SEPARATION OF POLAR AND RESONANCE SUBSTITUENT EFFECTS IN THE REACTION OF BENZALDEHYDES WITH HCN - CORRELATION BETWEEN RHO-R-RHO-REQ RATIOS AND CENTRAL ATOM REHYBRIDIZATION

Rate and equilibrium constants for the reversible reaction of HCN with six substituted benzaldehydes have been determined in aqueous solution and in 60% acetonitrile-40% water, at 25 °C, ionic strength 0.05. These rate and equilibrium constants give nonlinear plots against σ, σn, or σ+but are correlated using the equation log k/k0= ρσn+ σr(σ+- σn) in which polar and resonance effects are separated. For the reaction in water, ρf= 1.37, ρrev= 0.71, ρeq= 0.66 and ρrf= 0.89, ρrrev= -0.39, preq = 1.12; in 60% acetonitrile, ρf=1.66, ρrev= 0.78, ρeq= 0.88 and ρrf=0.32, ρrrev= -0.19, ρreq= 0.56. Within experimental error, the p values for the forward and reverse reactions are additive to give the observed equilibrium values. The equilibrium and kinetic α-deuterium isotope effects on the addition of HCN to benzaldehyde are KD/KH= 1.30, kD/kH= 1.20 in water and KD/KH= 1.25, kD/kH= 1.12 in 60% acetonitrile. Correcting the isotope effect for the ionization of the α-hydroxynitrile gives transition states that are 83 and 63% toward the addition product in the respective solvent systems. These estimates are virtually identical with the values of 79 and 57% obtained from the ratios ρr/ρreqin the two solvent systems, suggesting that ρris a direct measure of central atom rehybridization. The use of pr/preq ratios may therefore provide a useful supplement or alternate to the use of isotope effects in the study of transition-state structures. © 1979, American Chemical Society. All rights reserved.