Reaction of Oxygen Atoms with Carbonyl Compounds Part 3.-Ketene BY G. P. R. MACK AND B. A. THRUSH* University of Cambridge, Department of Physical Chemistry, Lensfield Road, Cambridge CB2 1EP Received 27th July, 1973 The reaction between excess atomic oxygen and ketene has been studied in a discharge flow system by product analysis and kinetically using e.p.r. and chemiluminescznce. This rate constant for the initial step, which yields predominantly HCO+ HCO was found to be (3.4 k 0.3) x 10' ' cm3 mol-' s-' at 293 K. The subsequent reactions are O+HCO -+ OH+CO O+HCO -+ H+COz H+HCO-t H2+CO OfOH -+ 02+H. Can, Gay, Glass and Niki used a mass-spectrometer to study the reaction of ketene with atomic oxygen in a discharge-flow system. They deduced that the initial step was addition of an oxygen atom to ketene, predominantly at the olefinic carbon atom ; the observed products (CO, C02, H20, H2C0, H2, H) could be explained by rapid decomposition of the adduct followed by reaction of its fragments with atomic oxygen and with ketene.We have investigated the 0 + CH2C0 reaction in the apparatus previously used for the O+CH3CH0 and O+H2C0 reactions 2* ; and show that the initial step yields predominantly HCO + HCO formed by the isomerisation of the initial adduct to a vibrationally excited glyoxal molecule which then decomposes. Our results do not support the view that ketene is an important intermediate in the reaction of excess atomic oxygen with acetaldehyde. EXPERIMENTAL The reaction was studied in the discharge flow systems previously de~cribed.~ Atomic concentrations were measured by e.p.r.spectroscopy or the air afterglow and the reaction products were analysed by gas chromatography. All experiments were carried out at 293 K. Ketene was prepared by the thermal decomposition of acetic anhydride at 820K. Acetone and unreacted anhydride were removed at 195 K in a trap packed with glass wool. The ketene was purified by bulb-to-bulb distillation and a residual impurity of ethane was estimated by gas chromatography to be 5-8 %. All errors quoted are one standard deviation. RESULTS AND DISCUSSION STOICHIOMETRY OF THE REACTION The number of oxygen atoms consumed per ketene molecule added and per hydrogen atom formed were measured by e.p.r. The values were plotted against initial ketene concentration and extrapolated to zero reactant concentration to obtain the limiting stoichiometries of reaction.For both medium (cu. 30ms) and long 187188 0-ATOM + CARBONYL REACTIONS (ca. 100 ms) reaction times good straight lines were obtained; these plots yielded 3.7k0.2 oxygen atoms consumed per ketene molecule at long reaction times. The ratio of oxygen atoms consumed to hydrogen atoms produced was obtained by similar extrapolation to zero ketene concentration as shown in fig. 1. It fell slightly from 2.1 +O. 1 at short reaction times (ca. 5 ms) to 1.8 f: 0.1 at medium reaction times. The hydrogen atom yield per reactant molecule dropped significantly as the reactant flow increased, but this effect was less than in the O+CH,CH02 and O+H2C03 reactions, so that for large ketene flows [HI> [O].The maximum concentration of hydrogen atoms generally occurred for [O] - 3[H]. - OO 1 2 3 G 1 010[CH2CO]o~mol ~ m - ~ FIG. 1.-Stoichiometry of hydrogen atom production at short reaction time (4.46 ms). [010 = 2.52 x mol STABLE PRODUCTS The analyses for carbon dioxide (table 1) show that 0.65+0.03 molecules of C02 per CH2C0 are formed at long reaction times. For shorter reaction times, where the reaction was not complete before quenching on the silver foil, the C02 yields are higher. Small amounts of formaldehyde and water could be detected under such conditions, but only traces of ketene suggesting that it was largely destroyed on the silver foil. TABLE 1 .