“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 1 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.