HYDRATION EQUILIBRIA OF 9-ACRIDINECARBOXALDEHYDE

Hydration rate constants and equilibrium constants have been obtained for 9-acridinecarboxaldehyde in aqueous solution. Under acidic conditions where the acridine is protonated, signals for the hydrate and free aldehyde forms can be observed as separate species in the H-1 NMR spectrum. Integration provides the hydration equilibrium constant K-h+ = [hydrate]/[aldehyde] = 2,5. Using an apparent acidity constant obtained from a spectroscopic titration curve, the rate-pH profile was fitted to provide the hydration constant for the equilibration of the neutral acridines, K-H = 0.07. This analysis also provides the acidity constants for the two acridinium ions, the aldehyde with pK = 3,78 and the hydrate with pK = 5,36. A comparison with the 4-pyridinecarboxaldehdye system reveals that the K-h+: K-h ratios for the acridine and pyridine are the same within experimental error, but that the acridine and acridinium aldehydes are 20-fold less hydrated than their pyridine analogs. A comparison with benzaldehydes reveals that, in their reactivities, the two heterocyclic aldehydes behave in a similar manner. Thus, for example, plots of log k(H) for the acid-catalyzed dehydration and hydrations versus log K-h for the equilibrium hydration show single correlation lines including the points for the benzaldehydes and heterocyclic aldehydes (but not the aliphatic aldehydes).