GAS–LIQUID CHROMATOGRAPHY OF TERPENES: PART VII. THE DEHYDRATION OF p-MENTHAN-8-OL

Abstract

Mixtures of cis- and trans-p-menthan-8-ol were prepared by hydrogenation of α-terpineol over platinum or palladium catalysts and the isomer ratios obtained were determined by gas–liquid chromatography. The dehydration of a 1:1 mixture, as well as of the pure stereoisomers, was studied with a number of reagents. Aqueous oxalic and acetic acids, fused potassium hydrogen sulphate, anhydrous oxalic, silicic, and boric acids, and "Florisil" promoted the typical acid-catalyzed dehydration, with p-menth-4(8)-ene and smaller amounts of isomeric p-menth-8-enes as the primary products, and p-menth-3-ene the final product owing to isomerization. With anhydrous silicic acid the latter was obtained in better than 80% yield. With non-acidic alumina, or thionyl chloride in excess pyridine, the main product was p-menth-8-ene (up to 90%); no isomerization to p-menth-3-ene was obtained. The possible reaction mechanisms are discussed.