2584
C.A. Arrington, Jr., and D. J. Cox
Arrhenius Parameters for the Reaction of Oxygen Atoms, O(3P), with Propyne C. A. Arrlngton, Jr.,* and D. J. Cox Department of Chemistry, Furman University, Greenvll/e,South Carolina 296 13 (Received May 12, 1975) Publication costs assisted by the Petroleum Research Fund and Furman University
The rate of reaction of oxygen atoms, O(3P), with propyne has been investigated in a flow system in the temperature range 298 to 600 K. The Arrhenius form of the rate constant is found to be k = 2.3 f 0.6 X 10-11e1950*700/RT.The effect of the methyl group on the activation energy is consistent with earlier interpretations of atomic oxygen reactions with alkenes.
Introduction There have been numerous studies of the kinetics of the reactions of ground state oxygen atoms with alkenes, both hydrocarbons and halogenated a1kenes.l It is generally accepted that in the addition of atomic oxygen to the double bond the oxygen atom is acting as an electrophilic reagent. Thus any substituent which enhances the “availability” of the P electrons of the alkene tends to increase the rate of the initial attack by oxygen atoms.2 Temperature studies have provided activation energies of these reactions, and the primary effect of the substituent appears to be manifest in a change in the activation energy of the reaction. While the reaction of atomic oxygen with acetylene has been extensively ~ t u d i e dthere , ~ has been little systematic investigation of the effects of substituents on the rate parameters of the reactions of oxygen atoms with alkynes. As is the case in the reactions with alkenes, the initial step is the addition of the oxygen atom to the multiple bond site. I t is to be expected that substituents will influence the rate parameters of the alkyne reactions in a way similar to that of the alkene reactions. In this paper we report the results of investigations of the kinetics of the reactions of ground state oxygen atoms with p r ~ p y n e . ~ Experimental Section The experiments were carried out in a discharge flow system using chemiluminescence to monitor the oxygen atom concentration. The main flow tube, a 25-mm Pyrex tube 1 m long, was cleaned with 10% H F to reduce wall recombination of atoms. Oxygen atoms were produced by the reaction of NO with nitrogen atoms produced in a microwave discharge. Reactant gases were added through a movable inlet jet so that reaction times could be varied by adjusting the distance of the inlet jet with respect to the fixed position of the photomultiplier tube. The molecular nitrogen flow rate was measured with a calibrated Matheson rotameter. Reactant gas and NO flow rates were measured with capillary tube flow meters using Poiseuille’s equation. Total system pressure was measured 10 cm upstream from the reaction zone by a McCleod gauge. The temperature of the reaction zone was controlled by a furnace which enclosed 60 cm of the main flow tube. The temperature in the middle of the flow tube was constant to within 2’ for a distance of 30 cm prior to the position of the photomultiplier tube. Temperatures were routinely measured with a thermocouple in the furnace, but suitable corrections were made using a calibration curve established by measuring The Journal of Physical Chemistry, Voi. 79, No. 24, 1975
the flow tube temperature with a thermocouple in the movable inlet jet. The intensity of chemiluminescence was monitored through a 1-mm slit in the furnace wall using either a Westinghouse WX-4288 photomultiplier tube or an RCA 1P28 tube. Both tubes were cooled with Dry Ice. Baird Atomic interference filters were used to select appropriate wavelength regions for study. Output of the phototube was amplified and measured by a Keithley Model 610B electrometer. Prepurified nitrogen obtained from Selox with a stated purity of 99.99% was used without further purification. Nitric oxide from Matheson was passed through an Ascarite trap to remove NOz. Propyne was obtained from Matheson and was found to have 4% acetylene as an impurity. The acetylene was not removed since acetylene reacts with oxygen atoms more slowly than propyne and the presence of a few percent acetylene would have a negligible effect on the rate of oxygen atom consumption. The total pressure for all experiments was in the range 1-3 Torr. Typically the oxygen atom concentrations were near 1014 atoms/cm3, and the reactant gas concentrations were from 1014to several times 1015 molecules/cm3. Before each series of measurements the system was allowed to equilibrate with the discharge and furnace on for a t least 1 hr and often for several hours before any measurements were taken. For a given temperature several decay curves were obtained for each reactant gas flow rate and several reactant gas flow rates were used. Linear flow velocities were between 300 and 700 cmhec.
