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The photochemistry of aldehydes in the gaseous phase Sifniades, Stylianos

Abstract

The object of the present work was the investigation of two separate topics, the investigation of the radical-radical termination step in the photochemical oxidation of acetaldehyde and the investigation of the photolysis of crotonaldehyde, both in the gas phase and at room temperature. A third topic, the photolysis of 3-butene-1-al, was investigated in the course of the work, in order to provide evidence in favour of the mechanism proposed for the photolysis of crotonaldehyde. In the first part of the work, mixtures of acetaldehyde and isotopically enriched oxygen containing argon as a reference gas were irradiated at 25°C with light at a wavelength of 3130 [formula omitted] and the reaction products were analyzed by gas chromatography and mass spectrometry. The major products were found to be peracetic acid and diacetyl peroxide. By following the rate of formation of these products, as well as the concentrations of the three possible kinds of oxygen in the reaction mixture, it was possible to verify McDowell and Farmer's mechanism for the photooxidation of acetaldehyde and establish a satisfactory mechanism for the chain terminating step of the reaction. In the second part of the work, crotonaldehyde was irradiated at 30°C with light at a wavelength of 2450-4000 [formula omitted], and the products analyzed by gas chromatography and infra-red spectroscopy. The only product in experiments of short duration was found to be 3-butene-1-al, thus establishing isomerization as the primary process of the reaction. In later stages CO, propylene and 1,5-hexadiene were formed. The following experimental laws were found to hold for the rate of formation of these products: d(3-butene-1-al)/dt = ႴIabs , Ⴔ = 0.095 ± 0.005 d(1, 5-hexadiene)/dt ∼ (Iabs².t)/(crotonaldehyde ) d(propylene )/dt ∼ (Iabs².t)/(crotonaldehyde)1.3-1.5 No detailed law was established for the rate of formation of CO, but whenever measured it was found to follow closely the propylene formation. These experimental laws were interpreted by means of a mechanism, in which crotonaldehyde participates in so far as it isomerizes to 3-butene-1-al. The latter was assumed to yield the three other products of the reaction through a mixed radical-molecular photochemical dissociation. Evidence was provided in favour of the proposed mechanism by studying the photolysis of 3-butene-1-al at 30°C and a wavelength of 2450-4000 [formula omitted]. The products of the reaction were found to be propylene, 1,5-hexadiene and CO, with their rates of formation obeying the following laws: d(1,5-hexadiene)/dt = Ⴔhexad. Iabs , Ⴔhexad = 0.135 ± 0.0318 d(propylene)/dt = Ⴔprop‧Iabs , Ⴔprop = 0.79 ± 0.05 CO formation was found to be approximately equal to propylene formation. These experimental laws were interpreted by means of a mixed radical-molecular photochemical dissociation of 3-butene-1-al, thus justifying the mechanism proposed for the photolysis of crotonaldehyde.

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