-CARBON DIOXIDE YIELDS total total yield pmol pmol C W CHzCO r010/ CHiCO/ co21 total pressure/ reaction 1O~~~OIol Torr timelms mol cm-3 [CH~COIO 1.85 122.0 4.6 29.8 31.5 20.0 0.64 1.85 122.0 4.2 21.5 39.4 26.8 0.68 1.85 122.0 4.1 25 .O 39.2 24.2 0.62 2.25 21.9 2.46 10.0 39.2 30.8 0.79 1.93 11.8 1.31 25 .O 30.1 22.7 0.75 KINETICS OF OVERALL REACTION As established by Carr et d.,l the reaction is first order in [O] and in [CH,CO].The rate coefficient was determined from the oxygen atom decays at fixed short reaction times when varying amounts of ketene were added, the integrated second- order rate equation was used in the form where nx = [O], - [O] and n is the stoichiometry of oxygen atom consumption. To obtain large decays, these runs were carried to comparable initial oxygen atom and ketene concentrations, and the stoichiometries measured with excess oxygenG . P .R . MACK A N D k. A . THRUSH 189 atoms cannot be substituted directly. Apart from the initial points, values of n = 2, 3 and 4 gave good straight lines (fig. 2) and it is not possible to deduce the correct value of n for high ratios of [CH,CO], to [O], on this basis. It is shown below that n = 3.0k0.2 under these conditions ; putting n = 3 yields a value of kl = (3.42$. 0.09) x 10l1 cm3 mol-l s-l at 293 K which is independent of total pressure over the range 0.9 to 2.7 Torr (table 2). The steeper oxygen atom decay with small [CH,CO], arises from a higher effective stoichiometry of oxygen atom consumption under such conditions. 1010[CH2C8]o/mol ~ r n - ~ [010 = 2.52 x mol crne3. 0, n = 2 ; A, n = 3 ; 0, n = 4. FIG. 2.-Kinetics of oxygen atom consumption for various stoichiometries. Reaction time = 4.46 ms.TABLE 2.-DETERMINATION OF THE RATE COEFFICIENTS OF 0-k KETENE run K11 K12 K2 K13 K4 K3 K5 K1 reaction t helms 3.27 5.24 4.46 6.42 5.68 6.57 8.08 4.91 1010t010/ mol cm-3 2.12 2.12 2.52 2.12 2.46 2.66 2.42 4.55 1010[CH2CO]o range/ mol cm-3 0.2-5.9 0.6-3.6 0.27-4.1 0.58-2.56 0.86-3.85 1.03-3.5 0.77-2.53 0.78-7.9 kl/crn3 mol-1 s-1 for n = 3 3.12 3.57 3.40 3.81 3.19 3.28 3.53 2.78 REACTION MECHANISM Although the C-H bond energy in ketene has not been determined, there is no reason to suppose that it is any lower than in ethylene which hits a slightly longer C-H bond (1.085 A as against 1.079 A). Thus abstraction of an H atom by 0 would be at least 25 kJ mol-1 endothermic giving a Boltzmann factor of less than The initial step must, as with ethylene, involve the addition of an oxygen atom followed by fragmentation of the excited adduct.Carr et al.' consider the following processes at 293 K even if the reverse process has no activation energy. 0 + CH,CO+CH,CO~-CCO + H20 + 46 kJ mol-1 (14 -+H,CO + CO + 408 kJ mol-1 (I@ -+CH, + C02 + 197 kJ mol-1 (1 4 +HCO + H + CO + 56 kJ mol-1 (14 -+HCO+HCO+ 138 kJ mol-l. ( 1 4190 0-ATOM + CARBONYL REACTIONS Apart from its inherent implausibility being a weakly exothermic four-centre reaction, the lack of water formation in the presence of excess atomic oxygen excludes any large contribution from reaction (la). With reaction (lb) as initial step, atomic hydrogen is produced only by the subsequent 0 + H2C0 reaction which is four times slower than (1).In the presence of excess atomic oxygen (1 c) would be followed by giving a stoichiometry of which agrees with neither of the measured stoichiometries. rapidly with ketene [CH,CO], which is not observed. be (Id) and (le) with carbon dioxide being formed in one of the subsequent steps O+CH2 -+ CO+2H+316 kJ mol-' (2) 2 0 + CH2C0 -+ CO + C02 + 2H Furthermore, CH2 reacts and this would give a reduced hydrogen atom yield at high Thus, none of these three processes can be important, and the dominant paths must 0 + HCO -+ OH + CO + 350 kJ mob1 0 + HCO -+ H + C02 + 455 kJ mol-' (3) (4) H+ HCO -+ H2 + CO + 358 kJ mol-' O+OH 3 H + 0 2 +71 kJ niol-l (5) (6) where k3 : k4 : k, = 0.46 : 0.54 : 4.0 and k6 = 3 x l O I 3 cm3 mol-' s - ' .