E. R. ZABOLOTNY AND H. GESSER
854
creases. This suggestion is supported by the fact that a t 20' yv for polyethylene oxide dimethyl ether 400 in which the hydroxyl groups are entirely absent is only 15.8 atm. deg.-l whereas that for polyethylene oxide 400 is 17.5 atm. deg.-l. Furthermore the limiting value to which yv for polyethylene oxide decreases a t any temperature is about the same as the limiting value to which it has been estimated that yv for polyethylene oxide dimethyl ether increasesaB The Entropy of Melting at Constant Volume.By interpolation from Table I the value of yv for high molecular weight polyethylene oxide a t its normal melting point (66O)* is 13.3 atm. deg.-l. Substitution of this value in eq. 1 together with the = 5.85 cal. deg.-l mole-l and AB, values (AS,), = 5.1 cc. mole-l gives (ASm)v = 4.22 cal. deg.-l per mole of repeating units. This is equivalent to (ASm)v = 1.41 cal. deg.-l per mole of chain atoms. Starkweather and Boyd2have described a method of calculating the entropy of melting of linear polymers from a lattice model. They write the entropy of melting a t constant volume per mole of chain atoms as (AS& = ASR 4- ASD = (A&
- 0.86R) f
Asn
(2)
The rotational isomerism contribution ASR is written as the sum of the gain in entropy due to rotational isomerism without regard to the lattice interferences (AS,) and the decrease per bond due to packing on the lattice (0.86 R ) . ASD is a term added to allow for the over restrictiveness of a lattice treatment and is called the long range disorder contribution. A reasonable value of ASD would be of the order of R cal. deg.-l per mole of chain atoms.
TTol.66
The authors have applied their theory t o polyethylene and polymethylene oxide. I n both these polymers the chain bonds are all alike, either C-C or C-0. But in polyethylene oxide there are two C-0 bonds and one C-C bond in each repeating unit. If the difference in potential energy between gauche and trans forms for the C-C bond is the same in polyethylene oxide as in polyethylene then at the melting temperature of polyethylene oxide (339OK.) the theory gives AS, = 1.57 cal. deg.-l per mole of C-C bonds. Similarly if the difference between gauche and trans forms for the C-0 bond is the same in polyethylene oxide as in polymethylene oxide then at the melting temperature of polyethylene oxide ASg = 0.89 cal. deg.-l per mole of C-0 bonds. Hence for the repeating (2 X unit in polyethylene oxide Ax, = 1.57 0.89) = 3.35 cal. deg.-l per mole of repeating units. This is equivalent to AS, = 1.12 cal. deg.-' per mole of chain atoms. When this value is used for AS, the value of ASD calculated by difference from eq. 2 is 2.0 cal. deg.-l per mole of chain atoms. The theory applied in this way thus gives a reasonable account of the entropy of melting of polyethylene oxide. According to this analysis the gain in entropy per chain atom due to rotational isomerism on melting is lower for polyethylene oxide than for either polymethylene oxide or polyethylene. The low value for polyethylene oxide is in agreement with the observation of DavisonQfrom infrared studies that the configuration of the O.(CH&O group in polyethylene oxide is substantially the same in the molten and crystalline states, so that only a limited amount of rotation must occur on melting.
+
(9) W. H. T, Davison, J. Chem. Soc., 3270 (1955).
THE REACTION OF ACTIVE NITROGEN WITH SIMPLE HYDROCARBONS" BY E. R. ZABOLOTSYAND H. GESSER Parker Chemistry Laboratory, University of Manitoba, Winnipeg, Canada Received October 8, 1961
The reaction of methane with active nitrogen produced by passing both nitrogen and nitrogen-argon mixtures through a condensed discharge was studied io an unheated reaction vessel. The reactions of ethane and acetylene with active nitrogen from the nitrogen-argon system also were investigated. Hydrogen cyanide and ammonia were found as products. It is concluded that excited argon atoms increased the chemical reactivity of active nitrogen by augmenting the percentage of excited species in active nitrogen.
