ACETYLENE PHOTOLYSIS
AT
1236 AND 1470 R
2929
It is unfortunate that the high-resolution nmr technique used in this work did not yield more results on IC4 such that a comparison could be made between IC4 and kH based on the above discussion. The results presented here show that in addition to the dispersion forces discussed by Grunwald as a factor which effects k~ is the specific proton-aromatic interaction. It is doubtful that aromatic-substituted amines should be used in any attempt to correlate values of k~ with parameters such as partial molar volume of a mixed set of aromatic and aliphatic amines.
somewhat unexpected. However, inspection of models of the compounds studied shows that there is considerable freedom of orientation of the substituents and that the orientation required for hydrogen bond formation between the water molecule and the benzene ring is not particularlg favorable. Therefore the value of kH might be expected to approach that of a tertiary aliphatic amine. I n fact the value of k~ for triethylamine has been reported as 3.8 X lo9 a t 25°.6 Also models indicate a favorable steric arrangement for the water molecule to participate in a bimolecular transfer.
Acetylene Photolysis at 1236 and 1470 A by Seiki Takita, Yuji Mod, and Ikuzo Tanaka Laboratory of Physical Chemistry, Tokgo Institute of Technology,Ohokayama, Meguro-ku, Tokgo, Japan (Received February 9,1969)
To investigate the depsndence of the excitation wavelength on the acetylene photolysis, the experiments both at 1236 and 1470 A were carried out. The systems in the present work were acetylene, acetylene-hydrogen, acetylene-rare gases, acetylene-nitrogen, and acetylene-methane with and without a small amount of nitric oxide. These results were compared with those obtained previously. It was found that rare gases, nitrogen, and perhaps hydrogen, too, hardly affected the excited state of acetylene. It was found that the ratio ethylene/l,3-butadiene increased in proportion to the reciprocal of the pressure in all systems except acetylene-hydrogen, where the following chain reactions may be contained.
C2H2
+ H +C2H**
C2Ha* +M +C2H3 C2H3(*)
+ Hz
---t
C2H4
+H
The ratio lca/lca'
C2H2*
+ C2H2 -% C4H2 + 2H ka'_ C4H2 + H2
seems larger at 1236 than at 1470 A.
1. Introduction Acetylene photosensitization of methane was already reported,' where the ratio of reaction rates of excited acetylene molecule with ground state of acetylene and methane was estimated as the order of lo2. In the present work some new experiments w2re performed in the acetylene-methane system a t 1470 A; also studies in the acetylene-hydrogen, acetylene-rare gases, and acetylene-n$rogen systems were carried out a t both 1470 and 1236 A, in addition to the acetylene system at 1236 A. These results are compared to each other. Acetylenemethane system was not studied at 1236 A, because the
absorption by methane a t this wavelength cannot be ignored. All products except diacetylene disappeared in all systems when a small amount of nitric oxide was added. The fact that no ethane was formed in the acetylenem e t h a n e N 0 system supports our previous results that the acetylene photosensitization reaction of methane is the type of
+
+ CH3 + H (or C2H3 + CH,)
CZHZ* CH, ---f CzHz
(1) S. Takita, Y. Mori, and I. Tanaka, J. Phys. Chem., 7 2 , 4360
(1968).
Volume 79,Number 9 September 1969
S. TAKITA, Y. MORI, AND I. TANAKA
2930 but not the type of
CzHz*
+ CH4
--+
CzHz
+ CH2 + Hz
because CH, radical is scavenged by NO but :CH2 radical is not scavenged.
2. Experimental Section Kr resonance lamp was made just like the Xe resonance lamp which was already described previously. The lamp emitted resonance radiation at 1236 (10.0 eV) , was and a t 1165 b (10.6 eV). The intensity a t 1165 % less than 20% of the intensity of 1236 A, which was estimated as 1Ol6 quanta/sec, and was considered to be unimportant. All gases such as C2H2,Hz, NO, and rare gases were obtained from Takachiho Coo,and all operating procedures were the same as those in the previous paper, except tbe reaction time which was prolonged to 15 min at 1236 A.
shows the relative rate of formation of each product against the pressure of acetylene. The increase of benzene is more remarkable a t 1236 b than that at 1470 A. Figure 2 shows that the relative quantum yield of diacetylene increases with increasing acetylene pressure, just like the figure given a t 1470-b photolysis. All products except diacetylene disappeared when a small amount of NO was added. Then the reaction mechanism (reactions 1-20) in the previous paper is summarized again.
