An understanding of the mechanism of the gas phase autoxidation of alkenes depends in turn on an understanding of the behaviour of the resulting unsaturated aldehydes in the process (1). However, little work has been done on the autoxidation of these species. It is known though, that the low temperature autoxidation of acrolein (produced during the oxidation of propene) is anomalously slow in comparison with the oxidation of acetaldehyde or propanal (2).
Chain branching for aldehyde oxidation is considered to be via
decomposition of the acylperoxide.
However, this route may not be so readily available for acrolein.
There is
an alternative route for acrylperoxy radicals, which can epoxidise acrolein instead of abstracting a hydrogen
atom.
In preliminary experiments, the epoxide of acrolein (oxiranyl carboxaldehyde) was identified in the
autoxidation of acrolein and in these studies the rate constant for the related reaction of epoxidation of
acrolein by acetylperoxy radicals (reaction I) has been examined at 383 K and found to be
log10(kI /dm3mol-1 s-1) = 4.1(+/-)0.5.
O
CH2CHCHO + CH3CO3 -> H2CCHCHO + CH3 + CO2 (I)
Acetylperoxy radicals were produced by the slow autoxidation of acetaldehyde in a static reactor in the presence of small quantities of 1-butene and acrolein. The rate of formation of oxiranyl carboxaldehyde relative to that of 1,2-epoxybutane was determined by GC FID, with product identification by GC MS (figure 1). A previously measured rate constant for the epoxidation of 1-butene by acetylperoxy radicals (3) allowed the rate constant for the epoxidation of acrolein to be determined.
This rate constant is more than two orders of magnitude faster than would be expected from the correlation that exists between the rate constant for epoxidation by peroxy radicals and the ionisation energy of the double bond (4) (figure 2).
The rate of hydrogen abstraction by acetylperoxy radicals from acrolein has also been examined for this work and found to be at least ten times slower than the corresponding abstraction from acetaldehyde. Work is currently being carried out to find whether the anomalously fast rate of epoxidation of acrolein by acylperoxy radicals allows the reaction to play a significant role during acrolein autoxidation.
References
1. Stark, M.S. and Waddington, D.J., Int J Chem Kinet, 27, 123-151 (1995).
2. Newitt, D.M., Baxt, L.M. and Kelkar, V.V., J Chem Soc, 1703-1710 (1939).
3. Selby, K. and Waddington, D.J., J Chem Soc Perkin Trans II, 1715-1718 (1975).
4. Ruiz Diaz, R., Selby, K. and Waddington, D.J., J Chem Soc Perkin Trans II, 360-363 (1977).
Figure 1. The growth of oxiranyl carboxaldehyde (solid circle) and 1,2-epoxybutane (o) during the co-oxidation of
acetaldehyde, acrolein and 1-butene.
Conditions: 383K, 400 mbar total pressure, 133 mbar
CH3CHO, 133 mbar O2, 13.3 mbar 1-C4H8, 13.3 mbar C2H3CHO, 107 mbar He.
Figure 2. Activation energy for the reaction of acetylperoxy radicals with alkenes related to the alkene ionisation potential of the alkenes. The plot shows the correlation that exists for mono-alkenes (o) and the anomalous behaviour of acrolein (solid circle).
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