Data Analysis and Results In these experiments each individual rate constant determination is affected directly or indirectly by more than 15 individual measurements or experimentally determined parameters. In addition the complicating features of mixing time and wall recombination can influence the measured rate constant even though precautions were taken to minimize these effects. As a result of all of these sources of experimental uncertainty we estimate that the measured rate constants should exhibit a relative uncertainty of f25%. Oxygen atom decay was followed under pseudo-firstorder conditions with propyne being in sufficient excess. Rate constants were determined from the slopes of log intensity vs. time plots. Since our measurements were of oxygen atom decay rates, the number of oxygen atoms removed following each
Arrhenius Parameters for the Reaction of Oxygen with Propyne
initial rate-determining reaction must be determined in order to calculate the rate constant for the initial reaction. It is generally assumed that the stoichiometry factor, the number of oxygen atoms removed per reactant gas molecule, is the appropriate factor to use, i.e., it is assumed that only one reactant gas molecule is removed for each ratedetermining elementary reaction. Brown and Thrush have studied the reaction of oxygen atoms with propyne using a flow system with ESR d e t e ~ t i o nFor . ~ conditions in which 0 atoms are in excess they found that the stoichiometry factor is between three and four oxygen atoms removed per propyne molecule removed. Herbrechtsmeier and Wagner report a value of 1.7 for the stoichiometry factor a t a pressure of 3 Torr.4 This value was measured with propyne as the reactant in excess and was found to be independent of temperature in the range 290-360 K. We have used this value, 1.7, for the stoichiometry factor in adjusting 0 atom decay rates to initial reaction rates. Relative oxygen atom concentrations were determined by measuring the intensity of CH chemiluminescence. The dependence of CH intensity on oxygen atom concentration was measured by varying oxygen atom concentration with a fixed excess of propyne and measuring the maximum CH intensity at a fixed reaction time. Log-log plots of CH intensity vs. 0 atom concentration measured at four temperatures from 315 to 520 K show that the intensity of CH emission is proportional to [0]2.1 over this temperature range. This dependence of CH intensity on 0 atom concentration is assumed to remain constant throughout the reaction. Any substantial change in this dependence would be reflected in a deviation from linearity of our pseudo-firstorder decay curves. The corresponding relationship for the reaction with acetylene is a power of 2 dependence of CH intensity on oxygen atom concentration.6 A complete study of CH chemiluminescence in the acetylene-atomic oxygen reaction covering the full range of reactant stoichiometries has shown that the intensity is a more complex function of reactant c~ncentration.~ Results of rate constant measurements are shown in Figure 1. The least-squares fit of the data in the Arrhenius plot, Figure 1,gives an activation energy of 1.95 f 0.7 kcal/ mol and a preexponential factor of 2.3 f 0.6 X cm3/ molecule sec. Discussion The results obtained for the reaction of atomic oxygen with propyne may be compared with other representative reactions shown in Table I. The lowering of the activation energy which can be attributed to the influence of the methyl substituent can be seen clearly in the comparison of ethylene, E , = 1.12 kcal/mol, and propene, E , = 0.1 kcal/ mol, and in the comparison of acetylene, E , = 3.15 kcal/ mol, and propyne, E , = 1.95 kcal/mol.8 The preexponential factor in each case is somewhat lower for the methyl-substituted reactant. It is thus apparent that the primary effect of the methyl substituent is the lowering of the activation energy for the reaction in which the oxygen atom interacts with the x electron system. The substituent effect on the activation energy of these reactions is best interpreted in terms of the influence of the substituent on the ionization potential of the x electrons. The methyl-substituted alkyne or alkene has a lower ionization potential than its parent compound. The oxygen atom, as an electrophilic reagent, reacts more readily with the substituted compound since the lesser energy required
2585
TABLE I: Table of Activation Energies and Ionization Potentials
-
Reactant
E a , kcal/mol
IP, eVf
C2H4
1.12"
CH,CH=CHZ cis-CH,CH=CHCH, (CH3),C=CHCH3 (CH&C=C (CH3)z CH,=CH-CH=CH,
0.08"
10.5 9.7 9.1
-0.33"
8.7 8.3
-1 .35" -1.57'
-0.76a
9.1 10.1
0.20"
C2F4 C3F6
C2Hz CH,C=CH HCN CHSCN ClCN
1.8a
11.1
3 .Oa 2 .Ob
11.4 10.5 13.7 12.2 12.5 14 .O
8.1' 6 .2a 6 -9' co 6 .ge a Cited in ref 1. This work. P. B. Davies and B. A. Thrush, Trans. Faraday Soc., 64, 1836 (1968). d B. D. Barton and D. J. Cox, unpublished results. e For the reaction 1 8 0 + C160 l e 0 + ClSO, S. Jaffe and F. S. Klein, Trans. Faraday Soc., 62, 3135 (1966). J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Drexl, and F. H. Field, Natl. Stand. Ref Data Ser., Natl Bur.
-
f
Stand., No. 26 (1969).
