Examinations of Dimethyl Sulfoxide-Phase Equilibrium Acidities of Selected Polycyclic Aromatic Compounds

Abstract

Comparisons of dimethyl sulibxide (DMSO)-phase pK _a's (in parentheses) for 10-cyano-9-methoxymethy-lanthracene (17.35), 10-cyano-9-phenoxymethylanthracene (14.9), 9-cyanomethoxymethylanthracene (20.4), and (α-methoxy) phenylacetonitrile (23.0) with literature DMSO-phase pK _a's for 9-methylanth-racene, 9-methoxymethylanthracene, 9-phenoxymethylanthracene, 9-cyanomethylanthracene, 10-cyano-9-methylanthracene, and phenylacetonitrile (31.1, 30.6, 30.2, 19.7, 20.0, and 21.9, respectively), indicate that: (a) α-methoxy and α-phenoxy substituents acidify 10-cyano-9-methylanthracene; (b) an α-methoxy moiety deacidifies 9-cyanomethylanthracene and phenylacetonitrile; and (c) 10-cyano substituents acidify 9-methylanthracene, 9-methoxymethylanthracene, and 9-phenoxymethylanthracene by 11.1, 13.25, and 15.3 pK _a units, respectively. These data are indicative of steric inhibition of resonance, resonance saturation, and effects due to cross conjugation, in carbanions derived from variously substituted 9-methylanthracenes. DMSO-phase pK _a for phenalene (18.2) and benzanthrene (20.2) have also been determined. In DMSO, phenalene is therefore 4.4 pK units more acidic than its isomeric analogue fluorene (22.6), while benzanthrene is 0.5 pK units less acidic than its isomeric analogue 1,2-benzofluorene (19.7). Specific solvation of the allylic-like phenalenide anion is a likely reason for its enhanced acidity, relative to benzanthrene.