EFFECTS OF ETHENE AND ETHANE ON THE PHOTOCHEMICAL PRODUCTION OF CO
2893
Effects of Ethene and Ethane on the Photochemical Production
of Carbon Monoxide from Acetone by A. S. Gordon and R. H. Knipe Research Department, Chemistry Division, Code 6069, Naval Weapons Center, China Lake, California 93666 (Received March 2, 10'70)
The effect of ethane and ethene on the photolysis of acetone has been studied from 30 to 350". Ethane increases the rate of CO production at lower temperatures, becoming inert at about 150'. Ethene is an inhibitor, its effectiveness decreasing with increasing temperature. A t about 150' it becomes inert, but then at higher temperature it increases the rate of formation of CO. Evidence is presented that the high-temperature photochemical reaction between acetone and ethene is a radical chain reaction, the chain length increasing with temperature. The reaction has been studied extensively at 350'. A mechanism is proposed which is in accord with the observations.
Introduction The effect of various gases on the photochemical production of CO from acetone has been reported by Caldwell and H0are.l They report an increase in yield which depends on the pressure and nature of the added gas, and the temperature. In this paper we would like to report an extension of the above study in which we concentrated on ethane and ethene as diluents because the "chemical" effects of quenching could be compared with the "collisional" effects. A new effect of ethene has been studied in some detail.
Materials and Apparatus Eastman Kodak spectroscopic grade acetone was used without purification since gas chromatographic analysis showed that no appreciable impurity was present. It was freed of air by the conventional freezepump-warm cycle. Phillips ethene had only a few hundredths of 1% ethane and methane impurities; Phillips ethane had no discernible impurity by our gas chromatographic analysis. The reactants were photolyzed in a heated cylindrical quartz vessel, 2-cm in diameter and 4-em long with flat end windows. A similar vessel filled with quartz shell tubing, with the long axes of the pieces of tubing parallel to the long axis of the reaction vessel, was used in experiments to evaluate the effect of surface. A medium-pressure mercury lamp (Hanovia) was the radiation source. It was used unfiltered or with a lime glass filter which sharply cut all the radiation below 2900 8, with -35% transmission a t 3130 8, and 100% at 3600 A. The intensity was varied with stainless screen filters. The reaction products were quantitatively transferred onto a 3 / ~ e in. X 6 ft 1.5% squalane on pelletex gas chromatograph column, temperature programmed from liquid NZto 100". The low-boiling products separated
on a pelletex column were trapped on a 3//16 in. X 8 f t Sieve 5A column, temperature programmed from liquid Nz to 110". The CO analyses in the presence of ethane have been shown to be about 10% low due to entrapment in the solid during chromatographic analysis. A correction is not applied because the experimental scatter of the results is of the same order of magnitude.
Results and Discussion In Figures 1 and 2, the ratio of the rates of CO formation from a constant concentration (2.36 X M) of pure acetone vapor in the presence of 2.37 X M of ethane or ethene is plotted as a function of temperature. To emphasize the dual effect of ethene (Figure 2) , the ratios plotted for the low-temperature range, RCO~~~~/R are C the O , ,inverse , ~ ~ , of the ratios plotted for the high-temperature range. Atlow temperature the effect of ethane (Figure 1) isto increase the rate of CO production; the effect decreases with increase in temperature until about 150', where the effect disappears. Since we did not survey the effect of pressure of ethane at any one temperature, our results cannot be compared in detail with those of Hoare and Ca1dwell.l Our results, in qualitative agreement with theirs, show that the effect of diluent decreases with increase in temperature. The photo rate of production of CO from acetone in the presence of ethene relative to that from pure acetone is plotted in Figure 2. At low temperatures (30 to 125") ethene is seen to be an effective quencher. Actually, its quenching effect is greater than indicated by the ratio, since the ratio indicates the overall effect, which is, in part, an increase in rate of formation of CO because ethene also behaves as a third body. The (1) J. Caldwell and D. E. Hoare, J . Amer. Chem. SOC.,84, 3987 (1962).
