Article pubs.acs.org/JPCA
Pyrolysis Reactions of 3‑Oxetanone Emily M. Wright, Brian J. Warner, Hannah E. Foreman, and Laura R. McCunn* Department of Chemistry, Marshall University, One John Marshall Drive, Huntington, West Virginia 25755, United States
Kimberly N. Urness Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: The pyrolysis products of gas-phase 3-oxetanone were identified via matrix-isolation Fourier transform infrared spectroscopy and photoionization mass spectrometry. Pyrolysis was conducted in a hyperthermal nozzle at temperatures from 100 to 1200 °C with the dissociation onset observed at ∼600 °C. The ring strain in the cyclic structure of 3-oxetanone causes the molecule to decompose at relatively low temperatures. Previously, only one dissociation channel, producing formaldehyde and ketene, was considered as significant in photolysis. This study presents the first experimental measurements of the thermal decomposition of 3-oxetanone demonstrating an additional dissociation channel that forms ethylene oxide and carbon monoxide. Major products include formaldehyde, ketene, carbon monoxide, ethylene oxide, ethylene, and methyl radical. The first four products stem from initial decomposition of 3-oxetanone, while the additional products, ethylene and methyl radical, are believed to be due to further reactions involving ethylene oxide.
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INTRODUCTION Small, oxygen-containing ring structures are commonly used in synthetic and pharmaceutical chemistry1−3 and may also occur as intermediates during the oxidation of hydrocarbon radicals.4−7 Cyclic structures exhibiting a high degree of ring strain are often prone to thermal decomposition, even at relatively low temperatures, due to their instability.8−10 The cyclic ketone 3-oxetanone (Figure 1) is an archetype of such
dissociation of 3-oxetanone is a Rice-Ramsperger-KasselMarcus (RRKM) analysis of the unimolecular rate constant for the photolysis of 3-oxetanone, producing ketene and formaldehyde.19 However, these RRKM calculations used the activation energy for the analogous reaction cyclobutanone → ketene + ethylene. An alternate dissociation pathway (3oxetanone → (CH2)2O + CO) was not considered because a similar reaction is ∼2 orders of magnitude slower than the ketene + ethylene channel in cyclobutanone. For further insight on the thermal decomposition of 3oxetanone, similar molecules may be considered. There are two documented photodissociation pathways of −CH3 or −C6H5 tetrasubstituted 3-oxetanone. The photolysis of tetraphenyloxetan-3-one was studied with benzene and methanol solutions radiated at 350 nm for ∼17 h.20 The tetramethyloxetanone was irradiated at 313 nm in various solvents with a 1450 W lamp.21 In both studies, one observed dissociation pathway is analogous to what has been predicted in the unsubstituted 3-oxetanone, producing a disubstituted ketene and a ketone. The second pathway results in a tetrasubstituted oxirane and carbon monoxide. Cyclobutanone, identical to 3-oxetanone except for the lack of an oxygen in the four-membered ring, dissociates predominantly to ketene and ethylene but can also make cyclopropane plus carbon monoxide. Early thermal decomposition experiments on cyclobutanone in static systems with reaction times of up to 10 min and temperature ranges of 333−
Figure 1. 3-oxetanone.
small, ring-strained compounds. Most studies of 3-oxetanone have focused on how ring strain and the electronic interactions of atoms in the ring relate to its planar equilibrium geometry.9,11−17 The occurrence of 3-oxetanone is suspected in the decomposition of the acetonylperoxy radical, an intermediate formed during the atmospheric processing of acetone.18 Atmospheric degradation of acetone produces the acetonyl radical, CH3C(O)CH2, which reacts with O2 to form the acetonylperoxy radical. Rate constants predicted by a canonical variational theory/small-curvature tunneling method show that the radical isomerizes then produces 3-oxetanone and hydroxyl radical. These are the dominant products in the 250−2000 K range. There is no significant branching to other products below 1200 K.18 The only published study of the © XXXX American Chemical Society
Received: May 12, 2015 Revised: June 23, 2015
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The Journal of Physical Chemistry A 373 °C22 and 360−406 °C8 found both reaction channels to be first order. A later study employing a heated quartz tube with much shorter residence times (1070 °C) and observed kinetics in those studies make the reaction seem unfavorable here. B. Ethylene Oxide and Carbon Monoxide Product Channel. Thermal decomposition of 3-oxetanone via Reaction 2 yields ethylene oxide and carbon monoxide. Both of these products were observed at pyrolysis temperatures of 600−1200 °C. The presence of CO is confirmed from the band at 2139 cm−1 (Figure 3); however, PIMS is unable to detect CO because the molecule’s IE42 (14.0 eV) is higher than the energy
of the photoionization laser. Bands at 3077, 3021, 3010, 1273, 879, 877, and 821 cm−1 are indicative of ethylene oxide47 production and are shown in Figures 4, 5, and 7. A feature at m/z = 44 (C2H4O+) is observed in the mass spectrum following pyrolysis at each temperature shown (Figure 2) indicating the presence of ethylene oxide. Some of the signal is due to dissociative ionization of the starting material, 3oxetanone, but it could also indicate acetaldehyde as a thermal product (IE = 10.2 eV).42 However, there is no FTIR evidence48 to confirm or disprove that acetaldehyde is a thermal intermediate, leaving ethylene oxide as the likely product. If ethylene oxide reacts unimolecularly before exiting the reactor, it will likely isomerize to form acetaldehyde32,33,49 as in Reaction 3. Since there is no appreciable evidence for acetaldehyde by FTIR in these experiments this molecule must undergo further dissociation as in Reactions 4 and 5 to D