The results of the active phosphorus-hydrocarbon Introduction The reactions of active nitrogen with a number reactions3 investigated in this Laboratory also led of hydrocarbons have been studied by many in- to the expectation that ammonia is an intermediate vestigators, in particular Winklerl and co-workers. product which might be found in large quantities The studies to date have for the most part utilized if the reactions were carried out at low nitrogen active nitrogen from a discharge through nitrogen. concentrations. Hydrogen cyanide generally is reported t o be the Experimental only major nitrogen containing compound in these The apparatus was essentially that described by Winkler reactions. Dewhurst,2however, found that signifi- and co-workers,4 the difference being that the discharge cant quantities of ammonia also were present. tube and the reaction system, a spherical 500-cc. bulb, were (*I The research for this paper was supported by the Defence Research Board of Canada, under grant number 95-30-28, Project D76-95-30-28. (1) H.G. Evans, G. R. Freeman, and C. A. Winkler, Can, d , Chem., 84, 1271 (1956). (2) H. A, Dewhurst, J . Phys. Chern., 68, 1976 (196Q)4
not poisoned. I n most of the experiments argon, a t a floiv rate of 3.0 X 10-3 mole/min., was used as a carrier gas to (3) E. R. Zabolotrip add H. Gesser, J. Am. Chem. S O ~ .81, , 6051 (1959). (4) D. Ai drmstrong Bnd 6.PI. Winkler, J. Phgs. Chem.4 60, 1100 fl966)i
May, 1962
855
REACTION OF ACTIVENITROGEN WITH SIMPLE HYDROCARBONS
sweep different concentrations of nitrogen through the discharge. All reactants were fractionated by bulb-to-bulb distillation. The condensable products and reactant were separated by a LeRoy still; the temperature was varied so that the substance being distilled had a vapor pressure of about 20 p . The amounts of reactant and products were measured in a combined Toepler pump and gas buret. Ammonia, identified by its infrared spectrum, was found to be associated with hydrogen cyanide as ammonium cyanide a t low temperatures and was distilled at -85”. At room temperature the ammonium cyanide was an equimolar mixture of ammonia and hydrogen cyanide.
50
2 45 x 40
.$ 35
% 30 “0 25
E
$20 3 15
Results Methane, ethane, and acetylene all were reacted with active nitrogen in an argon atmosphere. I n each case hydrogen cyanide and ammonia were found as the major gaseous products with little or no polymer formation. Trace amounts of ethane were found when methane was used as the reactant and correspondingly small amounts of butane and cyanogen were found when ethane was reacted. On the basis of the hydrogen cyanide formed, about 5% of the methane reacted a t 27’ when nitrogen was used without a carrier gas. The molecular mole/min., cornitrogen flow rate was 1.6 X responding to a reaction zone pressure of 1.30 mm. When argon was used as a carrier for nitrogen flowing at a rate of 33 X mole/min. (Ar/N2 > loo), to obtain a reaction zone pressure of 2.23 mm., the rate of formation of ammonia was increased fivefold and the yield of hydrogen cyanide was approximately doubled in comparison with experiments using pure nitrogen. The rate of production of hydrogen cyanide increased rapidly with increase in methane flow, but the ammonia remained relatively constant. The increase in the rates of formation of products in the argonnitrogen system might be attributed t o the fact that argon decreases the rate of recombination of nitrogen atoms thus effectively increasing the nitrogen atom concentration. However, the nitrogen atom flow rate in the nitrogen system (> 100 X mole/min.) as determined by the maximum yield of hydrogen cyanide from ethylene was greater than twice the molecular nitrogen flow rate in the nitrogen-argon system. Since blank experiments using only argon in the discharge indicated no hydrocarbon decomposition, the increase in the rates of formation of products in the argon-nitrogen system, as compared to the pure nitrogen system, must be due to collisions between excited argon atoms and nitrogen atoms and molecules. The results are shown in Fig. 1. Similar results, shown in Fig. 2, were obtained when the methane flow was kept constant a t about 20 X mole/ min. and the moleculBr nitrogen flow in the argon stream was varied. The pressure in the reactiorjt system was 1.45 mm. The increrhse in hydrogen cyanide production with increase in molecular hitrogen coneentration can be interpreted as POportional to the increase in the excited species concentration. One reason that the rate of production of ammonia was constant may be the fact that, a t least to a smaller extent, ammonia itself is used up as a r e a ~ t a n t . ~Thus the rate of production of am-
8 10 5
0 5 10 15 20 30 40 50 60 70 80 Methane flow, moleslmin. X 106. Fig. 1.-Relation between hydrogen cyanide and ammonia production with methane flow rate: m, HCN (arHCN (pure nitrogen system); gon-nitrogen mixture); 0, 0 , NH, (argon-nitrogen mixture); 0, NHs (pure nitrogen system).
-2
E$
:K,
,
,
,
,
,
,
0
10 20 30 40 50 60 70 80 Nitrogen flow, moles/min. x 106. Fig. 2.-Relation between product production and nitrogen flow rate in the reaction of methane with active nitrogen. m -HCN 0-NH,
1 2 3 4 5 Argon pressure (mm.). Fig. 3.-Relation between product production and reaction zone pressure for the reaction of active nitrogen with methane. 0
monia observed experimentally may be called an “equilibrium rate.” At flow rates of molecular nitrogen of about one pmole/mb., abmt 40% of the aitiogen reacted to form smmonia and hydrogen cyanide. When active nitrogen from the argon-nitrogen discharge was passed over ammonia, ethane, and ethylene a t -196’ in the “product trap,” no decomposition was a k v e d . I n order to find the optimum conditions for the production of ammonia both nitrogen and mrthane
E. R. ZABOLOTNYAND H. GRSSER
856
5 60
CH3
x 50 d
*g 40
2
4 30 E B 20
-
s g 10 & 5
10 15 20 25 30 35 40 45 50 Reactant flow, moles/min. X lo4. Fig. 4.-Relation between product production and hydrocarbon flow rate. Reactant HCN “3 0 CzHz 0 CzHs rn 0 0
respectively) were kept constant and the reaction pressure was varied by adjusting the flow of argon. The results are illustrated in Fig. 3. The yield of ammonia decreased much less rapidly with increase in pressure than did the yield of hydrogen cyanide. The maximum rate of hydrogen cyanide production may be taken to correspond to the maximum excited species concentration. The destruction of ammonia produced in the reaction of course also would be a maximum a t this point. The yields of ammonia and hydrogen cyanide for the active nitrogen reaction with ethane and acetylene are given in Fig. 4. The reaction zone pressure was 2.23 mm. and the molecular nitrogen flow rate was 33 X 10-6 mole/min. Lower hydrogen cyanide yields in the case of acetylene indicates that there was more polymer formation than in the methane and ethane reactions. I n the ethane reaction about 70% of the ethane destroyed was recovered as hydrogen cyanide but only 50% of the acetylene destroyed was recovered as hydrogen cyanide.
