Mechanistic Studies on the Three Component Synthesis of Tetrahydrobenzo[ b]pyran Catalyzed by Agar

The mechanism of reaction between 4-methoxybenzaldehyde 1, malononitrile 2, and dimedone 3 in the presence of agar as a green catalyst in a mixture of water and ethanol media as green solvents has been investigated by means of kinetic studies. The rate constant, order of reaction, activation energy and rate determining step (RDS) have been determined. The reaction’s overall order for the formation of tetrahydrobenzo[b]pyran confirmed to second-order kinetics and the partial orders with respect to the reactants 1, 2 and 3 were one, one and zero, respectively. The results showed that rate of reaction speeds up in solvent with high dielectric constant (water/ethanol, 2:1) and decreases with respect to lower dielectric constant (water/ethanol, 1:1) at all temperatures. The first step of the speculative mechanism was recognized as a RDS (K1) in accordance with the effect of solvent on the reaction rate and this was confirmed based upon the steady-state approximation.