The Journal of Physical Chemistry, Vo2. 74, No.16, 1970
2894
A. S. GORDON AND R. H. KNIPE
LIME GLASS X
UNFlLTEREO
0 0.8 % S C R E E N
I
2 b.
CO MIX CO P U R E
Figure 1. Ratio of rates of formation of CO from and a 10 ethane-1 acetone mixture acetone (CODUre) (CO,i,) as a function of temperature.
filter reduced the total actinic flux in addition to eliminating the shorter wavelengths. The hypothesis was tested by replacing the lime glass filter with a neutral density filter of stainless steel screen which transmitted 35% of the radiation. This increased the rate of CO production from twice that of pure acetone to over three times that for pure acetone under the same conditions. When a second stainless steel screen was inserted, giving an 85% overall reduction of intensity, the rate climbed to over five times the pure acetone rate under the same conditions, and for a screen with a transmission of 0.8% the ratio climbs to about sixteen. The results are characteristic of a chain reaction; the lower the flux the lower the steady-state radical concentration and the longer the chain length, since the chains are terminated by radical-radical reactions and carried by radical concentration to the first power.
LIME G L A S S X
UNFILTERED
0
0 8 % SCREEN
e
10-
C O PURE CO MIX
and a 10 ethene-1 acetone mixture Figure 2. Ratio of rates of formation of CO from acetone (CODUre) (CO,i,) as a function of temperature.
effect of ethene, as that for ethane, decreases with temperature until about 125-150", where there is no apparent effect. With increasing temperature a new phenomenon appears; the photo rate of CO production in the presence of ethene increases relative to the rate for pure acetone. When a lime glass filter is used, the effect of ethene on acetone photolysis is amplified so that a t 350" the rate of CO production is eight times that for pure acetone a t the same pressure as the acetone partial pressure in the mix; with unfiltered radiation the rate from the ethene-acetone mix is twice the pure acetone rate. Although this might argue for a wavelength effect, absorption spectra of the mixes showed no significant excess absorption at any wavelength in the presence of ethene in the temperature range of this study. The apparent inconsistency was resolved when it was recognized that the insertion of the lime glass The Journal of Physical Chemistry, VoL 74, No. 16, 1970
The effectof temperature was not studied above 350" because ketene is a product of acetone photolysis at higher temperatures increasing markedly with temperature. This reactive product would be an added complication. Since the quantum yield of CO from an acetoneethene mix can be many times unity at 350",a study of the effects of concentration, intensity, and surface was made at this temperature. The rate of CO produced was found to be moderately retarded by surface and independent of ethene concentration over a tenfold range (see Table I). The effect of light intensity and acetone concentration could be correlated with the amount of light absorbed as monitored by the rate of CO production from pure acetone a t a pressure equal to the partial pressure of acetone in the acetone-ethene mix under the same experimental conditions.
EFFECTS OF ETHENE AND ETHANE ON
THE
PHOTOCHEMICAL PRODUCTION OF CO
2895
~~
~
~~~
Table I: Effect of Some Olefins on the Relative Quantum Yield of CO from Acetone Photolysis a t 350" [Acetonel/
4RCOpure
[COlmix/ [COlpure
mo1"z sec/l.''2
[Acetone], M x 10-8
Reaction mixt, ethene-acetone
Time, sec
Surface/vol
Re1 intens
1.3 1.9 1.8 2.1 2.5 2.5 3.4 3.0 2.8 5.6 5.3 4.6 5.2 7.3 6.5 8.7 6.0 4.1 13 14 11 12 10 7.7 15.7 15.3 13.5
1.5 3.4 3.2 3.9 5.7 6.2 8.9 8.6 8.6 14.3 14.1 14.3 17.8 28 29 33 32 35 52 49 53 54 53 57 50 52 50
0.26 1.03 1.03 0.26 2.32 2.32 1.03 1.03 0.26 2.32 2.32 2.32 1.03 2.32 1.03 1.03 1.03 1.03 2.32 2.32 2.32 2.32 2.32 2.32 2.32 2.32 2.32
1 1 10 1 1 1 1 10 1 1 1 1 1 1 10 10 10 10 1 1 1 1 1 1 10 10
120 60 60 960 60 60 480 240 1800 30 60 240 600 300 420 420 1800 900 120 120 480 900 1800 900 90 90 90
1.6 1.6 1.6 1.6 1.6 7.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 7.6 1.6 1.6 1.6 1.6 1.6 7.6 1.6 1.6 1.6
100 100 100 14 100 100 14 14 L.G." 14 14 14 L.G." L.G." 1 1 1 1 1 1 1 1 1 1 L.G." L.G." L.G."
