REACTIONS OF DEUTERIUM ATOXSWITH OLEFIXSIN LIQUIDPROPANE -~
38 1
-
Reactions of Deuterium Atoms with Olefins in Liquid Propane at 90"K. Relative Rate Constants of the Addition
by T. T. Kassal and M. Szwarc Department of Chemistry. State University College of Forestry at Syracuse University, Syracuse 10, N e w York (Received J u l y 22, 1983)
The addition of D atoms to a series of olefins and dienes was studied at 90'K. The deuterium atoms were formed on a hot tungsten wire in the gas phase and thereafter diffused fnto liquid propane where a homogeneous reaction took place. The relative rate cortstants of D atom addition to propylene, butene-l, pentene-1, hexene-1, and 3-methylbutene-1 were found to be nearly identical, the addition to isobutene and butadiene were found to be 5 and 25 times faster than to the terminal olefins, while the addition to pentene-2 and hexene-2 were half as high. hllene was found to be less reactive than the internal olefins, The usual pattern of reactivities is maintained even a t this low temperature.
During the past five years, Klein and Scheer1-6have published a series of most interesting papers describing; the reactions of a variety of olefins with H atoms. The substrates were condensed a t temperatures below 100OK,, while the atoms were produced in the gas phase by the dissociation of €1, on a hot tungsten filament, Their results indicated a most unusual gradation of the reactivities of the investigated olefins and dienes. For example,2 the reactivtties toward H atoms a t 77°K. appeared to decrease as follows: propylene > butene-1 > isobutene > 3-methylbutene-1 >> butadiene, while hexene-1 did not react a t all. These findings aroused our interest since they are a t variance with the normally observed gradations in reactivities of olefine and dienes toward free atoms and radicals. The reactivity of a substrate was measured by the rate of hydrogen uptake, A p / A t , a t a constant amount of the hydrogenated substance. However, many other factors might influence the result of such an experiment, e.g., the roughness coefficient of the surface, the transmissioi~and solubility coefficients of the atoms, etc. In fact, the pronounced effect of the matrix on the apparent reactivity of an olefin was demonstrated in a recent paper by Klein and Scheer.:' It was decided, therefore, to develop a liquid system in which the investigated olefin or diene is dissolved and reacts under standard conditions. In such a system the rate of hydrogen uptake should correctly determine the relative reactivity of a substrate.
The Liquid Phase System. After an extensive investigation, it was found that liquid propane forms the most convenient solvent for low temperature experiments. Its melting point was reported to be 83"K., and carefully deaerated liquid propane containing a sufficient amount of solute remains fluid even a t 77 OK. Its inertness to D atoms at 90°K. was demonstrated by exposing the liquid to a high partial pressure of D a'toms for periods as long as 1000 sec. n'either decrease of pressure nor the appearance of HD was observed. The hydrogenation experiments were carried out iin an apparatus shown in Fig. 1. A flat bottomed tube served as a reactor in which the solvent and the rea8gent were condensed. T o standardize our procedure, we used the same amount of solvent in all the runs. The mixture was stirred magnetically a t a constant, although rather slow rate, and it was demonstrated that small variations in the rate of stirring did not affect the results. The bungsten filament used for production of D atoms was placed 12 cm. above the solvent's surface. R. Klein and M . D. Scheer, J . Am. Chem. SOC.,80, 1007 (1958). R. Klein and M . D. Scheer, J . P h y s . Chem., 62, 1011 (1958). M. D. Scheer and R. Klein, ibid., 63, 1517 (1959). (4) R . Klein, M.D. Scheer, and J. G. Waller, ibid., 64, 1247 (1960). (5) R. Klein and M. D. Scheer, ibid., 65, 324 (1961). (6) M. D. Scheer and R. Klein, ibid., 65, 375 (1961). (7) R. Klein and M. D. Scheer, ibid., 66, 2677 (1962). (1) (2) (3)
V o l u m e 68, S u m b e r 2
February, 1964
I
u
it I
V
-4CM
-
f
5
n,
Figure 1.
