THE REACTION OF ACTIVE NITROGEN WITH ETHANOL1

Transcription

1 THE REACTION OF ACTIVE NITROGEN WITH ETHANOL1 P. A. GAR TAG AN IS^ Physical Chemistry Defiartment, Ontario Research Foundation, Toronto, Ontario Received May 31, 1963 ABSTRACT The reaction of active nitrogen with ethanol has been investigated in the range 300 to 593 OK using a modified condensed-discharge Wood-Bonhoeffer fast-flow system. The only condensable products found in appreciable amounts were hydrogen cyanide and water. Hydrogen was the main noncondensable product. A very small amount of acetaldehyde was also formed along with traces of ethane, ethylene, methane, acetonitrile, cyanogen, and probably carbon monoxide. The overall activation energy is 3.4 kcal/mole. It is postulated that the mechanism consists of the formation of two fragments NCzH6 and OH, from which the condensable products result as follows: N + CzHsOH + NC2H6 + OH, NCzH6 + CH3 + HCN + H, A number of products found in trace quantities are produced by concomitant reactions of the hydrogen atoms with methyl radicals, and with ethanol as well as by disproportionation of ethyl radicals to produce ethane and ethylene. A preliminary study of the reaction of active nitrogen with isopropanol indicated that the energy of activation is in line with the energies of activation of methanol and ethanol. INTRODUCTION The reaction of active nitrogen with methanol has been discussed in a previous paper (1). The present investigation was initiated as a further step towards the study of the kinetics of the reactions between active nitrogen and various oxygen-containing organic compounds. The reaction with ethanol appeared to be of interest because of the opportunity for comparing the reactivity of the two lower members of the series. Some preliminary experiments were also performed with isopropanol to assess the trend of the reaction of active nitrogen with a higher alcohol. EXPERIMENTAL The apparatus and procedure used in carrying out the reactions were similar to those described by Gartaganis and Winkler (2), except that the spherical reaction vessel was replaced by a 100 cm reaction tube of 16 mm i.d. Thermocouple wells and observation windows were placed at regular intervals along the reaction tube. The products were identified by mass spectrometry and gas phase chromatography (Vapor Fractometer, Model 154, Perkin Elmer Corp. polyethylene glycol on Teflon powder; helium; 80 "C). Hydrogen cyanide was measured with silver nitrate and the water with Karl-Fischer reagent. The ratio of water to hydrogen cyanide was determined by the following method. The reaction products were trapped in methanol and titrated with Karl-Fischer reagent added to the trap through a septum from a microburette ending to a hypodermic needle. Ten milliliters 1 N potassium hydroxide were introduced with a syringe, the trap was then opened, 3 N ammonium hydroxide added, and the hydrogen cyanide determined with silver nitrate as usual. RESULTS The reaction was accompanied by a bright peach-colored flame, similar to those observed with hydrocarbons or methanol. As in the case of the latter compound the reaction of 'Presented at the 46th Annual Conference of the Chemical Institute of Canada, Toronto, June Present address: Dominion Tar E7 Chemical Co. Ltd., Central Research Laboratories, Senneville, P.Q. Canadian Journal of Chemistry. Volume 43 (1965)

2 936 CANADIAN JOURNAL OF CHEMISTRY. VOL active nitrogen with ethanol was found to be relatively simple inasmuch as very few products were produced and there seemed to be no extensive side reactions to conlplicate the kinetic considerations. Of course, intermediate steps of this type of reaction may be kinetically very complex. Hydrogen cyanide and water in approx. 2 to 1 ratio were the only condensable products found in appreciable amounts. Hydrogen was the main noncondensable product. A very small amount of acetaldehyde was formed along with sillall amounts (less than 2%) of ethane, ethylene, methane, acetonitrile, cyanogen, and probably carbon monoxide LO ETHANOL FLOW RATE, pmoles / s FIG. 1. Hydrogen cyanide production as a function of alcohol flowrate in the reaction of active ~litrogetl with ethanol. a, 593 "I<; 0,428 "I<; W, 300 OK. The yield of hydrogen cyanide, the only nitrogen-containing major product recovered at 300,428, and 593 OK at various flow rates, is shown in Fig. 1. Since it is now recognized that the principal chemically reactive constituent present in active nitrogen is the nitrogen atom in the 4S ground state (3), second-order rate constants for the nitrogen atom - ethanol reaction were calculated in a manner similar to that previously outlined for the methanol reaction (I). A Reynolds-number calculation (4) showed that streamline flow prevailed in the reaction vessel and, therefore, the following integrated second-order expression was used for the calculation of the rate constants, assuming that one nlolecule of ethanol yields two molecules of hydrogen cyanide, i.e. assuining n = 1/2. The pertinent kinetic data are shown in Table I. Another series of experiments with a different atomic nitrogen concentration was done earlier and the results agree with the information reported above. A number of exploratory experiments were performed using the same apparatus and experimental conditions to study the general characteristics of the reaction of active nitrogen with isopropanol. The results are presented in Fig. 2 and Table I (in this case