13.6
52
2.32
90
1.6
L.G.a
120 960 960 300
1.6 1.6 1.6 7.6
1 1 1 1
960
1.6
1
0 . 1 propene10 ethene1 acetone 0.1 propene10 ethene1 acetone Propene-acetone
1.9 1.4 1.3 9.2 12 a
52 54 54 57
2.32 2.32 2.32 2.32
45
2.32
1 1 5 9 He1 ethene1 acetone 1 CFaCHCH21 acetone
L.G. = Lime glass filter.
The ratio of rates of CO production from the acetoneethene mix to that from the same pressure of pure could be varied from a value acetone, CO,i,/CO,,,,, slightly greater than 1 to a value of 16, depending on the partial pressure of the acetone in the mix and the light flux. A simple mechanism which is consonant with the observations may be formulated
+ CO CH3 + CHaCOCHa +CH, + CH2COCH3 CH~COCHI+ C2H4 +CH2CH2CH2COCHa
CH~CH~CH~COCHZ +CHaClHCHzCOCHs (3') CH3CHCH2COCH3 --it CH,CH=CH2 CH&0 +CH3 CHzCOCHa
+ CHzCOCHa +
+ CO
(4) (5)
CH&OCH2CH&OCH3 (sa)
CH3COCH3 -% 2CH3
+CHaCH2CH2COCH2 CH~CH~CH~COCHZ
+ CHaCO
CH3
+
+ CH3 +C2Hs
(Bb)
CH3 CH2COCHa ----f CHaCHzCOCHa ( 6 ~ ) Assume that reactions 3, 3') 4, and 5 are very rapid relative to reaction 2 (ie., (cH2COCH3)>> (CH2CH2The Journal of Phyaieal Chemistry, Vol. 7 4 , No. 16,1070
2896
A. S. GORDON AND R. H. KNIPE
+
CHZCOCH,)or its isomers). Then Rcomi, = Rcopure k2(CH2COCH,)(CzH4) and dividing by RCOpure
I6
I
I I ETHENE / I K E T O N E 0 IO E T H E N E / I ACETONE
A
A
where R = rate. , A t steady state
LI
b
I ETHENE 10 ETHENE
I
I / /
I bCETOHE I KETONE
---
I
I
I
lo/
I/d
I
low i w l o e ~ 1.
WIIOCI
high ~ u i l o ~ i hlph I v r f o e I
I N C L U D E S E F F E C T O F PPPRECIABLE PROPENE G E N E R A T E D D U R I N Q T H E C O U R S E OF R E A C T I O N INCLUDES E F F E C T O I W I L L bMOUHT O F P R O P I N E IN R E A C l b N l G A S M l X l U R L w i l w s n o o i REICION TIME
+ 2keC(CHa)(CH&OCH,)
k~b(CHa)'
where the factors of 2 multiplying the k's take into account that two radicals are destroyed for each product molecule produced and ki(CHB)(CH&OCHa) = kz(CHzCOCHa)(CzH4)
+
+
~ ~ , , ( C H Z C O C H ~L)B ~~(CH~)(CH~COCH~)
/A
For long chains the propagation reactions 1 and 2 dominate those reactions which quench the acetonyl radical 6a and 6c, so that RcOmix RCOpure
-
1 +:
kl(CHaCOCH3) o ki( CHaCOCHa) keb k2(~z~4)
+
+
:
i
i
Aoetone/l/Rco,,,,. 20 ao
I 40
BO
Figure 3. Effect of [acetone]/[rate CO formation in pure acetone] 'I2on the relative rate of CO formation in acefone-olefin mixes at 350'.