In earlier experiments, a shield was introduced between the filament and the liquid to protect the solution from the heat radiation. However, it was found that this precaution is superfluous; even if the radiation slightly raised the temperature of the stirred liquid, the effect mould be undetectable within the limits of our accuracy. S o solvent evaporation was observed, since no cracking of the propane or the reagent on the hot filament was noticed. In our technique it was essential to maintain a constant and reproducible partial pressure of D atoms in each series of experiments. This required constancy of filament temperature, standardization of the initial pressure of D2, and preservation of a fixed geometry of our apparatus. It was found that the temperature of the mire was constant for a constant voltage drop across the filament. The latter was maintained a t the required level by a constant voltage transformer and by manually regulated Variacs. The heat mas removed The Journal of Physical Chemistry
from the filament mainly by radiation and by conductivity through the heavy leads; only a small fraction of the dissipated energy was carried away by the gas. The leads were kept a t constant tewperature by blowing air through the iriner space leading to the electrodes (see Fig. l), and the rest of the apparatus was therinostated by its immersion in the refrigerating liquid. The level of the coolant was kept constant during the experiment and during the period when the hydrogen prebsure \vas measured. A\lloreover,an asbestos plate and a rubber stopper isolated the refrigerating liquid from the outside, thus preventing any cooling of walls not immersed in the dewar (see Fig. 1). The constancy of the deuterium atom flux into the liquid was periodically checked by determining the Duptake of pentene-1 at a constant concentration of the olefin. LYhenever the filament had to be changed, the results were normalized by reinvestigating the addition to pentene-1 under a new set of conditions. Thorough degassing of the solvent and solute was essential for reliability and reproducibility of the experiments. Chemically pure propane was first deaerated by the conventional freeze and thaw technique and by high vacuum distillation. The investigated olefin was treated similarly. The hydrocarbons then were distilled to thermostated pipets and from there into the reactor. In the reactor the liquid mas stirred for about 1 hr. a t 9OoK. kthile the apparatus was connected to high vacuum pumps. I n a typical experiment, the investigated olefin and the solvent were condensed on the bottom of the reactor tube while the refrigerant covered only this part of the vessel that contained the investigated solution (about 1.25 em. above the bottom of the reactor) and the stopcock leading to the vacuum line was closed. The walls of the tube above the surface of the cooled liquid then were heated with a hot air blower while the level of refrigerant was maintained on the level of the solution. This procedure removed any olefins condensed on the wall. Thereafter, the dewar was raised to a position shown in Fig. 1 and additional refrigerant was added if necessary and the system was pumped for a while. The required amount of deuterium was admitted into the reactor and the filament switched on for about 1000 sec. During this time any olefin molecules adsorbed on the wall were hydrogenated and thereafter the drop of pressure reflected the genuine rate of reaction in the liquid phase. The pressure mas measured at various times and, of course, each reading was taken a few minutes after switching off the filament to allow the system to reach its standard state. A typical plot of A p us. time is shown in Fig. 2 . Three factors, namely, solubility, reactivity, and
REACTIONS OF DEUTERIUM ATOMSWITH OLEFINSIN I,IQUID PROPANE
D atom uptake
383
==
k,dd[Olef]
L=
f,F(z,r)dz
= G(r)
It is implied that the olefin concentration remains rn.T.8
.0417
constant throughout the whole liquid, an assumption which appears to be justified since the total amount (of atoms reacting in a single experiment is much smalller than the amount of a substrate present in a layer as m.f.= ,0175 thin as 5 X cm. . 1 1 1 1 1 1 1 1 1 1 1 1 1 1 It was shown that no HD is formed under our ex20 40 60 130 100 120 140 160 180 200220240260280xx)32o34x1 perimental conditions, and therefore the experimentally TIME (sec.) measured Ap/At is proportional to G ( r ) , the proportionality factor being determined by the volume of the Figure 2. 3-Methylbutene-1 + propane. reactor, the area of the liquid, and the temperature of the gas. Hence, if the same rate of pressure drop is observed in two experiments involving different subvolatility, impose a limitation upon the range of substrates, say (1) and (2)) the respective rl and r2 must be strates which could be investigated by this technique. identical, i.e. For example, ethylene was found too volatile to be studied in our system, whereas tetramethylethylene kadd, zlkadd, 1 [Orefl]/[Olefz](Ap/At = const.) (1) was not sufficiently reactive within the limits of its Equation 1 permits the determination of the relative solubility. rate constants of D atom addition t o a series of subCalculation of the Relative Rate Constants of D Atom strates. In our experiments pentene-1 was chosrn Additzon. At a fixed temperature of the wire and as the standard olefin and all the relative rate conpressure of Dz a stationary concentration of D atoms is stants refer to k a d d for pentene-1 as unity. Finall,y, established above the liquid propane. The partiail it should be noted that eq. 1 remains valid even if other pressure, p D , of the atoms may be calculated with a transport processes participate in the reaction, assumreasonable accuracy from Langmuir's equationS giving ing, of course, their reproducibility under standard the equilibrium constant, Kd of the dissociatioii