~ The initial ratios [O] to [CH,CO] used for the C02 analyses were such that the relation derived in Part 1 would predict CO, yields per CH2C0 molecule of 0.4 and 0.8 respectively for paths ( I d ) and (le) plus their subsequent reactions and 0.4 for path (lb) followed by complete oxidation of the formaldehyde formed. However, there was no evidence of a decrease in the stoichiometry of hydrogen atom production at short reaction times as would be required if hydrogen atom production depended on the slow O+H2C0 reaction. Reactions (Id) and (le) plus subsequent steps correspond respectively to 2.46 and 3.92 oxygen atoms consumed per ketone molecule to yield 2 hydrogen atoms. Thus the product analyses and stoichiometries show that (le) is the dominant initial process, probably with a contribution of about 25 % from The reac- tion of OH radicals with ketene has not been studied, but its rate constant is probably close to the value of 10l2 cm3 inol-I s-l found by Morris and Niki for OH+C2H4, and to the similar rate constant of O+C2H4,6 as there is close parallelism in the reaction rates of 0 and OH with ~ l e f i n s .~ The high rate constants of OH + CH3CH0 and OH+H,CO are almost certainly due to abstraction at weak aldehydic C-H bonds which are absent in ketene. In the 0 + CH2C0 reaction any OH formed will be removed almost exclusively by reaction (6). (Id). Reactions of ketene with other species are almost certainly negligible. The reaction H+CH,CO-,CH,CO~'+CH,+CO+ 122 kJ mol-i (7) has a rate constant O+CH,CO, and its importance is further reduced by the rapid subsequent step (8) which consumes 0 and regenerates H.At the higher initial ratios of [CH2CO] to [O] used in the kinetic studies, the first effect of the increase in [H]/[O] ratio as the reaction proceeds on the observed stoichiometry will, therefore, not involve the initial step but affect competition between H and 0 for HCO, since k , = 4(k3 +k4). of 8 x 1O1O em3 mol-l s-l, one quarter that deduced here for 0 + CH3 -+ H2C0 + H + 286 kJ mol-I (8)G . P. R . MACK A N D B . A . THRUSH 191 The relative values of k3, k4 and k, quoted above were combined with the average values of [O] and [HI to calculate the stoichiometries of oxygen atom removal (n) at different points on the kinetic plots used to calculate kl. Values for a typical run are shown in table 3.It can be seen that n = 3.0k0.2 over the region used to determine the rate coefficient, but at low ketene additions, where few H atoms are formed but the decrease in [O] is too small to measure accurately, the stoichiometry of oxygen atom consumption is higher, as illustrated by the steeper initial decrease in [O] in fig. 2. Applying this value of n to all the kinetic runs yields kl = (3.4k0.3) x loll cm3 mol-1 s-l at 293 K. TABLE 3 .-CALCULATION OF EFFECTIVE STOICHIOMETRY FOR EACH EXPERIMENTAL POINT IN RUN K1 (all concentrations in lo-’* cm3 mol-’ s-l) 0.784 2.96 1.60 0.534 3.67 0.267 3.58 1.791 1.97 2.58 0.765 2.99 0.383 3.27 2.708 1.33 3.22 0.810 2.46 0.41 5 3.10 3.397 1.06 3.49 0.801 2.20 0.401 3.1 1 4.133 0.86 3.70 0.760 1.97 0.380 2.99 5.392 0.60 3.96 0.686 1.65 0.343 2.93 6.146 0.51 4.04 0.687 1.52 0.344 2.87 7.839 0.36 4.19 0.610 1.28 0.315 2.83 “WCOlo [Ole,ptl [OIreacted [Hlexptl <O> (H> n This is somewhat lower than the value of kl = 5.3 x loll cm3 mol-1 s-l from the mass spectrometric study of the ketene decay by Carr et aZ.l ; however, these workers found iz = 2.0k0.5 and the observed rates of oxygen atom consumption in the two systems agree well.Their observations that the C02 from the l80 + CH2C160 reaction contained 20 % each of C1*02 and C1602, and their finding of large yields of water not detected in our experiments suggest that their results were affected by heterogeneous reactions of ketene in the sampling zone. CONCLUSIONS Our results are consistent with reaction (le) to yield HCO+HCO being the dom- inant primary step in the O+CH2C0 reaction. The short-fall in the COz yield suggests a smaller contribution from (Id) which yields HCO+H+CO.This latter process is to be expected to accompany (le) since the appearance of at least 60 % of the energy released in (Ie) as internal energy of either HCO fragment would lead to its dissociation. At low pressures CH3 + CHO are the products of the reaction of oxygen atoms with eth~lene.