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Photochemical and themal reaction of crotonaldehyde and 3-butenal Sifniades, Stylianos

Abstract

In the first part of this work the thermal reaction [Formula omitted] was studied thermodynamically and kinetically in the gas phase and at the temperature range 150 to 210°C. The equilibrium composition was found to be very unfavourable to 3-butenal which constitutes 0.35% of the mixture at 150°C and 1.04% at 210°C. The enthalpy of the reaction was estimated to be 7.20 ± 0.09 kcal/ mole. In the kinetic study both the forward and reverse reactions were found to be heterogeneous and of the first order with respect to surface area and pressure of reactant. The rate constants obeyed the relations [Equation omitted] k₁ was determined in a vessel with surface to volume ratio equal to 1.2 and k₋₁ in a vessel with ratio equal to 4.1. The significance of the experimental Arrhenius parameters was discussed in terms of the theory of absolute rates as applied to surface reactions and of a mechanism based on a fast adsorption-desorption of the Langmuir type. In the second part of the work the photochemical isomerization of crotonaldehyde to 3-butenal was studied in the gas phase and at the temperature range 25 to 140°C. Exciting radiation of the wavelengths 3130, 3340 and 3660 Å was used. It was found that the quantum yield of the isomerization obeys the Stern-Volmer equation 1/ϕ = a + bP ein/mole at the temperature and wavelength range studied. The pressure, P, of crotonaldehyde was varied from 0.4 to 34 mm Hg. The value of parameter a varies from 11 to 37 ein/mole with the low values observed at 3130 Å and the high values at 3660 Å. The parameter b varies from 0.19 to 7.5 ein.mole⁻¹ (mm Hg)⁻¹ with the low values also observed at the shorter wavelength. At constant wavelength b decreases with increasing temperature. A mechanism was discussed according to which the excited singlet '(n,π* ) is the reacting species and it was shown that a "strong collision" deactivation and a classical energy distribution function predict qualitatively the dependence of b on temperature and wavelength. An attempt to predict this dependence in a quantitative manner failed. A refinement to the "strong collision" mechanism using the same energy distribution function could not be tested numerically because of computational difficulties. An alternative mechanism was discussed involving a cis-trans equilibrium of crotonaldehyde in the ground state and it was shown that the parameters a and b may be interpreted in more than one way. In the third part of the work the photolysis and photochemical oxidation of 3-butenal were studied in the gas phase and the temperature range 25 to 140°C. In the photolysis exciting radiation of 3130 and 3340 Å was used. The products were carbon monoxide, propylene and biallyl. They were found to obey the relation CO ≤ propylene + 2.biallyl The overall quantum yields taken as equal to ϕ (propylene) + 2 ϕ (biallyl) obeys the Stern-Volmer equation (1). The Value of the parameter a varies from 0.994 to 1.226 ein/mole and that of b from 1.15 x 10⁻² to 3.75 x 10⁻² ein. mole⁻¹ (mm Hg)⁻¹ . The lowest values for both parameters are observed at 3130 Å and 140°C and the highest at 3340 Å and 25°C. The significance of parameter a was discussed and it was shown that its dependence on temperature and wavelength can be predicted within the limits of the experimental error by using the classical energy distribution function. In the photochemical oxidation radiation of 3130 Å was used. The major products with the quantum yields at 25°C shown in paranetheses were: carbon monoxide (3.0-3.4), allyl alcohol (1.1-2.4), acrolein (1.2-1.9), carbon dioxide (0.4-1.0), propylene (0.4-0.5), peroxide (0.20-0.27) and ethylene (0.22-0.27). Variation of the experimental conditions at constant temperature had little effect on the quantum yields. Increase of temperature to 140°C resulted in decrease of the yield of allyl alcohol and acrolein and increase of the yield of all the other products. A mechanism was discussed which explains the results in a qualitative way.

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