilibrium values are independent of acetic acid concentration, then these values can be applied to an aqueous medium. Table I11 summarizes the values of KAcocl as well as those of kacocl. The kacoc, values are also plotted against 1/T in Fig. 7. From this curve, it can be seen that the Nchlorination of the amide by chlorine acetate requires a relatively small activation energy of about 5 kcal/mole. While the principal assumption made, namely that the value of KAcocl determined in concentrated acetic acid is valid in dilute acetic acid, is reasonable, it has not been proved and this calculated activation energy should be regarded with caution. Thus, the Nchlorination at near room temperatures of the amide in aqueous acetic acid solution by chlorine acetate has a lower apparent activation energy than that by molecular chlorine, and has a rate constant about 100 times as great. Hypochlorous acid is rather unreactive by comparison. Similar conclusions have been reported by de la Mare et al. (10) in a study of the chlorination of toluene.

7 WAYMAN AND THOMM: NCHLORINATION OF SECONDARY AMIDES. I 2567 Acknowledgments J. Chem. Eng. 46, 282 (1968). 4. P. B. D. DE LA MARE, J. T. HARVEY, M. HASSAN, and This work was supported in part by the S. VARMA. J. Chem. Sot (1958). Defence Research Board of Canada (Grant No. i: g: z; ~ ~ ~ ~ ~ k ~ A A. ~ ~ ~ b ). One of us (E.W.C.W.T.) would like to VERNON. J. Chem. Soc (1954). thank the National Research Council of Canada 7. G.STANLEY and J..SHORTER. J. (3m1. Sot. 246 (1958). for a Scholarship awarded since P. B. D. DE LA MARE and M. HASSAN. J. Chem. SOC (1958). 1. R. P. MAUGER and F. G. SOPER. J. Chem. Soc P. B. D. DE LA MARE, I. C. HILTON, and C. A. (1946). VERNON. J. Chem. Soc (1960). 2. C. R. EDMOND and F. G. SOPER. J. Chem. Soc P. B. D. DE LA MARE, I. C. HILTON, and S. VARMA. (1949). J. Chem. Soc (1960). 3. M. WAYMAN, H. SALAMAT, and E, J. DEWER. Can. 11. A. A. JAKOWIN. 2. Physik. Chem. 29, 613 (1899).