~ This behaviour corresponds to intersystem crossing and isomerisa- lion of the initial adduct to yield a highly vibrationally excited molecule of the C2H40 isomer with the lowest heat of formation, followed by its unimolecular decomposition. This arises because the steady state concentration of an isomer at any particular (high) energy is proportional to the density of its internal energy levels at that energy, and as the level density rises sharply with internal energy the most stable isomer predominates.Another example is the formation of CH2 + CO in the 0 + C2H2 reaction where the intermediate ketene molecule can be trapped in rigid matrices.ll On this basis the products of the 0 + CH2C0 reaction at low pressures should correspond to the rup- ture of the weakest bond in glyoxal which is the most stable C2H202 isomer. This192 O-ATOM + CARBONYL REACTIONS process would yield two HCO radicals in agreement with our conclusion that process (le) predominates, and that the observed products come from the subsequent rac- tions (3), (4), (5) and (6).The high stoichiometries of oxygen atom consumption and the CO, yields obs- erved here are not consistent with ketene being a significant intermediate in the reac- tion of excess atomic oxygen with acetaldehyde, reported in the preceding paper, since this would require higher stoichiometries of oxygen atom consumption and lower CO, yields than are observed in the 0 + CH3CH0 reaction. We thank the Ministry of Defence for an E.M. Research Contract and Dr. L. Phillips of E.R.D.E. for helpful discussions. R. W. Carr, I. D. Gay, G. P. Glass and H. Niki, J. Chem. Phys., 1968, 49, 846. G. P. R. Mack and B. A. Thrush, J.C.S. Firaday I, 1974,70, 178. G. P. R. Mack and B. A. Thrush, J.C.S. Fwahy I, 1973,69,208. H. Niki, C. McKnight and B. Weinstock, Abs. 154th Ann. Mtg.Amer. Chem. Soc., 1967, V110. C. B. Moore and G. C. Pimentel, J. Chem. Phys., 1963,38,2816. J. M . Brown and B. A. Thrush, Trans. Faraday Suc., 1967, 63, 630. ' W. Braun, A. M. Bass and M. Pilling, J. Chem. Phys., 1970, 52, 5131. M. A. A. Clyne and B. A. Thrush, Proc. Roy. SOC. A, 1963, 275, 544. E. D. Morris and H. Niki, J. Phys. Chem., 1971, 75, 3640. I. Haller and G. C. Pimentel, J. Amer. Chem. Soc., 1962, 84, 2855. l o R. J. CvetanoviE, J. Chem. Phys., 1955, 23, 1375. Reaction of Oxygen Atoms with Carbonyl Compounds Part 3.-Ketene BY G. P. R. MACK AND B. A. THRUSH* University of Cambridge, Department of Physical Chemistry, Lensfield Road, Cambridge CB2 1EP Received 27th July, 1973 The reaction between excess atomic oxygen and ketene has been studied in a discharge flow system by product analysis and kinetically using e.p.r.and chemiluminescznce. This rate constant for the initial step, which yields predominantly HCO+ HCO was found to be (3.4 k 0.3) x 10' ' cm3 mol-' s-' at 293 K. The subsequent reactions are O+HCO -+ OH+CO O+HCO -+ H+COz H+HCO-t H2+CO OfOH -+ 02+H. Can, Gay, Glass and Niki used a mass-spectrometer to study the reaction of ketene with atomic oxygen in a discharge-flow system. They deduced that the initial step was addition of an oxygen atom to ketene, predominantly at the olefinic carbon atom ; the observed products (CO, C02, H20, H2C0, H2, H) could be explained by rapid decomposition of the adduct followed by reaction of its fragments with atomic oxygen and with ketene. We have investigated the 0 + CH2C0 reaction in the apparatus previously used for the O+CH3CH0 and O+H2C0 reactions 2* ; and show that the initial step yields predominantly HCO + HCO formed by the isomerisation of the initial adduct to a vibrationally excited glyoxal molecule which then decomposes.Our results do not support the view that ketene is an important intermediate in the reaction of excess atomic oxygen with acetaldehyde. EXPERIMENTAL The reaction was studied in the discharge flow systems previously de~cribed.