620
D. M. WILESA N D C. A. WINKLER
Vol. 61
TIIE REACTION OF HYDROGEN ATOMS WITH PHOSPHINE’ I ~ YTI. M. WILES~ AND C.A. WINKLER Contributionf r o m the Physical Chemistry Lnboratory, McGill University, Montreal, Canada Received December 8, 1068
Red phosphorus and molecular hydrogen mere the only products of the reaction between hydrogen atoms and hosphine. It is postulated that hydrogen abstraction is followed by combination of PH, radicals to give the products. $he atomic h drogen concentration was estimuted by measuring the extent of the reactions of HBr and HI with hydrogen atoms under tXe same experimental conditions.
Introduction The reactions of atomic hydrogen with a large number of organic compounds have been investigatfed under a variety of experimental conditions. I t is generally accepted that the primary reaction with saturated hydrocarbons is hydrogen abstraction whereas with alkyl halides and alkenes it is thought to involve halogen atom abstraction and hydrogen atom addition, respectively. Reactions between atomic hydrogen and gaseous inorganic compounds have not been extensively studied in quantitative experiments, although the qualitative effects upon many inorganic substances have been reported in the literature. Phosphine has been subjected to the action of hydrogen atoms produced in an electrodeless discharge3 in a search for higher hydrides of phosphorus, but no sttempt was made t o determine the products of the reaction or its mechanism. In the present work the reaction of hydrogen atoms with phosphine was studied in a Wood-Bonhoeffer fast-flow system. A limited examination of the corrcsponding reactions with hydrogen iodide and hydrogen bromide was also made with a view to obtaining a measure of the atomic hydrogen concentration in the system. Experimental The apparatus and the expcrimental techniques were similar to those described in an earlier paper from this Laborat,ory.‘ The preesure in the apparatus before each ex eriment was always less than 6 X lo-* mm. Hydrogen Prom a cylinder wtw passed through a trap immersed in liquid nitrogen and was admitted through a capillary flow meter to both sides of a V-shaped discharge tube. The total pressure of atomic and molecular hydrogen in the reaction system was 0.87 mm., with a molecular hydrogen flow rate of 1.24 x lo-‘ mole/sec. Experiments were made a t high temperature with an electrical heater surrounding the reaction vessel and the lower 5 cm. of the tube that connected it to the discharge tube. The temperature was measured with a glass-enclosed, copperconstantan thermocouple located just below the reactant inlet in the center of the reaction vessel. The inner surfaces of thc discharge tube and reaction system were poisoned with 2% metaphosphoric acid solution. All experiments lasted 100 seconds. Phosphine wm prepared using three different methods: i) by decomposing phos honium iodide with dilute base; (ii) by treating calcium pfosphide with dilute acid; (iii) by treating white phosphorus with boiling aqueous potassium hydroxide solution. In each case, hydrogen was puased throu h the reaction flaek, and the mixture of h drogen and hos \orus hydrides prtssed through G N H8I and 2$% k O I f solutions, through two traps immersed in freezing ethanol, and filially through a liquid nitrogen trap to con( 1 ) With Bnanoial aasiatance from the Nntionel Renearch Council
of Canada. (2) Holder of National Research Council Studentships, 1955-1958, 1956-1957. (3) IC. Q. Denbigh, Trans. Faraday Soc., 85, 1432 (1030). (4) P. A. Gartagnnis and C. A. Winkler, Can. J . Chem.. 54, 1467 (1956).
denRe the phosphine. Prior to use, the condensate waa dietilled twice from a tra cooled to -130’. Phosphine flow ratem were controlled e y maintaining a constant pressure behind a capillury ’et. T h e flow meter was calibrated by distilling the gas cohected during “blank” experiments into a small removable trap that contained dilute silver nitrate solution. The precipitated silver was removed by filtration and excess silver nitrate waR determined by the Volhard method.’ Estimation of unreacted phosphine by the eame procedure enabled the amount of reaction in each experiment to be determined. Hydrogen iodide0 and hydrogen bromide’ were pre ared by standard methods. The gases were carried in a sgeam of dry hydrogen through two traps at -100“ before they were condensed with liquid nitrogen. They were purified further by bulb-to-bulb distillations and care was taken in subsequent experiments to revent their contact with mercury. Hydrogen halide low rates were rneaaured in “blank” experiments by distillin the condensate from the product trap (kept a t 100” for %Br, -60’ for HI) into a removable trap containing standard NaOII solution and back-titrating the oxcess alkali with standard acid. The extent of reaction was determined by collecting and memurin unreacted hydrogen halide in the same manner. %thy1 chloride (b.p. rade) was obtained from Ingram and Bell, Ltd., Montrea!, and waa vacuum distilled before use. The extent of its reaction with hydrogen atoms WMI estimated from the amount of hydrogen chloride produced.