CzH2
+ hv
--.t
CzH2*
(I&)
C2H2* +C2Hz (+ hv) C2H2* +CzH CzH
+ CzH2
---t
+H
(CdH%*)--+ C4H2
I
(1)
+H
(2) (31
3. Results and Discussion A . The Acetylene System (ut 1636 A ) . The results
obtained in the photolysis a t 1236 b are similar to those obtained at 1470 An1 Some products such as ethylene, diacetylene, l,&butadiene, benzene, and polymer were also observed in the reaction a t 1236 b. Figure 1
!&
401
IO
20
30
LO
PRESSURE OF ACETYLENE
50
60
(Torr )
Figure 1. Relative rate of formation of each product in the photolysis at 1236 d against the pressure of acetylene.
In addition to the reactions 6, 6’, and 7, the following reaction may be probable.
C2H2*
0
10
20
30
40
50
60
70
PRESSURE OF ACETYLENE
Figure 2. The pressure dependence of the quantum yield ratio ( v P / v dof diacetylene in the photolysis at 1236 d. The Journal of Physical Chemistry
+ CzH2 +CzHs + C2H
(6”)
This reaction mechanism was once proposed by Cochran, et U Z . , ~ who observed esr signals of ethynyl free radical prepared by the photolysis of acetylene using a radiofrequency discharge in H2 gas as the light source. Authors, however, stated in their report that this mechanism was speculative and would require further experi(2) E. L. Coohran, F. J. 40, 213 (1964).
Adrian, and V. A. Bowers, J . Chem. Phys.,
ACETYLENE PHOTOLYSIS AT 1236 AND 1470 A
2931 PRODUCT YIELD [ xlO-’Frno[
12
10
10
8
8
6
6
4
4
2
600
400
200
0
800
PRESSURE OF HYDROGEN ( T o r r
(
at
1470A
0
200
400
600
PRESSURE OF HYDROGEN ( Torr (at 1236A )
)
800
1
Figure 3. The pressure dependence of the product yield against the pressure of hydrogen [acetylene (2.1 Torr)hydrogen system] : 0, C4H2; 0 , C2H4; a,1,3-C4Hs; @, C4H4. (a) The photolysis at 1470 A; (b) the photolysis at 1236 A.
mental verification. Since in the present work there are no positive data which will require this reaction mechanism, we will consider 6, 6’, and 7 as the reaction of excited acetylene. The steady-state rate approximation leads to R[CdHnl
where I, means the light absorption by acetylene. After some approximations the following two limiting cases are considered R’[C&] % kzkira/(ki f kz)(k3 Rh[C4HzI
(k6
+
k6’)1a/(k6
+
+ + k6’ + k6
(22)
k4)
k7)
(23)
where R1[CaH2] and Rh[C4HI]are the rate of formation of diacetylene at low pressure limit and high pressure limit of acetylene, respectively. Therefore, eq 21 is consistent with Figure 2 if Rh[C4H2]> R ~ [ C ~ H ~ ~ . B. CZHz(g.1 Torr)-Hz System. I n this system a remarkable increase of the rate of formation of ethylene was observed, and small amounts of ethane and n-butane were also detected. This indicates that the amount of hydrogen atoms produced in the CzH2-H2
system is much larger than that produced in the acetylene system, The rate of formation of eath product VS. hydrogen pressure a t 1236 and at 1470 A is shown in Figures 3a and b, and from these data the ratios of - R [CPH21) / R [ C I K ~ ]are Plotted 21s. hydrogen pressure in Figure 4, where CpRp means the summation of the reaction rates of all products which were detected. Therefore, this value corresponds to the ratio of the over-all amount of hydrogen atoms and that of diacetylerie produced in the reaction system. This figure shows that C2H3radicals become to be important for the formation of reaction products much more than C2H radicals whose contributions are, on the contrary, decreased. Thus the source of hydrogen atoms should not be only reactions 2 and 3, but also the following reactions which compete with reactions 3-7 have to be added.
