Kinetics and mechanism for oxime formation from 4-dimethylamino-benzaldehyde and 4-trimethylammonio-benzaldehyde iodide

The following lines of evidence establish that oxime formation from 4-dimet hylaminobenzaldehyde and 4-trimethylammoniobenzaldehyde iodide occurs with a simple two-step mechanism. The pH-rate profile for the reaction of 4-trim ethylammoniobenzaldehyde iodide exhibits, in order of decreasing pH, a nega tive deviation at pH near 2.0, corresponding to a transition from rate-dete rmining step carbinolamine dehydration with acid catalysis to the uncatalyz ed carbinolamine formation. In the case of the reaction of 4-dimethylaminob enzaldehyde, the pH-profile exhibits, in order of decreasing pH, a positive deviation at pH near 3.5 and then a negative deviation at pH near 2.0. The se deviations have been interpreted in terms of i) transition of the rate-d etermining step, and ii) protolytic equilibrium of the substrate. (C) 1999 John Wiley & Sons, Inc.