~ Atomic concentrations were measured by e.p.r. spectroscopy or the air afterglow and the reaction products were analysed by gas chromatography. All experiments were carried out at 293 K. Ketene was prepared by the thermal decomposition of acetic anhydride at 820K. Acetone and unreacted anhydride were removed at 195 K in a trap packed with glass wool.The ketene was purified by bulb-to-bulb distillation and a residual impurity of ethane was estimated by gas chromatography to be 5-8 %. All errors quoted are one standard deviation. RESULTS AND DISCUSSION STOICHIOMETRY OF THE REACTION The number of oxygen atoms consumed per ketene molecule added and per hydrogen atom formed were measured by e.p.r. The values were plotted against initial ketene concentration and extrapolated to zero reactant concentration to obtain the limiting stoichiometries of reaction. For both medium (cu. 30ms) and long 187188 0-ATOM + CARBONYL REACTIONS (ca. 100 ms) reaction times good straight lines were obtained; these plots yielded 3.7k0.2 oxygen atoms consumed per ketene molecule at long reaction times.The ratio of oxygen atoms consumed to hydrogen atoms produced was obtained by similar extrapolation to zero ketene concentration as shown in fig. 1. It fell slightly from 2.1 +O. 1 at short reaction times (ca. 5 ms) to 1.8 f: 0.1 at medium reaction times. The hydrogen atom yield per reactant molecule dropped significantly as the reactant flow increased, but this effect was less than in the O+CH,CH02 and O+H2C03 reactions, so that for large ketene flows [HI> [O]. The maximum concentration of hydrogen atoms generally occurred for [O] - 3[H]. - OO 1 2 3 G 1 010[CH2CO]o~mol ~ m - ~ FIG. 1.-Stoichiometry of hydrogen atom production at short reaction time (4.46 ms). [010 = 2.52 x mol STABLE PRODUCTS The analyses for carbon dioxide (table 1) show that 0.65+0.03 molecules of C02 per CH2C0 are formed at long reaction times.For shorter reaction times, where the reaction was not complete before quenching on the silver foil, the C02 yields are higher. Small amounts of formaldehyde and water could be detected under such conditions, but only traces of ketene suggesting that it was largely destroyed on the silver foil. TABLE 1 .-CARBON DIOXIDE YIELDS total total yield pmol pmol C W CHzCO r010/ CHiCO/ co21 total pressure/ reaction 1O~~~OIol Torr timelms mol cm-3 [CH~COIO 1.85 122.0 4.6 29.8 31.5 20.0 0.64 1.85 122.0 4.2 21.5 39.4 26.8 0.68 1.85 122.0 4.1 25 .O 39.2 24.2 0.62 2.25 21.9 2.46 10.0 39.2 30.8 0.79 1.93 11.8 1.31 25 .O 30.1 22.7 0.75 KINETICS OF OVERALL REACTION As established by Carr et d.,l the reaction is first order in [O] and in [CH,CO]. The rate coefficient was determined from the oxygen atom decays at fixed short reaction times when varying amounts of ketene were added, the integrated second- order rate equation was used in the form where nx = [O], - [O] and n is the stoichiometry of oxygen atom consumption.To obtain large decays, these runs were carried to comparable initial oxygen atom and ketene concentrations, and the stoichiometries measured with excess oxygenG . P . R . MACK A N D k. A . THRUSH 189 atoms cannot be substituted directly. Apart from the initial points, values of n = 2, 3 and 4 gave good straight lines (fig. 2) and it is not possible to deduce the correct value of n for high ratios of [CH,CO], to [O], on this basis.It is shown below that n = 3.0k0.2 under these conditions ; putting n = 3 yields a value of kl = (3.42$. 0.09) x 10l1 cm3 mol-l s-l at 293 K which is independent of total pressure over the range 0.9 to 2.7 Torr (table 2). The steeper oxygen atom decay with small [CH,CO], arises from a higher effective stoichiometry of oxygen atom consumption under such conditions. 