+
+
CZH HZ-+ C2Hz H (or CzH3) (24) CzH2* Ht -+ CzH3 H (or CzHz 2H) (25)
+
+
+
The pressure dependencies of the relative quantum yield of diacetylene were obtained by the same method described in the previous paper, and were plotted in Figures 5a and b, together with the data obtained in Figures 3a and b. Since the quantum yield of diacetylene in the acetylene-hydrogen system is almost constant at the pressure of hydrogen higher than 300 Torr, the precursor of diacetylene at this pressure region should not be the radiVolume YS, Number 9 September 1969
S. TAKITA, Y. MORI,AND I. TANAKA
2932
6
2
0
200 4 00 PRESSURE OF
600
H2
800
(Torr)
0
200 400 600 800 PRESSURE OF HYDROGEN I Torr 1 Figure 4. The pressure dependence of the ratio (Z,R, R[Cr&])/R[ClH2] against the pressure of hydrogen [acetylene (2.1 Torr)-hydrogen system] : 0, at 1470 d; 0, at 1236 d.
l b
1.0
l
0-•
0.8
cal C2H but the excited state of acetylene, as will be described in the following. If reaction 25 is efficient, the yield of diacetylene cannot be constant at high pressure of hydrogen; then reaction 25 may not be important. These estimations are proved by the fact that the addition of argon, which would not react with ethynyl radicals, scarcely affected the rate of the formation of diacetylene, as is shown in Figure 5b. Although the formation of diacetylene in the acetylene-hydrogen system is reduced to about 0.6 in the photolysis a t 1236 d, it is reduced to 0.4 in the photolysis a t 1470 d, and it may be concluded that the role of the excited staJe of acetylene is more important at 1236 than a t 1470 A. Even when nitrogen was added, however, the relative rate of formation ofdiacetylene remained constant, which is shown in Figure 6; then .the precursor of diacetylene at high pressure - of. .nitrogen should still be the same excited state of acetylene which is accompanied with reaction 6. Since the rate of ihe formation of ethylene increases continuously as the pressure of hydrogen increases, as is shown in Figure 3, the following reaction will be required. C2H3
+ Hz
-
CzH4
+H
(26)
Although the reactions 10, 11, 14, 15, and 26 compete with each other for the consumption of vinyl radicals, it is estimated that reaction 26 is dominant when the pressure of hydrogen is high, because the rates of reactions 26 and 15 are estimated as 109,6-6.4/@ and l o s e 3 1. mol-' sec-l14 respectively, where 0 is 2.303RT. Therefore, reaction 26 becomes comparable to reaction 15 at the pressure of hydrogen of about 4000 Torr, when the pressure of acetylene is 2.1 Torr. The increase of ethylene yield in this system, however, is more remarkable than that expected from the rate of reaction 26. Since the initial vinyl radical formed from The Journal of Physical Chemistru
0.6
0.4
-
0
200
600
400
PRESWE OF M
8 00
( Torr )
Figure 5. The pressure dependence of the quantum yield ratio ((pP/'pp.1) of diacetylene in the photolyses at 1470 and a t 1236 d against the pressure of hydrogen: (a) acetylene (2.1 Torr)-hydrogen system, at 1470 A; (b) 0, acetylene (2.1 Torr)hydrogen system; 0, acetylene (2.1 Torr)-argon system, at 1236 d.
+
the addition reaction of CzHz H in this system is considered to be in a highly vibrationally excited state with about 41-kcal/mol excitation energy, the reaction of hot vinyl radical may be important. C2H3*
+ Hz
--3
+H
C2H4
(27)
The difference between two curves of the rate of formation of ethylene, shown in the Figures 3a and b, may be caused by reaction 6 which will contribute to the formation of hydrogen atoms a great deal. Since the ratio - RICaHII)/RICaHI~ is always larger at 1236 than at 1470 d (Figure 4),it will be concluded thpt the ratio of ks/kw is larger at 1236 than at 1470 A. This is reasonable because the energy of pboton at 1236 A is about 1.6 eV larger than that at 1470 A. Relative quantum yields of diacetylene in the photolysis a t 1470 in the acetylene (2.1 Torr)-methane system are also plotted in Figure 7 against the pressure of methane. This figure is essentially different from those
(xpRp
(3) S. W. Benson and G. R. Haugen, J . Phys. Chem., 71, 4404 (1967). (4) S. W. Benson and G. R. Haugen, ibid., 71, 1735 (1967).
ACETYLENE PHOTOLYSIS AT 1236 AND 1470 A
I
2933
0
PRESSURE OF N2 ( Torr )
Figure 6. The pressure dependence of the relative rate of formation of each product against the pressure of nitrogen in the photolysis at 1236 A [acetylene (2.1 Torr)-nitrogen system].
L
5
20
P-'x 10' ( T o r i ' )
1.0
0
: -
0.8
>
5
10
0
Figure 8. The plot of the ratio of (R[C~H+J - R[CrHd])/ R[l,8-C,HLI]against the reciprocal of the pressure (the pressure of acetylene is 2.1 Torr).
0.6
L
z
Q
0.4
W
2 0.2
3!
