Formation Pathways toward 2- and 4-Methylbenzaldehyde via Sequential Reactions from Acetaldehyde over Hydroxyapatite Catalyst

Condensation reactions of biomass derived C₂ and C₄ aldehydes form both ortho- and para-tolualdehydes (2-MB and 4-MB, respectively). The complete reaction network and the detailed mechanisms, however, have not been fully described. Here, analysis of the products formed by sequential condensation reactions of acetaldehyde and 2-butenal suggests that 2- and 4-MB products form via aromatization of 2,4,6-octatrienal and of highly reactive acyclic intermediate(s) formed via self-addition of 2-butenal, respectively. The exact positions at which C−C bonds form between C₄ co-reactants to create 4-MB products were investigated by using reactant mixtures containing combinations of 2-butenal, 2-butenol, and 3-methyl-2-butenal as model reactants. The last two reactants can form products that may be assigned to specific reaction pathways (not distinguishable during self-addition of 2-butenal) and also have decreased reactivity at specific carbon atoms. The analysis of the products suggests that 4-MB species form via 2-butenal self-addition by nucleophilic attack of the α-C to the carbonyl-C. Additionally, Diels–Alder reactions (between C₆ and C₂ intermediates) do not contribute in any significant manner to the formation of 4-MB. These findings complete the description of the reaction network that forms 2- and 4-MB from acetaldehyde on hydroxyapatite.