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Title: Thermal decomposition of 2-fluoroethanol: Single pulse shock tube and ab initio studies
Journal: J. Phys. Chem. A
Volume: 107
Page(s): 9782 - 9793
Year: 2003
Reference type: Journal article
Squib: 2003RAJ/RED9782-9793
Reaction:
C2H5OH → C2H4 + H2O
Reaction order:
1
Temperature:
1000 - 1200
K
Rate expression:
7.67x1014 [s-1] e-270328 [J/mole]/RT
Category: Theory
Data type:
Ab initio
Comments:
NOTE This is speculative reaction to explain formation of C2H4. Authors imply that C-OH bond fission channel (with identical rate expression) may be more likely.
Combined experimental and quantum chemical study of the thermal decomposition of 2-Fluoroethanol or CH2F-CH2OH. Measured decomposition of 2-Fluoroethanol in a shock tube at temperatures of 1000-1200 K and pressures of 13-23 atm. Typical concentrations of 500-1000 ppm 2-Fluoroethanol in Argon. Products detected with GC/FID. Quantum calculations using density functional theory (DFT) with B3LYP/6-311++G(d,p) method. Rate expressions from transition state using TST calculations.
Direct measurements from decomposition products of HF and H2O elimination. Possible C-OH bond fission channel measured indirectly by modeling formation of C2H4 from CH2FCH2OH -> CH2FCH2* + OH and CH2FCH2* -> C2H4 + F with the first step rate C-O bond fission rate determining and the second step C-F beta elimination fast. Also proposed another channel to explain C2H4 formation with HOF elimination or CH2FCH2OH -> C2H4 + HOF. The authors seem to prefer the first C-O bond fission channel, the barrier of about 86 kcal/mol is very similar to the barrier for CH3OH -> CH3 + OH of about 90 kcal/mol. In addition, the authors could find no transition state for elimination of HOF using the quantum calculations. Proposed HOF elimination channel is by analogy to known HOI elimination channel from CH2ICH2Cl. However, C-F bond is significantly stronger and thus HOF elimination is much less likely.
Good agreement (1 kcal/mol) between experimentally derived barriers for HF and H2O elimination and those from the quantum calculations. Also calculated CH3CH2OH -> C2H4 + H2O and compared with rate expression in literature. In this study, also the enthalpy of formation of 2- Fluoroethanol, not available elsewhere, was predicted using MP2/6-311++G(d,p) energies.
Note that initial CH2=CH2(OH) product from HF elimination channel quickly isomerizes to CH3-CHO.
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Rate constant values calculated from the Arrhenius expression:
| T (K) | k(T) [s-1] |
|---|---|
| 1000 | 5.82E0 |
| 1025 | 1.29E1 |
| 1050 | 2.74E1 |
| 1075 | 5.62E1 |
| 1100 | 1.12E2 |
| 1125 | 2.16E2 |
| 1150 | 4.04E2 |
| 1175 | 7.37E2 |
| 1200 | 1.31E3 |