The effect of polar and non-polar solvents on the optical rotatory powers of certain saturated and unsaturated terpenic hydrocarbons

Abstract

(1) The optical rotatory powers and rotatory dis- persions of five terpenic hydrocarbons, viz. d- menthene, d- limonene, d - pinene, d- pinane and d- carane ( ?), have been examined in dilute solution in various polar and non-polar organic solvents.

(2) The rotatory powers of the weakly polar, un- saturated hydrocarbons, d- menthene and d- pinene are shown to be mainly related to the polarity of the solvent medium. The rotatory power of d- limonene, on the other hand is chiefly con- trolled by the refractive index of the solvent medium. In the former cases a few displacements occur and are explained on the basis of a minor refractivity effect being present. Similarly in the latter case, a minor polar effect appears to be present. In order to bring out the above relationships clearly, it is necessary to com- pare solvents of a similar type, by grouping them into an aromatic and an aliphatic series.

(3) The rotatory power of the non-polar saturated hydrocarbon, d- pinane has been examined in solution in thirty -six solvents of widely varying types. It is shown that the rotation is governed by the refractive index of the sol- vent medium, very slight displacements occurring due to the presence of a minor polar effect. In this case there is no need to divide the solvents into various groups according to their chemical types.

(4) Several physical theories of optical activity are examined from the point of view of their application to the experimental results obtained for d- pinane.

(5) Owing to the known difficulty of effecting a satisfactory separation of unsaturated hydro- carbons from saturated cyclopropane derivatives, some doubt is cast upon the identification as d- carane of the hydrocarbon obtained by dis- tilling pulegone hydrazone over caustic potash. An examination of the rotatory power of this product in various solutions reveals irregular variations.

(6) The effect of temperature variations upon solutions of the above compounds in a strongly polar and a non-polar solvent respectively has been determined. The observed results in the majority of cases, may be explained on the basis of the changes in the degree of solute -solvent association and in the refractive index of the mixture. Similar explanations hold for the variation in rotatory powers of the above compounds with changes in concentration in certain solutions.