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ethylene oxide is 10.56 eV.42 The (CH2)2O+ parent ion does not appear until the sample is heated significantly. At 900 °C, pyrolysis products emerge at m/z = 15 and 42, from ions of CH3 and ketene, respectively. Features at m/z = 29 and 43 are the result of dissociative ionization of vibrationally excited ethylene oxide and/or acetaldehyde.40 Ethylene was also observed following pyrolysis of 3oxetanone, with vibrational bands at 2995, 947, and 1440 cm−1 (Figures 4, 5, and 6); however, CH2CH2+ does not appear in the PIMS spectra of Figure 2 because the PIMS experiments were performed at a lower concentration than the MI experiments. Additionally, the photoionization laser is just under the threshold for ionization of ethylene,42 10.51 eV, so the fraction of hot nascent ethylene molecules undergoing ionization could be small. Ethylene is likely produced by the reaction of ethylene oxide with a hydrogen atom. Lifshitz and Ben-Hamou32 observed ethylene as a product of ethylene oxide pyrolysis in their shock-tube studies at 560−930 °C. Their results indicate that the reaction does not involve the isomerization of ethylene oxide to acetaldehyde but the addition of a hydrogen atom to the oxygen prior to dissociation to C2H4 + OH. The PIMS spectra in Figure 9 do show weak but discernible signal at m/z = 28, particularly at 1200 °C, pointing to ethylene oxide decomposition as a source of ethylene.
Figure 8. PIMS spectra following pyrolysis of ketene46 at 300−1300 °C obtained with a 10.5 eV photoionization laser.
form CH3 + CO + H. No FTIR bands were observed to indicate the appearance of CH3; however, PIMS provides evidence of CH3+ (IE = 9.8 eV),42 which appears at m/z = 15 in Figure 2. Similar experiments on the pyrolysis of acetaldehyde34,40 show that it can also dissociate to form ketene + H2 or acetylene + water, via bimolecular reactions of the starting material with hydrogen atoms. It is difficult to determine whether the secondary reaction of ethylene oxide leading to ketene is actually occurring because ketene is also a product in Reaction 1. No evidence is seen for the formation of acetylene or water. The H2O observed in Figure 6 was merely background signal, which was determined by integrating the FTIR peaks for the argon-isolated H2O bends. The intensities were the same following pyrolysis of 3-oxetanone and following the heating of pure argon for similar deposition times and conditions. To provide additional evidence for secondary decomposition of ethylene oxide, PIMS was performed following its pyrolysis (Figure 9). The spectrum for ethylene oxide heated to 50 °C is void of signal, which is unsurprising, given that the IE of
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CONCLUSIONS The pyrolysis of 3-oxetanone mixed in argon or helium at temperatures of 600−1200 °C produces the anticipated products H2CO + H2CCO, as well as ethylene oxide + CO. Secondary reactions involving ethylene oxide lead to the products CH3, H, and CH2CH2, in addition to CO and H2CCO. While it would be desirable to quantify the product branching for the two primary reactions of 3-oxetanone, it is impractical to measure the relative amounts of products from the two primary reactions. While formaldehyde or ketene could easily be quantified via FTIR, ethylene oxide undergoes further reactions, which also can lead to carbon monoxide. Therefore, it would be impossible to know how much ethylene oxide or carbon monoxide came directly from pyrolysis of 3-oxetanone. The detection of two pyrolysis pathways in 3-oxetanone in these experiments provides motivation to revisit the RRKM analysis of 3-oxetanone with accurate transition-state energies. The dissociation of 3-oxetanone requires temperatures such that it should not occur in the atmospheric processing of acetone via the acetonyl radical. However, 3-oxetanone and its dissociation reactions should be considered in combustion reactions that include the acetonyl radical as an intermediate.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 304-696-2319. Fax: 304-696-3243. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research. This work was also supported by a Faculty Start-up Award from The Camille and Henry Dreyfus Foundation. B.J.W. acknowledges a fellowship from the SURE Program
Figure 9. PIMS spectra following pyrolysis of ethylene oxide at 50, 900, and 1200 °C obtained with a 10.5 eV photoionization laser. Pyrolysis was performed on 0.03% samples of ethylene oxide in 1500 Torr of He carrier gas. E
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funded through the West Virginia Research Challenge Fund and administered by the West Virginia Higher Education Policy Commission, Division of Science and Research, Grant No. HEPC.dsr.11.24; AMEND 1. E.M.W. thanks the chemistry alumni of Marshall Univ. for a SURF summer research fellowship. K.N.U acknowledges funding from the National Science Foundation (CHE-0848606 and CHD-1112466). The authors thank AnGayle Vasiliou and Barney Ellison for providing the PIMS spectra of ketene in Figure 8.
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