THERMAL DECOMPOSITION OF PROPIONALDEHYDE

The kinetics of the thermal decomposition of propionaldehyde are very similar to those of acetaldehyde. However, in spite of the simple stoichiometry, many problems of the kinetics and mechanism were unresolved right up to very recent times. 

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The thermal decomposition of propionaldehyde is a homogeneous reaction in a seasoned vessel - however, in a clean one, the reaction proceeds more slowly. 

First evidence for the occurrence of free radicals in the thermal decomposition of propionaldehyde was obtained by the para-ortho hydrogen technique . Sworski and Burton investigating the reaction between 850 and 950 °C by the lead-mirror technique, identified methyl and ethyl radicals and also determined their concentration ratio. They showed that the concentration of C2H5 exceeds that of CH3 in the temperature range studied and that the [CH3]/[C2H5] ratio increases with increasing temperature (Table 4). 

Hydroxymethylmethyldiazirine (209 unprotonated) formed propionaldehyde as the sole product by thermal nitrogen extrusion 4-hydroxy-l,2-diazaspiro[2.5]oct-l-ene (218) formed a mixture of cyclohexanone (73%), cyclohexenol (21%) and cyclohexene oxide (5%). Thermal decomposition of difluorodiazirine (219) was investigated intensively. In this case there is no intramolecular stabilization possible. On heating for three hours to 165-180 °C hexafluorocyclopropane and tetrafluoroethylene were formed together with perfluorofor-maldazine 64JHC59). 

In epoxidation, the propene-to-CHP molar ratio is 10 1, the reaction temperature is 60 °C and the pressure is sufficient to maintain propene in the liquid phase. The feed to the epoxidation reactor must contain less than 1% water in order to limit the hydrolysis of PO to glycol. The reaction is catalyzed by a proprietary, silylated, titanium-containing silicon oxide catalyst. The conversion of CHP is greater than 95%. Selectivity for PO based on hydroperoxide is 95%, whereas selectivity based on propene is around 99%. By-products of the reaction are aldehydes, such as acetaldehyde and propionaldehyde, alcohols (methanol and propene glycol), ketones and esters (e.g., acetone and methyl formate). The catalyst fixed-bed is structured into multiple catalyst layers, with heat exchangers in between the layers. This prevents excessive increases in temperature due to the exothermal reaction that would cause both thermal decomposition of the hydroperoxide and consecutive reactions of PO. 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

Neither the thermal nor the cobalt-catalyzed decomposition of 3-butene-2-hydroperoxide in benzene at 100 °C. produced any acetaldehyde or propionaldehyde. In the presence of a trace of sulfuric acid, a small amount of acetaldehyde along with a large number of other products were produced on mixing. Furthermore, on heating at 100°C., polymerization is apparently the major reaction no volatile products were detected, and only a slight increase in acetaldehyde was observed. Pyrolysis of a benzene or carbon tetrachloride solution at 200°C. in the injection block of the gas chromatograph gave no acetaldehyde or propionaldehyde, and none was detected in any experiments conducted in methanol. 

The addition of BF3-OEt2 to an a-phosphorylated imine results in the 1,3-transfer of a diphenylphosphinoyl group, with resultant migration of the C-N=C triad. This method is less destructive than the thermal rearrangement. The decomposition of dimethyldioxirane in acetone to methyl acetate is accelerated with BF3 OEt2, but acetol is also formed. Propene oxide undergoes polymerization with BF3-OEt2 in most solvents, but isomerizes to propionaldehyde and acetone in dioxane. ... 

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