Solvent effects on circular dichroism induced in aroylarenecarboxylic acids by optically active amines
When 2-, 3-, or 4-benzoylbenzoic acid, 2-(2,4,6-trimethylbenzoyl)benzoic acid, or 2-acetylbenzoic acid and (–)-(R)-amphetamine, (–)-(R)-N-methylamphetamine, (±)-(R)- or (–)-(S)-α-phenylethylamine, or (±)-(S)-s-butylamine were dissolved together in equimolar proportions in the non-polar solvents benzene or carbon tetrachloride at 25°, induced circular dichroism was observed even at low concentrations. The logarithm of the molecular ellipticity θ, of the mixture of 2-benzoylbenzoic acid and amphetamine inversely correlates with the ET value of the solvent used. A contact ion pair derived from salt formation between the keto-acid and the optically active amine may be responsible for the phenomenon. In polar solvents such as alcohol the induced c.d. effect is reduced as expected from increased dissociation of the ion pair.