conditions of the experiments. process Dz $ 2D a t the temperature of the wire, T,, Results as a function of T,, the temperature of the gas, and W, the total rate of atom destruction by all the procThe drop of pressure A p was measured for various esses but their combination on the wire. The equatime intervals At a t a constant concentration of the tion has the form Kdlss = ( T w / T 8 ) 1 / 2 ( p-D const. X substrate and a t fixed operating conditions of the apW T , ' / 2 ) z / p ~and z , it may be shown that W is sufficiently paratus. From the resulting linear plot, illustrated m small to make p~ only slightly smalller than the value Fig. 2, Ap/At was determined for a particular concentriacalculated on the basis W = 0. tion of a subslmte. By keeping the operating condiAs a result of the diffusion of the attomsinto the liquid tions constant but varying the substrate's concentratioin, a stationary st>ateis established in which the concentraa functional relationship was established for ~ p / ~ t tion of the atoms, C,, in a layer at depth rt: is given by and the concentration, as exemplified by Fig. 3 and 4. the differential equahion, B(d2C,/dx2) = IC,CZ2 tThe graphs, such as Fig. 4, allow the determination f,kadd [Olef IC,. Here, 3 denotes the diffusion constant of the concentrations of various substrates correspondof D atoms in the liquid, IC, is their bimolecular rate ing to a constant Ap/At, and hence give the values of the constant of recombination, kadd is t,heir rate constant respective k a d d . Alternatively, these constants are of addition to a substrate present a t a concentration given by the initial slopes of the lines shown in Fig. 4. [Olef 1, and f, is a coeficient, determined by C, and k a d d . All the data obtained by this technique are collected [Olef], which gives the fraction of the radicals formed in Table I. It should be noticed that in a series of in a layer at depth x that become hydrogenated substrates of nearly identical reactivity, the experithrough the addition of a second D atom. The solumental points in graphs of AplAt us. [Olef]fall on a comtion of this equation, with the appropriate boundary mon line; see, e.g., Fig. 4. Such lines perhaps give the conditions, gives C, as a function of 5 and of the variable best experimental evidence for closely similar reactivity parameter r =: Ic,dd[Olef]. Denoting this function by of such olefins as propylene, butene-1, pentene- 1, the symbol F(z,r) we may express the rate of D atom uptake by the integral (8) I Langmuir, J . A m . Chem. SOC.,3 7 , 417 (1915). Volume 68,A4rumbeTD
February, 1964
hexene-1, 3-methylbutene-1, etc., or pentene-2 and hexene-2.
Discussion
.I
.2
.5
.4
.3
.6
.7
MOLE FRACTION OF BUTENE-I
Figure 3. Total flux of D atoms a t low filament temperature.
AT 9O0K.
A 0 V 0
7
BUTENE- I PENTENE- I HEXENE-I 3 METHYL-BUTENE-I
f
Ap'At
6
5 4
3 2
The results shown in Table I or in Fig. 4 prove that even a t this low temperature the a-monoolefins possessing the terminal group -CH=CH2 are equally reactive toward D atom addition. The small variations in the observed rate constant may be real, although they are within the experimental uncertainties. Isobutene was found to be 4-5 times as reactive as the olefins of the previous class and butadiene was the most reactive substrate in the investigated series of compounds. The normal pattern of reactivity, as illustrated by their methyl affinitiesg determined in the temperature range O-lOOo, is preserved even a t 90°K. The olefins with internal double bonds -CH=CHare by a factor of two less reactive than the terminal olefins, i.e., the reactivity per carbon center seems to The low reactivity decrease by a factor of -4. of allene parallels its low methyl affinity'O which was found to be lower than that of propylene or butene-1. It seems, therefore, that the lorn temperature does not introduce any anomalies in kinetic behavior. Acknowledgment. We wish to thank the Office of Ordnance Research Durham, for financial support of this work through Grant DA-ORD-31-124-61-G72.
Appendix
I
.01
.02
-03
.04
.OS
mole froctlon
Figure 4. All lines with the exception of that observed for isobutene represent extrapolations of initial tangents. The curvature is observed only for isobutene, since in other experiments the rate, A p / A t , was too low to give appreciable deviation from linearity.
Table I : The Relative Rat,e Constants of D Atom Addition to Olefins in Liquid Propane at 90"K., the Rate Constant of Addition to Pentene-1 Being Taken as Unity k80
Olefin
Propylene Butene-1 Pentene-1 Hexene-1 3-Methplbutene-1 Isobutene Butadiene Pentene-2 Hexene-2 Allene
The Journal of Physical Chemistry
(relative)
1 0
0.95 (1.00) 1.4 1 2 4-5 25 0 5 0.5 -0.4
The Transmission Coe$cient for D Atom Penetration through the Surface of Liquid Propane. Increase in the olefin concentration leads eventually to trapping of all the atoms that penetrated through the liquid surface. Under these conditions CO, the concentration of the atoms in the highest layer of the liquid, falls down to zero. For practical reasons, experiments leading to trapping of all the atoms may be performed only a t low temperature of the wire; otherwise, the required concentration of the scavenger would be impossibly high. The pertinent experiments were performed a t T , = 14OO0K. and are presented graphically in Fig. 3. The plateau value corresponds toeflux of 2.2 X mole of D/cm.2 sec., the concentration in the gas phase was calculated from Langmuir's equation to be 5 X lo-'* mole of D j ~ m .and ~ , hence the transmission coefficient for D atom penetration through the surface of liquid propane a t 90OK. is about The existence of a plateau proves that the reaction takes place in the liquid and not Qnthe surface. (9) M. Feld and M. Szwsro, J . Am. Chem. Soc., 8 2 , 3791 (1960). (10) A. Rajbenbach and M . Sswarc, Proc. Roy. SOC. (London), A251, 394 (1959).
