Acetaldehyde, bond dissociation energy

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

The carbonyl group appears to withdraw electrons from the adjacent vinyl carbons and therefore increase the dissociation energy of the Cd—bond. The methyl C—H bond strength in acetaldehyde, CH3CH=0 to CH2 CH=0 + H of 96.4 kcal mol [115] shows resonance stabilization of the methyl radical with the carbonyl of ca. 4.5 kcal-mof relative to the 101 value of ethane. 

If product formation proceeded via an intermediate long-lived ion whose dissociation were governed by competitive channeling of energy into the possible dissociation coordinates, one would expect the complex to fragment at the weakest bond—i.e., in the most thermodynamically favorable direction. This is clearly not the case. The most exothermic reaction (17) does not dominate the fate of the complex formed by collision of methanol ion with acetaldehyde. Similarly, one would expect Reaction 4 to be favored and Reactions 5, 6, and 7 to occur with about equal probability. 

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