Under pseudo first order condition, we have measured rate constant k between substitute benzylphosphonates I (R = H, Me, Cl, Br) and p-, m-substituted benzaldehydes II (R1 = H, 4-Me, 4-MeO, 4-Cl, 3-NO2) and found that the reaction is of over all third order, first order in phosphonate, ethoxide and aldehyde resp. Correlation of the logarithmic k's with a series of the corresponding Hammett s values gives satisfactory linear relationship, which shows that electron-withdrawing substituents on both benzaldehydes and benzylphosphonates impose rate enhancement effect upon the reaction, giving a reaction constant p, of approx. 2.0 and 3.4. Correlation has likewise been made between Lnk and the corresponding inverse temps. (27.0?.1癈, 35.4?.1癈, 45.4?.1癈) at which the reactions proceed to give activation parameters Ea and DSG. From these values it is suggested that the reaction follows the usual two step mechanism with the formation of four-membered cyclic intermediate as rate determing step.

Key words: REACTION KINETICS, REACTION MECHANISM, BENZYL GROUP, REACTION RATE, BENZALDEHYDE