The panorama of base-catalysed isomerisations of terpenes is an important part of aroma chemistry. Major contributions in this area are presented here under sections on hydrocarbons, alcohols, aldehydes, ketones, acids, esters, and epoxides.
p-Menthenes. Pines and Eschinazi introduced sodium-organosodium catalysts, for example sodium-'benzylsodium' catalyst (prepared by treating an excess of sodium in toluene with o-chlorotoluene) for isomerising the title compounds. One of their main findings is that when (+)-p-menth-l-ene (1), trans-p-menth-2-ene (2) or p-menth-3-ene (3), is refluxed at 168-175° for 20-22 hrs with the catalyst, the isomerisate is an equilibrium mixture of (3) (63%), (1) (32%), and p-menth-8(9)-ene (4) (5%). The rate of racemisation of (+)-(1) is relatively faster than that of its rate of isomerisation and (+)-trans-(2) reorganised to (+)-(1). There is no formation of p-cymene (5). The mechanism proposed involves intermediate carbanions.