Titrations in non-aqueous media. Part V. Acid-base behaviour of substituted 2-amino-3-cyanofurans, 2-amino-3-cyanothiophenes and 2-amino-3-cyanopyrrole and their Schiff bases with salicylaldehyde and 2-hydroxy-1-naphthaldehyde

Abstract

The acid-base behaviour of substituted 2-amino-3-cyanofurans, 2-amino-3-cyanothiophenes and 2-amino-3-cyanopyrrole and their Schiff bases with salicylaldehyde and 2-hydroxy-1-naphthaldehyde have been investigated. The results are as follows. The amines and their Schiff bases cannot be titrated potentiometrically as bases with perchloric acid in nitrobenzene and acetic acid solvents, but they can be titrated as acids with tetrabutylammonium hydroxide in pyridine. This is due to the electron-withdrawing effect of the cyano group situated at the 3-position to the rings. The electron-withdrawing effect of the cyano group in the Schiff bases can extend to the benzene ring of the salicylaldehyde through the imino group. This renders the benzene ring more acidic, although the same effect does not make any appreciable difference to the acidity of the naphthalene ring of 2-hydroxy-1-naphthaldehyde as the electron-withdrawing effect of the cyano group is nearly in balance with the electron-withdrawing effect of the naphthalene ring. The naphthalene ring is on average 1.8 pK_a′ units more acidic than the benzene ring. The electron-withdrawing order of the rings is pyrrole > furan > thiophene. The electron-withdrawing effect of the furan ring is stronger than that of the benzene ring.