1010[CH2C8]o/mol ~ r n - ~ [010 = 2.52 x mol crne3. 0, n = 2 ; A, n = 3 ; 0, n = 4. FIG. 2.-Kinetics of oxygen atom consumption for various stoichiometries. Reaction time = 4.46 ms. TABLE 2.-DETERMINATION OF THE RATE COEFFICIENTS OF 0-k KETENE run K11 K12 K2 K13 K4 K3 K5 K1 reaction t helms 3.27 5.24 4.46 6.42 5.68 6.57 8.08 4.91 1010t010/ mol cm-3 2.12 2.12 2.52 2.12 2.46 2.66 2.42 4.55 1010[CH2CO]o range/ mol cm-3 0.2-5.9 0.6-3.6 0.27-4.1 0.58-2.56 0.86-3.85 1.03-3.5 0.77-2.53 0.78-7.9 kl/crn3 mol-1 s-1 for n = 3 3.12 3.57 3.40 3.81 3.19 3.28 3.53 2.78 REACTION MECHANISM Although the C-H bond energy in ketene has not been determined, there is no reason to suppose that it is any lower than in ethylene which hits a slightly longer C-H bond (1.085 A as against 1.079 A).Thus abstraction of an H atom by 0 would be at least 25 kJ mol-1 endothermic giving a Boltzmann factor of less than The initial step must, as with ethylene, involve the addition of an oxygen atom followed by fragmentation of the excited adduct. Carr et al.' consider the following processes at 293 K even if the reverse process has no activation energy. 0 + CH,CO+CH,CO~-CCO + H20 + 46 kJ mol-1 (14 -+H,CO + CO + 408 kJ mol-1 (I@ -+CH, + C02 + 197 kJ mol-1 (1 4 +HCO + H + CO + 56 kJ mol-1 (14 -+HCO+HCO+ 138 kJ mol-l.( 1 4190 0-ATOM + CARBONYL REACTIONS Apart from its inherent implausibility being a weakly exothermic four-centre reaction, the lack of water formation in the presence of excess atomic oxygen excludes any large contribution from reaction (la). With reaction (lb) as initial step, atomic hydrogen is produced only by the subsequent 0 + H2C0 reaction which is four times slower than (1). In the presence of excess atomic oxygen (1 c) would be followed by giving a stoichiometry of which agrees with neither of the measured stoichiometries. rapidly with ketene [CH,CO], which is not observed. be (Id) and (le) with carbon dioxide being formed in one of the subsequent steps O+CH2 -+ CO+2H+316 kJ mol-' (2) 2 0 + CH2C0 -+ CO + C02 + 2H Furthermore, CH2 reacts and this would give a reduced hydrogen atom yield at high Thus, none of these three processes can be important, and the dominant paths must 0 + HCO -+ OH + CO + 350 kJ mob1 0 + HCO -+ H + C02 + 455 kJ mol-' (3) (4) H+ HCO -+ H2 + CO + 358 kJ mol-' O+OH 3 H + 0 2 +71 kJ niol-l (5) (6) where k3 : k4 : k, = 0.46 : 0.54 : 4.0 and k6 = 3 x l O I 3 cm3 mol-' s - ' . ~ The initial ratios [O] to [CH,CO] used for the C02 analyses were such that the relation derived in Part 1 would predict CO, yields per CH2C0 molecule of 0.4 and 0.8 respectively for paths ( I d ) and (le) plus their subsequent reactions and 0.4 for path (lb) followed by complete oxidation of the formaldehyde formed.However, there was no evidence of a decrease in the stoichiometry of hydrogen atom production at short reaction times as would be required if hydrogen atom production depended on the slow O+H2C0 reaction. Reactions (Id) and (le) plus subsequent steps correspond respectively to 2.46 and 3.92 oxygen atoms consumed per ketone molecule to yield 2 hydrogen atoms. Thus the product analyses and stoichiometries show that (le) is the dominant initial process, probably with a contribution of about 25 % from The reac- tion of OH radicals with ketene has not been studied, but its rate constant is probably close to the value of 10l2 cm3 inol-I s-l found by Morris and Niki for OH+C2H4, and to the similar rate constant of O+C2H4,6 as there is close parallelism in the reaction rates of 0 and OH with ~ l e f i n s .