0
s
1
41 2
v
,
IO
20 I / T x IO3
30
Figure 1. Arrhenius plot for measured rate constants of 0 pyne: (I) value obtained by Brown and T h r ~ s h . ~
+ pro-
for removal of a x electron results in a lower activation energy for the reaction on which a x bond is broken and electron density is transferred to the oxygen atom. The general correlation of activation energy for reaction with 0 atoms and ionization potential for several T bonded compounds may be seen in Table I and Figure 2. The readily apparent correlation between E , and IP for those reactions suggests that the transition states must involve electron transfer from the system to the oxygen atom.g While it is not possible to identify the site of 0 atom addition from the kinetic data, it is worthwhile to examine several points relevant to the question of site of addition. Calculations of electron densities in propynelO indicate an electron density on the acetylenic carbon adjacent to hydrogen which is greater than the electron density on the carbon atom in acetylene. Furthermore, we would expect the biradical intermediate to have a somewhat greater stability for addition at H-C (I) rather than CH3-C (11) beThe Journal of Physical Chemistry, Vol. 79, No. 24, 1975
c. A. Arrington, Jr., and D..J. Cox
2586
tones.13 On the basis of observed scrambling of 13C isotopes they have shown that the intermediate keto carbenes I11 and IV undergo interconversion. While the potential ener-
91
0 HCh
7i
0 ClCh
0 CO
0
0 CH3Ch
CH,--C-
II
0
c -H
I11
-
s 3-I
I1
CH,-C-C-H IV
gy surface of these singlet species is different from the corresponding surface for the triplet species formed in the O(3P) reaction with propyne, it is probable that the internal energy of the 0 propyne intermediate is sufficiently high to allow it to undergo rapid interconversion among the various structural isomers of the complex before undergoing fragmentation. It should be noted that at higher pressures or in a condensed medium the C3H40 intermediate will be relaxed into one of the stable forms, methyl ketene (CHs(H)C= C=O) or acrolein (CH2=CH-C(H)=0).14 One must not, however, infer anything about the site of initial attack from the structure of the final products in view of the extensive rearrangement that can occur in the intervening intermediate. under the low pressure conditions of our experiments the C3H40 intermediate will fragment to give the products shown in reactions 1-5. The reactive intermediates formed in these reactions will undergo rapid reaction with oxygen atoms resulting in the removal of several oxygen atoms subsequent to the initial 0 C3H4 reaction.
+
Y
w" 20
'i c 78
7
9
s
I
IO
I1
,
i2
13
I
14
I
15
1.P (eV) Figure 2. Correlation of ionization potentials and activation energies for reactions of 0 with P electron containing molecules.
0.
*O
I
I HC=C-CH,
+
H-C=C-CH, I1
I cause of the stabilization of the odd electron by the methyl group. However extended Huckel calculationsll appear to. indicate little or no difference on the energies of the two intermediates. Two experimental studies have a bearing on the discussion at this point. Gutman and coworkers have observed the initial products of the O+ propyne reaction in a crossed beam experiment.12 The major channels are
0
+ CH,CCH
--
--
C,H,O + H C,HO CH, CO (C,H,)* C2H, + HCO C,HZO + CH,
+
+
(1) (2) (3)
(4) (5)
This multiplicity of reaction pathways along with the observation of scrambling of labeled hydrogen atoms indicates the presence of a series of intermediates capable of rapid interconversion prior to fragmentation. It is not possible to establish a unique addition site on the basis of their results. In the second study Strausz and coworkers have investigated the role of oxirene intermediates in the photochemically induced Wolff rearrangement of several diazoke-
The Journal of Physical Chemistry, Vol. 79, No. 24, 1975
Acknowledgment. The support of this work by Research Corporation and the Petroleum Research Fund of the American Chemical Society is gratefully acknowledged. References and Notes Results of numerous studies are compiled In J. T. Heron and R. E. Huie, J. Phys. Chem. Ref. Data, 2, 467 (1973). R. J. Cvetanovic, Adv. Photochem., I,115 (1963). ( a )G. S.James and G. P. Glass, J. Chem. Phys., 50, 2268 (1969);(b) A. A. Westenberg and N. DeHaas, J. Phys. Chem., 73, 1181 (1969). The results of a study of thls reaction using mass spectrometric detection have recently come to our attention: P. Herbrechtsmeier and H. Gg. Wagner, 2. Phys. Chem., 93, 143 (1974).The activation energy reported in this paper, €, = 2.00kcal/mol, is in excellent agreement with our own. (5)J. M. Brown and B. A. Thrush, Trans. Faraday SOC.,63,630 (1967). (6)C. A. Arrlngton, W. Brennen, G. P. Glass, J. V. Michael, and H. Niki, J. Chem. Phys., 43, 1389 (1965). (7)K. A. Quickert. J. Phys. Chem., 76, 625 (1972). (8) Values taken from ref 1. (9)The charqe transfer nature of the transition state was suggested by R . .. J. Cvetansvic in ref 2. (IO) J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory", McGraw-Hill, New York, N.Y., 1970,p 118. (1 1) I. G. Csizmadia, H. E. Gunning, R. K. Gosavi. and 0. P. Strausz, J. Am. Chem. SOC.,95,133 (1973). (12)J. R. Kanofsky, D. Lucas, F. Pruss, and D. Gutman, J. Phys. Chem., 78, 311 (1974). (13)J. Fenwick, G. Frater, K. Ogi, and 0. P. Strausz, J. Am. Chem. Soc., 95, 124 (1973). (14)See related work on 0 2-butyne reported in ref 1 1 and in H. E. Avery and S.J. Heath, Trans. Faraday SOC.,66, 512 (1972).
+