Our experiments at 350" show that (CzHI)concentraat 350" do not permit quantitative evaluation has no appreciable effect on the ratio R C O ~ ~ ~ / R C O ,experiments ,~, tion of the reaction of acetonyl and ethene. Our only over the range of C2H4 concentrations studied. To interpretation is that the acetonyl radical adds rapidly rationalize this observation with the mechanism, to ethene and that the sequence of isomerization reackz(CzH4)must be larger than kl(CH3COCH3). Thus tions and radical decompositions to form a molecule of RcOmix kl(CH3COCH3) CO and a methyl radical is so fast that all the inter%l+ 1/2k6al/i mediate radicals are held a t relatively low steady-state RCOpure RCOpure concentration. As may be seen in Figure 3, the experimental points The simple mechanism proposed above assumes that fall on a reasonably good straight line when plotted as a methyl radical is the only alkyl radical in the chain. os.O(CH&OCH3)/RcopUre '/a. function of R C O ~ , ~ / R C ~~~, Actually, it has been shown4 that methyl radical in the Since a range of almost 16-fold is encompaseed by the presence of ethene at 350" sets up long chains in which experiments, the proposed mechanism is given some n-propyl, and probably n-pentyl, and n-hexyl radicals substantiation. The slope of the line in Figure 3, are present in addition to methyl radicals. We assume kl/ke'/~ is 0.3 1.'" sec-'/', easily within the all the alkyl radicals abstract H from acetone with the combined errors of this and other work2 where JCI/JC~~/' same specific rate constant. Work on methyl and has been measured by a different technique in a lower n-propyl abstraction3 indicates that the energy of temperature range. It should be noted that for values activation for abstraction of H is greater for methyl of N 1, the chain length is short. In than for propyl radical while the A factor is less for this region, the assumption regarding the dominance of propyl. The factors compensate to some extent, and a t propagation over quenching is not valid. However, 350°, specific rate constants are probably the same since the value 1 is a lower bound for R C O , , , ~ ~ / R C O ~ ~ ~ ~ , within a factor of 5 , with that for methyl being the deviations from the linear relationship indicated in larger. Figure 3 due to this cause would not be detectable The parallel reaction chain initiated by the addition within the precision of the experimental values. of methyl radicals to ethene introduces considerable Endrenzi and LeRoy,3 in an analysis of the products of the same complicated system of acetone and ethene (2) H. Shaw and 8. Toby, J. Phys. Chem., 72,2337 (1968): E. Whittle in a lower temperature range, come to the conclusion and E. W. R. Steacie, J. Chem. Phys., 2 1 , 9 9 3 (1953). that the Arrhenius factors for addition of methyl and (3) L. Endrensi and D. J. LeRoy, J. Phys. Chem., 71, 1334 (1967). acetonyl radicals to ethene are the same, even though (4) A. S. Gordon and J. R. McNesby, J. Chem. Phys., 31, 853 the acetonyl radical has some resonance energy. Our (1959). The Journal of Physical Chemistry, Vol. 74, No. 16, 1970
PHOTODISSOCIATION OF HYDROQUINONE DERIVATIVES complexity into the interpretation of the hydrocarbon products. However, propene has been observed to be a major product of the reaction as would be predicted by both chain mechanisms. The CO-producing mechanism predicts that
