"In-water" Dehydration Reaction of an Aromatic Diol on an Inorganic Surface.

The effect of a synthetic saponite surface on the "in-water" dehydration reaction of diol was examined using 4-formyl-1-methylquinolinium salt (MQu) as a substrate. The equilibrium between aldehyde (MQu-Aldehyde) and diol (MQu-Diol) was affected by the surrounding environment. The equilibrium behavior was observed by H nuclear magnetic resonance (NMR) and UV-vis absorption measurements. Although MQu was completely in the form of MQu-Diol in water, the equilibrium almost shifted to the MQu-Aldehyde side when MQu was adsorbed on the saponite surface in water. In addition, the MQu-Aldehyde ratio depended on the negative charge density of saponite. The factors that determine MQu-Aldehyde: MQu-Diol ratio were discussed from the thermodynamic analysis of the system. These data indicate that the electrostatic interaction between the charged saponite surface and MQu stabilized the aldehyde side enthalpically and destabilized it entropically. The major reason for these results is considered to be the difference in adsorption stabilization between MQu-Aldehyde and MQu-Diol on saponite surfaces.