Theoretical approach of the mechanism of the reactions of chlorine atoms with aliphatic aldehydes

In this work, we use quantum chemistry (DFT-UBH&HLYP/6-311G(d,p)) coupled to kinetic calculations to obtain a detailed view of the mechanism of the Cl + aldehyde reactions when the size of the aldehyde increases. A series of linear and ramified aliphatic aldehydes: CH3C(O)H, CH3CH2C(O)H, CH3CH2CH2C(O)H, CH3CH2CH2CH2C(O)H, (CH3)2CHC(O)H and (CH3)3CC(O)H has been considered. For all these aldehydes, we have compared the various capture possibilities of H atoms by Cl atoms, from the alkyl chain and from the aldehydic group. In agreement with experimental data, our calculations show that acetaldehyde and propionaldehyde + Cl reactions almost exclusively proceed via abstraction of the aldehydic hydrogen atom. For larger aldehydes, this abstraction is no longer predominant, in agreement with recent experimental studies on n-butanaldehyde and n-pentanaldehyde. In this work, it has been shown that, aside from the aldehydic group, two factors may influence the reactivity of each site of the alkyl chain, the position of the carbon atom (from which the H atom will be abstracted) with respect to the CO group, and the nature of this carbon atom: primary (–CH3), secondary (>CH2) or tertiary (>CH−). The carbonyl influence extends to carbon atoms in β position for secondary sites and more for primary sites. For the tertiary carbon site in isobutyraldehyde, its reactivity is larger than the secondary ones, even if it is in α position.