3 GARTAGANIS: REACTION OF ACTIVE NITROGEN WITH ETHANOL 937 n in the reaction rate forillula is l/3). The molecular nitrogen flow rate was 16.3 pmoles/s, and the atomic nitrogen flow rate, determined from the reaction with ethylene at 590 OK, was 1.06 pmoles/s. TABLE I Second-order rate constants at various temperatures and other kinetic data for the reactions of atomic nitrogen with ethanol and isopropanol (pressure, 0.5 mm Hg; atomic nitrogen flow, molecular nitrogen flow, Alcohol HCN flowrate production k Streamline flow Average Temp. E P at (@moles/s) (@moles/s) (l/mole s X log k ("10 (kcal/mole) 500 "K Ethanol Isopropanol 0.22 ISOPROPANOL FLOW RATE, ).Imoles / s FIG. 2. Hydrogen cyanide production as a function of alcohol flowrate in the reaction of active nitrogen with isopropanol. 0, 593 "I<; 0, 300 OK.

4 938 CANADIAN JOURNAL OF CHEMISTRY. VOL. 43, 1965 DISCUSSION From the results of the present investigation, it is apparent that ethanol resembles methanol closely in its primary reaction with nitrogen atoms. The following steps account for the formation of hydrogen cyanide. Ill N + CzHaOH -+ NCzHs + OH, PI NCzHa 4 HCN + CH3 + H, [31 CH3 + N +HCN + 2H. It appears from spin considerationsreferred to earlier (1) that atomic rather than molecular hydrogen is produced by reactions [2] and [3]. Combination of the hydroxyl fragment of reaction [I] with this atomic hydrogen in the presence of a third body produces water: [41 OH + H -+ H20. On the other hand the reaction OH + N 4 HNO does not seem to occur to any appreciable extent. Recombination of hydrogen atoms by three-body collisions with ethanol molecules and with nitrogen (5) provide the only noncondensable product found in large amounts: El H $H +Nz-+Hz+Nz. It was again noticed that the limiting value of the rate of hydrogen cyanide formation does not correspond to the maximum nitrogen atom concentration indicated from the ethylene reaction and the same phenomenon was observed with isopropanol. It is probable that, in addition to the above reactions, homogeneous catalyzed recombination takes place with no product formation. [GI N -I- N + C2H60H -+ Nz -I- CzHbOH, [71 N + NCzHaOH -+ Np + CzHaOH. It should be noted, however, that while reactions [6] and [7] are possible, catalytic recombination has fallen somewhat into disrepute in active nitrogen reactions. The earlier temperature dependence of hydrogen cyanide in the ethylene reaction, for example, now appears to have been an experimental artifact characteristic of the spherical reaction vessels used (7). A reaction similar to eq. [7] has been postulated for ethylene by Kelly and Winkler (6). The formation of the small amounts of secondary hydrocarbon by-products probably resulted from reactions of various fragments with atomic hydrogen or with themselves, viz. PI CH3 + H -+ CHI, [9I CZHS + H -+ [lo] [Ill CZHB, 2CH3 -+ CzHa, 2C2H5 4 C2H4 + CzH6. Conceivably ethyl radicals could be produced from the NCzHs fragment directly or as a result of further attack by atomic nitrogen. [I21 NCzHb 4 N f CPHJ~ 1131 N NCzHr, -+ N2 + CnH6. Reaction [13] seems most unlikely, as there is no reason to expect the NCzH5 complex to have an appreciable existence before decomposing into hydrogen cyanide plus fragments.

5 GARTAGANIS: REACTION OF ACTIVE NITROGEN WITH ETHANOL 939 Finally, hydrogen abstraction by hydrogen atoms or methyl radicals could account for the production of traces of acetaldehyde from ethanol. CONCLUSION As a result of the present investigation it can be concluded that ethanol molecules react with atomic nitrogen by C-0 bond scission to form radicals that produce in a few simple steps hydrogen cyanide, water, and hydrogen, the principal products of this reaction. Some "wastage" of nitrogen atoms takes place and is probably due to homogeneous catalyzed recombinations, the alcohol and molecular nitrogen acting as the third body. The energy of activation for methanol, ethanol, and isopropanol reactions was calculated between 3.2 and 3.5 kcal/mole. ACKNOWLEDGMENTS The author is indebted to Dr. A. G. Harrison for mass spectrometric analyses, and to Messrs. M. J. Sole and S. M. Yee for assistance in the experimental work. REFERENCES 1. M. J. SOLE and P. A. GARTAGANIS. Can. J. Chem. 41, 1097 (1963). 2. P. A. GARTAGANIS and C. A. WINKLER. Can. J. Chem. 34, 1457 (1956). 3. G. G. MANNELLA. Chem. Rev. 63, 1 (1963). 4. J. R. PARTINGTON. An advanced treatise on physical chemistry. Vol. 1. Longmans, Green 5. H. GESSER, C. LUNER, and C. A. WINKLER. Can. J. Chem. 31, 346 (1953). 6. R. KELLY and C. A. WINKLER. Can. J. Chem. 37, 62 (1959); 38,2514 (1960). 7. A. N. WRIGHT, R. L. NELSON, and C. A. WINKLER. Can. J. Chem. 40, 1082 (1962). Co.