REVERSIBLE HYDRATION OF 2- AND 4-PYRIDINECARBOXALDEHYDES .3. SOLVENT DEUTERIUM ISOTOPE EFFECTS

The solvent deuterium isotope effects associated with the general acid, general base catalyzed hydrations of 2- and 4-pyridinecarboxaldehydes in veronal, phosphate, and acetate buffers have been determined. The kinetics for these reactions in both H2O and D2O were followed at 0.0° using a spectrophotometric method from which ratios of kH/kD were deduced for the spontaneous reaction, for hydronium and hydroxide ions as well as for the general acid-base components of the buffers employed. The data show that for the hydration of 4-pyridinecarboxaldehyde the kinetic deuterium isotope effects are 4.2 for water, 2.9 for acetate ion, 2.6 for acetic acid, 2.2 for monohydrogen phosphate, 1.3 for hydronium ion, and 0.9 for hydroxide ion. The deuterium isotope effects associated with the hydration of 2-pyridinecarboxaldehyde were generally found to be about 25% lower. The effects are compared with similar data pertaining to the mutarotation of glucose and the hydration of acetaldehyde.