“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 ¹H 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.