STERICALLY CONGESTED MOLECULES .12. ACIDITY CONSTANTS OF STERICALLY SHIELDED PROTONATED IMINES IN HYDROXYLIC SOLVENTS

NMR spectra in combination with pH measurements are shown to provide a simple and convenient procedure for determining the basicities of imi nes which are either slowly hydrolyzed (4, 5) or completely stable but inconveniently weak bases (11-13). The method has the advantage that it does not require any precise knowledge of the concentrations of the substrates or reagents. N-Alkyl (4, 15) and N-unsubstituted imines (5 , 8, 9, 14) show quite similar basicities with a weak solvent dependen ce (from pure water toi 99.5% methanol) which is akin to that of pyrid ine. The N-aryl imines 10-13 are less basic by ca. 4 pK units. It is c oncluded that spatial solvent exclusion by bulky substituents has rema rkably little effect on the pK(a)(') values.