-
Results and Discussion Figure 1 shows the reaulta obtained in the reactions of HI, HBr and C&HaClwith atomic hydrogen at 65”. The hydrogen halides reacted with 3 timea as many hydrogen atoms as did the alkyl halide and the fact that hydrogen iodide and hydrogen bromide reacted to the same extent is a good indication that both consumed all the hydrogen atoms. The following elementary steps must have occurred in the hydrogen halide reactions. H
X
+ HX +HI + X + X + M +Xn + M H + Xz +HX + X
(1)
(2) (3)
The activation energy for reaction 1 is approximately 1 kcal./mole.8 Reaction 3 has a small but significant activation energy, practically the same as for reaction 1. According t o Williams and Ogg,S the ratio k& is 3.5 for HI and is essentially independent of temperature. For HRr the ratio kt/ki is 8.4.8 However, since hydrogen astomconsumption is apparently complete with excess of either hydrogen halide, the consumption of H atoms by I2 or by Brs under these conditions seems to represent a negligibly small percentage of the total H-atom con(5) I. M. Kolthoff and E. l3. SandP11. “Textbook of Quantitative Inorganic Analysin.” 3rd ed., The hlacmillan Co., New York, N. Y., 1952. (6) A. I. Vogel, “A Textbook of Practical Organic Chemistry,” Longmans. Green rtnd Co., New York. N. Y., 1951. (7) J. W. Mrllor. “Mrllor’a Modern Inorgnnic Chemintry,” revisad cd., Longmans, Green and Co., New York. N. Y., 1939. (8) J. C. Morris and R . N. Peaae, J . Chem. Phva., 5 , 706 (1935). ( 8 ) R. R. Williams, Jr., and R. A. Oge, Jr., ibid., 16, 091 (1947).
L
h
REACTION OF HYDROGEN ATOMSWITII PHOSPHINE
May, 1'357
con trntion available. It is assumed, therefore, that the plateau value for the amount of reactant destxoycd may bc taken as B measure of the hydrogen atom concentration. On this basis the flow rate of atomic hydrogen in the reaction system was mole/scc. and t,he ethyl chloride reaction was 32% complctc a t 65' (cf. 10). It is of interest that n previous study of the IIBr-H atom reactionll indicnt,cd 100% coilsumption of hydrogen atoms. Thc only products of the reaction of phosphine with atomic hydrogen were molecular hydrogen and red phosphorus. The latter was deposited in a iiniforrn layer on the inside wall of the reaction vessel and it changed from a light golden to a doep amt)cr color as the thickness increased. When hydrogen atoms reacted with this coating it was consiirncd completely in such a manner that the clearly defined horizontal boundary between clean and ~~l~ospliorua-covered glass moved gradually down hhe reaction vcssd nnd fcw hydrogen atoms appe:ircd t o exist beyond the phosphorus level. This prooc!ss resulted in conversion of the red phosphorus tmophosphine and a small amount of diphosphine. T h o rcactions P r e d + H --f PHI* (4) PkI
+ HI +PH,
(5)
would a,acount for thc formation of phosphine, and tthc small timount of disphosphine could have been formed by PHI 2PZI9
+ H --+ PHa + HZ + M --+PzHc + M
1'113
I'Iiz
__
+H 1'112 + 112 + PtIz +Pi + 2Hz --t
1'2
-+- l'wd
0 HI 0 Her
A C,H,CI
--o--e-bgllh-
f
41
a
(6) (7)
13ccniiso of the trace of PzHd formed, the conversion of the layer could not be used to determine quantitatively the amount of phosphorus produced in the phosphine-n,tomic hydrogen reaction. The action of hydrogoii atoms on red phosphorus to produce phosphinc was observed by LangmuirLswho found that the process would proceed either in the gas phase or on the vessel wall. The conversion of phosphorus to phosphine observed in the present work occurred at an appreciable rate for temperatures at which the vapor pressure of red phosphorus is quite low. Under these conditions there must have been a considerable proportion of wall react,ion. The results obtained in the reaction of phosphine with H atoms a t three temperatures are given in Fig. 2. It is apparent that phosphine reacts with all thc available hydrogen atoms at 73", since the plateau corresponds closely to the maximum amounts of hydrogen iodide arid hydrogen bromidc destroyed undcr similar .conditions. It is pro\)able that phosphine reacts according to the following mechanism. (8) (9)
(10)
(10) 11. M. Chadwcll arid T. Titani. J . Am. Chem. Soc., 611, 1363 ( 1033).
(11) 14;. Crcini:r, J. Curry and M . I'olaiiyi, Z. phusik. Chsm., Bas, 445 (1933). (12) Tlic vslurs nf IicaLn of forination found in Circiilnr 600, U. S.
National Uurcaii of Standards wcre uaod lor specins occiirring in thirr and subsoqucnt rcaotions. Values of 30 and 55 kcal./urolc were estiinatcd l o r 1'11s and 1'11 radicals. ronpcctivcly. (13) I . Laiigiiiuir, J. A w . Chem. Suc., 34, 1310 ( I D I P ) .
621
PH3 FLOW
RATE
MOLE / SEC. X IO'.
Fig. 2.
The abstraction of hydrogen is typical of the reactions of atomic hydrogen and, because of its exothermicity, reaction 8 is the most reasonable primary step. Reactions 9 and 10 are substantiated by evidence obtained in investigations of the photochemical and mercury photosensitized decomposition of phosphine. Melville, el aZ.,14studied the photochemical decomposition of PDa and PHS and found evidence that PHn radicals decompose to give molecular hydrogen. Melville and Gray,16 in a study of the polymerization of phosphorus, obtained experimental evidence that PZ molecules condense to red phosphorus so that equations 8 to 10 can account for the products found in the present study. The following reactions are also possible, although they appear to occur only to a slight extent a t room temperature.
+ H +PH + Hz + PH2 +PI + I i + Hz PH + H + P + Ha
PHn
I"
PH
+ I"+
Pz
+ Hz
(11) (12) (13)
(14)
Reactions 1 1 and 13 should lower the plateau a t 73" relative to that representing the available hydrogen atoms, but this was not observed. It is therefore reasonable to assume that, at phosphine flow rates corresponding to complete consumption of hydrogen atoms, the unstable PH2 species re(14) H. W . Melville, J. L. Bolland and 13. L. Roxburgb, Proc. R o y . Soc. (London), A160, 406 (1937). (15) H. W. Mclvillo and 5. C. Gray, Trans. Faraday SOC.,81, 271
(11130).
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