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

Abstract

h e final product owing t o isomerization. IVith anhydrous silicic acid t h e latter was obtained in better than 807, yield. it ith non-acidic alumina, or thionyl chloride in excess pyridine, the main product was p-menth-8-ene (up t o 907,) ; no isomerization t o p-menth-3-ene was obtained. T h e possible-reaction mechanisms are discussed. I N T R O D U C T I O NDehydration of suitable terpene alcohols provides a simple means for sj-nthesizing unsaturated terpenes required as reference compounds in gas-liquid chromatography (GLC). I11 Part I1 of this series (I) the analysis of the mixture of p-menthadienes and oxides obtained in the acid-catalyzed dehydration-of a-terpineol (p-menth-1-ene-8-01) was described. More recently ( 2 ) , the author h a s reported results obtained in the dehydration of a number of terpene alcohols, including a-terpineol and 9-menthan-8-01 (dihydro-a-terpineol), with a simple non-acidic reagent, viz., base-modified alumina. The dehydration of 9-menthan-8-01 by acidic and lion-acidic dehydrating agents has now been studied in detail and optimum conditions for the preparation of p-menth-3-, -4(8)-, and -8-ene by this means have been determined. Since cis or trans elimination of the ele~nents of water can proceed equally well with p-tnenthan-8-01, it was hoped that such a study would also provide data on the relative ease of the formation of exocyclic double bonds in cyclohexane derivatives.In 1910, Bkhal (3) hydrogenated a-terpineol over nickel catalyst by the SabatierSenderson method and obtained what was then thought to be cis-p-menthan-8-01. Shortly afterwards Wallach (4) obtained the solid trans isomer (n1.p. 35' C) using colloidal palladium catalyst. Both authors reported that dehydration with acetic acid containing sulphuric acid gave p-menth-4(8)-ene. Perkin and Pickles ( 5 ) dehydrated p-menthan-8-01 'For Part V I , see Phytochenzistry, 1968 ( i n press 41, 1983 (m.p. 35' C) with fused potassium hydrogen sulphate and reported the isolation of pmenth-8-ene. However, Kotz and Busch (6) obtained 87% of theory of p-menth-3-ene when this reaction was carried out in a copper autoclave a t 200' C for 24 hours. Zeitschel and Schmidt (7) isolated trans-p-menthan-8-01 from American pine oil and claim to have obtained the pure cis and trans isomers by fractional distillation. However, their cis isomer was a liquid and gave a phenylurethane, m.p. 90-92' C. Keats (8), on the other hand, obtained solid cis-p-menthan-8-01, m.p. 25' C (phenylurethane n1.p. 114' C), and attempted to correlate the stereochemistry of both isomers b17 conversion, via the chloride, t o cis-and trans-p-menth-8-ene. The above discrepancies were resolved w11e11 in 1960 Eastman and Quinn (9) succeeded in obtaining both isomeric alcohols pure (n1.p. 35' and 47' C) by fractional distillation through a highly efficient distillation column, and showed that both the pure isomers gave phenylurethanes of m.p. 115-117' C, whereas mixture of these gave phenj-lurethanes of m.p. 91-97' C. In this study small aliquots of a-terpine...