~ The high rate constants of OH + CH3CH0 and OH+H,CO are almost certainly due to abstraction at weak aldehydic C-H bonds which are absent in ketene. In the 0 + CH2C0 reaction any OH formed will be removed almost exclusively by reaction (6). (Id). Reactions of ketene with other species are almost certainly negligible. The reaction H+CH,CO-,CH,CO~'+CH,+CO+ 122 kJ mol-i (7) has a rate constant O+CH,CO, and its importance is further reduced by the rapid subsequent step (8) which consumes 0 and regenerates H. At the higher initial ratios of [CH2CO] to [O] used in the kinetic studies, the first effect of the increase in [H]/[O] ratio as the reaction proceeds on the observed stoichiometry will, therefore, not involve the initial step but affect competition between H and 0 for HCO, since k , = 4(k3 +k4).of 8 x 1O1O em3 mol-l s-l, one quarter that deduced here for 0 + CH3 -+ H2C0 + H + 286 kJ mol-I (8)G . P. R . MACK A N D B . A . THRUSH 191 The relative values of k3, k4 and k, quoted above were combined with the average values of [O] and [HI to calculate the stoichiometries of oxygen atom removal (n) at different points on the kinetic plots used to calculate kl. Values for a typical run are shown in table 3. It can be seen that n = 3.0k0.2 over the region used to determine the rate coefficient, but at low ketene additions, where few H atoms are formed but the decrease in [O] is too small to measure accurately, the stoichiometry of oxygen atom consumption is higher, as illustrated by the steeper initial decrease in [O] in fig.2. Applying this value of n to all the kinetic runs yields kl = (3.4k0.3) x loll cm3 mol-1 s-l at 293 K. TABLE 3 .-CALCULATION OF EFFECTIVE STOICHIOMETRY FOR EACH EXPERIMENTAL POINT IN RUN K1 (all concentrations in lo-’* cm3 mol-’ s-l) 0.784 2.96 1.60 0.534 3.67 0.267 3.58 1.791 1.97 2.58 0.765 2.99 0.383 3.27 2.708 1.33 3.22 0.810 2.46 0.41 5 3.10 3.397 1.06 3.49 0.801 2.20 0.401 3.1 1 4.133 0.86 3.70 0.760 1.97 0.380 2.99 5.392 0.60 3.96 0.686 1.65 0.343 2.93 6.146 0.51 4.04 0.687 1.52 0.344 2.87 7.839 0.36 4.19 0.610 1.28 0.315 2.83 “WCOlo [Ole,ptl [OIreacted [Hlexptl <O> (H> n This is somewhat lower than the value of kl = 5.3 x loll cm3 mol-1 s-l from the mass spectrometric study of the ketene decay by Carr et aZ.l ; however, these workers found iz = 2.0k0.5 and the observed rates of oxygen atom consumption in the two systems agree well.Their observations that the C02 from the l80 + CH2C160 reaction contained 20 % each of C1*02 and C1602, and their finding of large yields of water not detected in our experiments suggest that their results were affected by heterogeneous reactions of ketene in the sampling zone. CONCLUSIONS Our results are consistent with reaction (le) to yield HCO+HCO being the dom- inant primary step in the O+CH2C0 reaction. The short-fall in the COz yield suggests a smaller contribution from (Id) which yields HCO+H+CO. This latter process is to be expected to accompany (le) since the appearance of at least 60 % of the energy released in (Ie) as internal energy of either HCO fragment would lead to its dissociation.At low pressures CH3 + CHO are the products of the reaction of oxygen atoms with eth~lene.~ This behaviour corresponds to intersystem crossing and isomerisa- lion of the initial adduct to yield a highly vibrationally excited molecule of the C2H40 isomer with the lowest heat of formation, followed by its unimolecular decomposition. This arises because the steady state concentration of an isomer at any particular (high) energy is proportional to the density of its internal energy levels at that energy, and as the level density rises sharply with internal energy the most stable isomer predominates. Another example is the formation of CH2 + CO in the 0 + C2H2 reaction where the intermediate ketene molecule can be trapped in rigid matrices.ll On this basis the products of the 0 + CH2C0 reaction at low pressures should correspond to the rup- ture of the weakest bond in glyoxal which is the most stable C2H202 isomer. This192 O-ATOM + CARBONYL REACTIONS process would yield two HCO radicals in agreement with our conclusion that process (le) predominates, and that the observed products come from the subsequent rac- tions (3), (4), (5) and (6). 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