We observe the propene rate to be at least tenfold the rate from the above relation, showing that the chain initiated by methyl adding to ethene is a t least ninefold faster than the chain producing CO. This implies that methyl and other alkyl radicals are important chain carriers in the reaction and thus the proposed mechanism represents a considerable simplification. It may be noted that some of the points in Figure 3 are considerably below the line. Propene has three easily abstractible hydrogens and can act as a chain quencher. The hypothesis was tested in three ways. (1) The same mixture was fun for different lengths of time. As time increased the points fell further below the line. (2) Propene-acetone mixes ( 1 : l and 5 : l ) were run under conditions where ethene-acetone mixes would give COmix/COpure ratios up to 14. The ratios were less than 2, independent of the mix ratio,
2897 confirming that propene is a strong chain inhibitor in the system. (3) Ethene-acetone mixes containing a small amount of propene were run a t short times under conditions where high COmix/COpure ratios are found. A significant decrease in CO yield was observed. The easily abstractible H atoms in propene were blocked by using l,l,l-trifluoropropene in place of the propene. As may be seen by reference to Table I, a 1:1 mix produced a COmix/COpure ratio very close to that for ethene-acetone. I n addition, a l,l,l-trifluorobutene was observed with a rate roughly equal to Rcomix- Rcopure. On the basis of the mechansism, the butene should be CF3CH2CH=CH2, but isomeric identification was not attempted. Since propene has such a large effect on the R C O ~ ~ ~ ~ / R ratio, C O ,the ,,~~ experimental values at even the shortest times are somewhat low and the slope of the experimental line is less than the true slope. Most of the studies were made in a reaction vessel with a surface/volume = 1.6. I n order to ascertain the effect of surface, some studies were made in a reaction with a s/v = 7.6. As may be noted in Table I and Figure 3, the bulk of such reactions show that the surface lowers the COmix/COp,re ratio, probably by acting as a radical trap.
Mechanism of Photodissociation of Hydroquinone Derivatives
by Hikoichiro Yamada, Nobuaki Nakashima, and Hiroshi Tsubomura Department of Chemistry, Faculty of Engineering Science, Osaka Uniuersity, Toyonaka, Osaka, J a p a n (Received January 81, 1970)
The mechanism of the photodissociation of hydroquinone and some of its derivatives irradiated in the nearultraviolet region in the matrix of ethanol at 77'K was studied by use of the electronic absorption spectra of these radicals. The dependences of the rates of radical formation on the light intensity and the effect of intermittent irradiation on the radical yield were investigated, and it was concluded that the side-chain photolysis of these compounds occurs through the biphotonic, two-step process oia the lowest triplet state of the parent molecule. It was also concluded that these reactions may occur unimolecularly. A new procedune was proposed for the treatment of the intermittent irradiation data.
It is well known that the near-ultraviolet irradiation of benzene derivatives in rigid matrices produces two types of unstable species. One is the radical due to the fission at the p bond of the side ~ h a i n , l -and ~ another is the cation due to the ejection of an electron into the m a t r i ~ . 4 - ~One of the present authors has pointed outBthat these types of photochemical products from various aromatics are related with their ionization potentials, namely the photoionization is predominant for aromatics having low ionization potentials such as
tetramethyl-p-phenylenediamine,while photodissociation is predominant for aromatics having rather high ionization potentials such as anisole. (1) G.Porter and E. Strachan, Trans. Faraday Soc., 54, 1595 (1958). (2) E.J. Land, G. Porter, and E. Strachan, ibid., 57, 1885 (1961). (3) B. Brocklehurst, W.A . Gibbons, F. T. Lang, G. Porter, and M. I. Savadatti, ibid., 62, 1793 (1966). (4) G. N. Lewis and D. Lipkin, J . Amer. Chem. Soc., 64, 2801 (1942). (5) G. N. Lewis and J. Bigeleisen, ibid., 65, 520 (1943). The Journal of Physical Chemistry, Vol. 74, No. 16, 1970