Discussion Since ammonia is formed as a product from the active nitrogen reaction with acetylene it would seem likely that one of the primary processes is a hydrogen atom abstraction to form an NH radical. The reaction CHr N 3 CHs N H (1) has a minimum activation energy of 14 kcal. for ground state nitrogen atoms. It therefore becomes necessary to assume that either excited nitrogen atoms or molecules are responsible for the initial reactions. Excited atoms, which are known to exist in active nitrogen in very small concentrations,~Jmay react with the hydrocarbon as*
+
CHI
+
+ N (ID) +CHa + NH + 40 ked. CHs + CH, +CpHa
(2) (3)
(6) H. P. Broida and 0. 8. Lutes, J . Chem. Phga., 84, 484 (1956). (7) H.I. Schiff, Ann. N. Y. Acad. Sci., 61, 518 (1957). (8) Heats of reaction are based on: D(N-H) = 87 kcal. G. Pannetier and A. G. Gaydon. J . chim. phye.. 48,221 (1951). D(CHrH) 101 kcal. T. L. Cottrell, “The Strength of Chemical Bonds,” Sec. Ed., Butterwortha, Washington. D. C.,1958. D(Nd = 225 kcal. J. M.Hendrie, J . Cham. Phys., 22, 1503 (1954). NPD) = 55 kcal. “Atomia Energy Levels,” National Bureau of Standards, Circular 467.
-
Vol. 66
+
+
CH, N(”) +[CHsN] +HCN Hz (4) N(“) + [CHsN] -+ HCN 2H (4a) NH NH -+ N2 H2 175 kcal. (5) NH Hz R/I --+ NH3 90 kcal. (6)
+
+ + +
+ +
+
+
The heat of reaction for reaction 2 is based on the formation of a ground state XH radical. With increase in concentration of NII radicals reaction 5 would be favored over reaction 6. According to the above mechanism, the hydrogen concentration would be at least as great as the yield of hydrogen cyanide, which is always greater than the yield of ammonia. Reaction 6, unfortunately, does not conserve spin and must be considered to be very slow. The formation of ammonia then would have to occur by reactions such as NH NHz
+ H + M + NHa + M + H + M +XH3 + M
(7) (8)
The initial reaction CH,
+ N +[CHiN] +HCN + Ht + H
(9)
postulated by WinklergO1Oundoubtedly also occurs. I n a nitrogen discharge system, reaction 7 would be the predominant first step. I n the nitrogen-argon discharge system, however, reactions 2 and 7 would become competitive. Reaction 2 would be favored with decrease in molecular nitrogen flow rate. The mechanism for the ethane reaction would involve similar steps C2I-16 CaH6
C2HI + N(2D) +CzHi + NH + 45 kcal. + N +[CzHsN] +CH3 + HCN + H + N +[CzHeN] +CH3 + HCN + Hz
(10) (11)
(12)
Reaction steps, corresponding to eq. 4 and 9, in the acetylene-active nitrogen reaction, unfortunately cannot be written. This of course would mean that the hydrogen produced in reaction 5 is the only hydrogen available for reaction 6. The small differences in the rates of formation of ammonia would seem to indicate that ammonia production depends to a greater extent on reaction 5 than on reactions of the type corresponding to eq. 4. It can be argued that the formation of an NH complex is not necessary even with acetylene. A complex of the type [C2H7N]might, on interaction with itself, yield ammonia in a one-step reaction without intervention of NH. The nitrogen atom is far removed from the hydrogen atoms and the reaction 2[CzHzN] +ISH3 + C,H (13) seems very unlikely if the radical models are considered. However, reactions between CZHzN radicals could produce NH or NH2 radicals. If one could prove that reactions of this type did occur to a significant extent it might not be necessary to assume hydrogen atom abstraction. Reactions of the type 2[CHaN] +NHa
+ [CJLN]
(14)
may occur in the methane-active nitrogen reaction but certainly to only a small extent since the lifetime of the [CH,N] radical would be very short. (R) H. Blades and C. A. Winkler, Can. J . Chem., 29, 1022 (1951). (10) P. Gartaganis and C. A. Winkler, %bid., 84, 1457 (1956).
