Anethum graveolens), and also in mandarin orange peel oil. R-(−)-Carvone is also the most abundant compound in the essential oil from several species of mint, particularly spearmint oil (Mentha spicata), which is composed of 50–80% R-(−)-carvone. Spearmint is a major source of naturally produced R-(−)-carvone. However, the majority of R-(−)-carvone used in commercial applications is synthesized from R-(+)-limonene. The R-(−)-carvone isomer also occurs in kuromoji oil. Some oils, like gingergrass oil, contain a mixture of both enantiomers. Many other natural oils, for example peppermint oil, contain trace quantities of carvones. History Caraway was used for medicinal purposes by the ancient Romans, but carvone was probably not isolated as a pure compound until Franz Varrentrapp (1815–1877) obtained it in 1849.Handwörterbuch der reinen und angewandten Chemie [Concise dictionary of pure and applied chemistry] (Braunschweig, (Germany): Friedrich Vieweg und Sohn, 1849), vol. 4, pages 686-688. [Notes: (1) Varrentrapp purified carvone by mixing oil of caraway with alcohol that had been saturated with hydrogen sulfide and ammonia; the reaction produced a crystalline precipitate, from which carvone could be recovered by adding potassium hydroxide in alcohol to the precipitate, and then adding water; (2) Varrentrapp's empirical formula for carvone is incorrect because chemists at that time used the wrong atomic masses for the elements; e.g., carbon (6 instead of 12).] It was originally called carvol by Schweizer. Goldschmidt and Zürrer identified it as a ketone related to limonene, and the structure was finally elucidated by Georg Wagner (1849–1903) in 1894. Preparation The dextro-form, S-(+)-carvone is obtained practically pure by the fractional distillation of caraway oil. The levo-form obtained from the oils containing it usually requires additional treatment to produce high purity R-(−)-carvone. This can be achieved by the formation of an addition compound with hydrogen sulfide, from which carvone may be regenerated by treatment with potassium hydroxide in ethanol and then distilling the product in a current of steam. Carvone may be synthetically prepared from limonene via limonene nitrosochloride which may be formed by treatment of limonene with isoamyl nitrite in glacial acetic acid. This compound is then converted into carvoxime, which can be achieved by refluxing with DMF in isopropanol. Refluxing carvoxime with 5% oxalic acid yields carvone. This procedure affords R-(−)-carvone from R-(+)-limonene. The major use of d-limonene is as a precursor to S-(+)-carvone. The large scale availability of orange rinds, a byproduct in the production of orange juice, has made limonene cheaply