W 1
II:
0
PRESSURE OF CH, (Torr) Figure 7. The pressure dependence of the quantum yield ratio ( ~ p , / p ~ .of l ) diacetylene against the pressure of methane [acetylene (2.1 Torr)-methane system at 1470 81.
obtained in the various reaction system; that is, the quantum yield of diacetylene goes on to decrease with increasing the pressure of methane. This indicates the existence of some reaction process which competes with reaction 6 and will strongly support the acetylene photosensitization of methane as was proposed in the previous paper. C. Disproportionation and Recombination of Vinyl Radicals. A few studies on the recombination and disproportionation reactions of vinyl radicals in the gas phase have been reported. Tickner and LeRoy5 produced vinyl radicals from vinyl bromide and iodide by using the sodium diffusionflame technique and observed products attributable to disproportionation and recombination reactions. Trotman-Dickenson and Verbeke6 studied the pyrolysis of mercury divinyl in a toluene carrier flow system a t temperatures between 502 and 642", where they detected acetylene and 1,3-butadiene, which indicate the disproportionation and recombination reactions of vinyl radicals, among the products. Similar results were obtained by Gunning in the photolysis of mercury divinyl.' As for the ratio of the dis-
proportionation and recombination reactions of vinyl radicals, it has been obtained as 1.1by Weir,* where the vinyl radicals were prepared by methyl radical sensitized decompositions of acrylaldehyde and vinyl formate a t 175". I n the previous paper we reported that the ratio of 1,3-butadiene and ethylene depended on the pressure of the system. I n the present work we also observed its pressure dependency in the acetylene-rare gas and acetylene-nitrogen systems. The results are shown in Figure 8. Since the vinyl radicals formed by the addition reaction of hydrogen to acetylene are in a highly vibrationally excited state, the deactivation of vinyl radicals may affect the rates of formation of the products which will have some relation with vinyl radicals in their formation processes. The deactivation of vinyl radicals, however, can not explain the present data unless it is considered that the ratio of disproportionation and recombination depends on the energy possessed by vinyl radicals, which may not be essentially different from the deactivation of C & , Therefore the following mechanism may be preferable.
*.
+ CzHz C2H3* + M H
--j
---+
C2H3*
C2H3
+M
(28) (29)
According to Benson and Haugen the pyrolysis of 1,3Tiokner, J . Chem. Phys., 19, 1247 (1951). (6) A. F. Trotman-Dickenson and G. J. 0. Verbeke, J . Chem. SOC., 2580 (1961). (7) A. G. Sherwood and H. E. Gunning, J . Phys. Chem., 69, 2323 (1965). (8) N. A. Weir, J . Chem. SOC., 6870 (1965). ( 5 ) D. J. LeRoy and A. W.
Volume 79,Number 9 September 1966
S. TAKITA, Y. MORI,AND I. TANAKA
2934 butadiene, which was recently investigated by Skinner and S ~ k o l o s k icould , ~ be interpreted by the decomposition of 1,a-butadiene to give 2CzH3, and this will indicate that the back-reaction of reaction 11 is important. Moreover reaction 12 will not be important because the should be in an electronically ground state. Then the steady-state rate approximation becomes
+ h[Ci"l[Ci"]
(30)
- R [ c ~ H=~ ]ho[CzH31a
(31)
RIC~H =~kio[C2%l2 I Then R[C&l On the other hand
The left hand of eq 34 in the various systems is plotted in Figure 8 against the reciprocal of the third body's pressure. All lines approximately coincide in their intercepts, which are obtained as about 3, and are in good agreement with eq 34. The experimental results show that heavy atoms are more efficient than light atoms and also molecules are more efficient than atoms as the deactivator of C4H6*. These results may be reasonable because C4Hs*should be in a vibrationally excited state and vibration-vibration energy transfer is considered to be more probable than vibration-translation energy transfer. From the slope of the lines in Figure 8, the relative efficiency of each gas as a deactivator of C4Ha* obtained. The results are shown in Table I. Table I
Since the yield of vinylacetylene, which is one of the minor products, does not increase as the pressure of the system increases, and also the yield of 1,3-butadiene is much less than that of vinylacetylene in the acetylene 2.1 Torr system, reaction 17 may be neglected. Then
M
He Ne Ar
Nz CHI CaHe
From (31) and (33)
a
1.0 1.1 4.7
7.7 12.0" 84.1-
Reference 1.
(9) 6.B. Skinner and E. M. Sokoloski,
(1960).
The Journal of Physical Chemistry
Efficiency a8 a deactivator of CIHO*
J. Phya. Chem.,
64, 1028
