Solvation of esters and dialkyl carbonates

Abstract

Infrared spectra in the CO and C—OR stretching-mode regions have been studied for dilute solutions of various simple esters and carbonates in a range of pure solvents, most work being carried out on methylacetate and dimethylcarbonate. There is a linear correlation between these modes, but their shifts are opposed, protic solvents inducing low-frequency shifts for the CO bands and high-frequency shifts for the C—OR bands. It is argued that this means that hydrogen-bonding occurs only at the carbonyl oxygen. Only one solvate was detected in aprotic solvents, but in alcoholic and aqueous solutions two distinct solvates were detected. For methanol solutions one band is assigned to monohydrogen-bonded ester and the other to non-bonded esters, whilst for aqueous solution one band is assigned to mono- and the other to di-hydrogen-bonded esters. Changes in the concentrations of these solvates with solvent composition have been monitored for a range of mixed solvents.

Shifts in the ¹³C resonance for the ¹³CO carbon of acetone and methylacetate depend linearly on the infrared shifts, provided the weighted mean values are used for solutions exhibiting two bands. However, the sensitivity of the ¹³C shift decreases on going from acetone, via methylacetate, to dimethylcarbonate, the last being almost completely insensitive. This trend follows the energy of the n→π^* absorption of these compounds.