THE REACTION OF ACTIVE NITROGEN WITH PROPANE' Can. J. Chem. Downloaded from by on 04/05/18

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1 THE REACTION OF ACTIVE NITROGEN WITH PROPANE' ABSTRACT 'The reaction of nitrogen atoms with propane has been found to produce hydrogen cyanide as the main product, together with smaller amounts of acetylene, ethylene, and ethane, which were recovered at all propane flow rates. Cornplete consumption of nitrogen atoms was not attair~ed at any propane flow rate used at 63OC., but was attained at 250 C. for ratios of propane to nitrogen atonls greater than 1.3. An activation energy of 5.6 f 0.6 kcal. and a steric factor between and lop3 was estimated from second order rate constants. The reaction of active nitrogen with propane has been discussed briefly in a recent review of active nitrogen - hydrocarbon reactions (13). A detailed examination of the reaction products and their dependence on propane flow rate is reported in the present paper. As before, it is assumed that the chemically reactive species in active nitrogen is atomic nitrogen (3,4). EXPERIMENTAL The apparatus used in this investigation was similar to that described in earlier papers from this laboratory (3,4). Preliminary experiments showed that hydrogen cyanide was the major product, and that ethylene, acetylene, ethane, and methane were formed in much smaller quantities. The vapor pressure - temperature relations for acetylene and propane are not sufficiently different at low temperatures to allow their fractionation in the LeRoy distillation apparatus used (6). Therefore, a portion of the unreacted propane was codistilled with ethylene and acetylene at -140 C. until the equilibrium vapor pressure decreased from that of the mixture to that of propane at this temperature. Analysis with a mass spectrometer gave the composition of this combined Cz-C3 hydrocarbon fraction. The remaining propane fraction, containing only a trace of acetylene, was separated from hydrogen cyanide by distillation at -120 C. Hydrogen cyanide was distilled at -70 C. and condensed 011 the surface of 10 ml. of 1 N potassium hydroxide which had been previously degassed and frozen in a detachable trap immersed in liquid nitrogen. The trap, closed with a stopcock, was removed and its bottom end plunged into hot water. This melted the alkali solution, which then dissolved the hydrogen cyanide before it could melt and polymerize. Titration of the resulting solution with standard silver nitrate gave the cyanide content. Analysis of noncondensable products, methane and hydrogen, was not made. The n~olecular nitrogen flow rate was maintained constant at 5.83 X mole per sec.; this gave a pressure of 1.35 f 0.02 and 1.42 & 0.01 mm. of 1 hlanuscript rcccived Decenrbcr 21, Contribution front the Physical Chctnistry Laboratory, McGill Univcrszty, illontreal, Qzre., with financial assistance from thc Natioizal Rescarch Council of Canada. * Holder of a National Research Council Studentship. Holder of a National Research Co~rncil Studentship. Present address: Polymer Corporation, Sarttia, Ont.

2 352 CANADIAN JOURNAL OF CHEMISTRY. VOL. 32 mercury at 63 f 3" and 250 f 5 C. respectively. Experiments were made at each temperature with propane flow rates ranging from 5 X lod7 to 2 X 10-5 mole per sec. From 15 experiments with ethylene at 100" and at 260 C., under experimental conditions corresponding to complete removal of the available nitrogen atoms (ll), the atomic nitrogen flow rate was estimated to be 5.20 f 0.50 X mole per sec. RESULTS AND DISCUSSION The results of the investigation are summarized in Fig..l. Hydrogen cyanide accounted for 90-97% of the propane consumed and the remainder was recovered as ethylene, acetylene, and ethane. The average carbon and hydrogen balances were 101% and 92% respectively TOTAL C, FIG. 1. Rate of formation of various products in the nitrogen at propane reaction, as a function of propane flow rate. The yield of hydrogen cyanide from the reaction at 63 C. increased gradually with increasing propane flow rate, and appeared to pass through a maximum without complete consumption of the nitrogen atoms available. The presence of the maximum may result, in part, from dilution effects which reduce the reaction time significantly at high propane flow rates. Moreover, under these conditions the reaction flame was localized at the top of the reaction vessel where the surface to volume ratio is relatively large and loss of nitrogen atoms or nitrogen atom - propane complexes at the surface might lead to the observed decrease in hydrogen cyanide production.

3 ONYSZCHUK El' :ILL. REA CT1O.V OF NITROGEN WITH PROPrl YE 353 For the reaction at 250 C., on the other hand, the yield of hydrogen cyanide remained constant at 5.2 X mole per sec. beyond a critical propane flow rate of about 7 X mole per sec. This value corresponds to the average atomic nitrogen flow rate determined from the ethylene experiments. It may be concluded that when the flow rate of propane exceeds the nitrogen atom flow rate by a factor of 1.3 or greater, the reaction proceeds to complete removal of the available atomic nitrogen. The curves representing production of C.1 hydrocarbons (mainly ethylene and acetylene) offer further support to the conclusion that nitrogen atoms arc con~pletely consunled at 250' but not at 63'C. Since ethylene and acetylene react rapidly with nitrogen atoms, a continued increase in -the amounts of these hydrocarbons would be expected above the critical propane flow rate. This was realized experimentally for ethylene, but only at the high reaction temperature. On the other hand, if nitrogen atoms are available for reaction at all propane flow rates, the curve for C2 hydrocarbon production should parallel that for hydrogen cyanide formation. This appears to be true for the reaction at 63OC. Of particular significallce was the recovery of ethylene ancl acetylene in the region of incomplete consumption of nitrogen atoms. The reaction of nitrogen atoms with these unsaturates is about ten times more rapid than the reaction with propane, if it is assumed that the initial attack of a nitrogen atoll1 on a hydrocarbon molecule is rate controlling. Since ethylene, acetylene, and propane would be expected to compete for nitrogen atoms according to their relative reactivities, the recovered unsaturates should represent only a fraction of the total amounts formed. Conservative calculations indicate that most of the hydrogen cyanide was probably derived from ethylene and acetylene formed by the primary attack of nitrogen atoms on propane. The general reaction mechanism which has been tentatively suggested for the paraffin hydrocarbon - nitrogen atom reaction (13), and which appears to be satisfactory for the ethane reaction (3), cannot adequately explain the presence of ethylene and acetylene among the products of the propane reaction. The recovery of these unsaturates in appreciable amounts in the region of excess nitrogen atoms implies their formation in a primary reaction step, rather than from secondary reactions involving hydrogen atoms and free radicals. A number of possible reactions might be suggested, but many of these are energetically unfavorable while others involve the improbable formation of "hot" radicals from the collision complex. A thermochemically favorable reaction of a type by which ethylene might be formed by decomposition of the propane-nitrogen atom collision complex would be N + C3Hg -3 Collision complex -+ HCN + CHCH3 + Hz + H. [I] The ethylidene radical previously postulated in the nitrogen atom - propylene reaction (10) and in other reactions (1,2) is thought to transform rapidly to ethylene, CH3CH -+ CH2=CH2 An alternative primary step might produce acetylene directly by a similar exothermic reaction,

4 354 CANADIAN JOL'RiVAL OF CHEMISTRY. VOL. 32 N + CaHs -, Collision complex + HCN + C2H2 + 2H2 + H. 121 Nitrogel1 atom reaction with the ethylene and acetylene produced by reactions [I] and [2] could occur as outlined in previous papers (11, 12). Both reactions [I] and [2] are admittedly inconlpatible with the Rice-Teller principle of least motion (8). However, this principle has been applied to processes that involve energies of about 100 kcal. or less, while in the nitrogen atom reactions, the high energy of formation of the C=N bond ( kcal.) is effectively released within the reaction conlplex and inight result in considerably more intracoinplex rearrangement than would otherwise be expected. The small quantities of ethane detected only in the experiments at 250 C. probably resulted from the hydrogenation of ethylene by atomic hydrogen (7). Second order rate constants were calculated for the propane-nitrogen atom reaction in a inanner similar to that previously outlined for the ethane reaction (3). In Table I are recorded the data obtained for experiments corresponding Propane flow DATA Tetnperalure = 250 f 5OC. TABLE I USED FOR THE CALCULATION OF SECOND ORDER RATE CONSTANTS formed M~le.sec.-~ X 106 Reaction time, sec. Ratio of propane* consu~ned to at. N I I I I I *The amount of propane consumed was calczrlated from the total a?nounl of HCN and C? hydrocarbons prodziccd. to incomplete consun~ption of nitrogen atoms and used for the calculntion of second order rate constants when both streamline and turbulent flow conditions are assumed. The average values of the rate constants, together with the corresponding activation energies, E, and steric factors, P, in the expression k = p2e-"irt are shown in Table 11. The collision number, 2, was calculated using collision diameters of 3.0 A and 4.2 A for atomic nitrogen (5) and pro- k,

5 ONYSZCHUK ET AL.: REACTION OF NITROGEN WITH PROPANE TABLE I1 KINETIC CONSTANTS FOR TFIE NITROGEN ATOM - PROPANE REACTION I I I I k, 1 P 1 E, assumed ~.mole.-~sec.-~ 1 1 kcal. Streamline Turb~llent pane (9) respectively. Actual flow conditions within the reaction vessel probably lie somewhere between the two extremes assumed. ACKNOWLEDGMENTS Grateful acknowledgment is made to Dr. R. A. Back, who concurrently investigated the nitrogen atom reaction with the butanes, for many helpful discussions of the reaction mechanism. The authors are also indebted to Dr. H. I. Schiff of this department for mass spectrometer analyses. REFERENCES 1. BAWN, C. E. H. and DUNNING, W. J. Trans. Faraday Soc. 35: BAWN, C. E. H. and MILSTED, J. Trans. Faraday Soc. 35: BLADES, H. and WINKLEK, C. A. Can. J. Chem. 29: GREENBLATT, J and WINKLER, C. A. Can. J. Research, B, 27: GREENBLATT, J. H. and WINKLER, C. A. Can. J. Research, B, 27: LEROY, D. J. Can. J. Research, B, 28: LEROY, D. J. and I~AHN, A. J. Chern. Phys. 15: RICE, F. 0. and TELLER, E. J. Chem. Phys. 6: TITANI, T. BLIII. Inst. Phys. Chern. Research (Tokyo), 8: TRICK, G. S. and WINKLER, C. A. Can. J. Chern. 30: VBRSTEEG, J. and WINKLER, C. A. Can. J. Cllem. 31: VERSTEEG, J. and WINKLER, C. A. Can. J. Chem. 31: WINKLER, C. A. and SCHIFF, H. I. Discussions Faraday 'Soc. 14: