Essential oils citronellal from

Synthesis from Citronellal. Citronellal can be hydrogenated to citronellol by the use of special catalysts and/or special hydrogenation techniques, e.g. [47]. The citronellal which is used as starting material may originate from synthetic production or from isolation of essential oils. Citronellal from citronella oil yields (+(-citronellol the corresponding material from citronellal from Eucalyptus citriodora oil is racemic. Pure (+(-citronellol is also obtained from (+)-citronellal which is produced as an intermediate of (-(-menthol (see p. 55-58). By this asymmetric technology, pure (-(-citronellal and therefore pure (-(-citronellol is also available. 

Synthesis of (+)- and ( )-Citronellol from the Citronellal Fraction of Essential Oils. (+)-Citronellal is obtained by distillation of Java citronella oil and is hydrogenated to (+)-citronellol in the presence of a catalyst (e.g., Raney nickel). Similarly, (zb)-citronellol is prepared from the ( )-citronellal fraction of Eucalyptus citriodora oil. 

Isolation from Essential Oils. (+)-Citronellal is obtained from citronella oils by fractional distillation. ( )-Citronellal is isolated from Eucalyptus citriodora oil when necessary, it is purified by using an addition compound, e.g., the bisulfite derivative. 

Individual compounds can be isolated from essential oils containing one or only a few major components by distillation or crystallization. Examples are eugenol from clove oil, menthol from commint oil, citronellal from Eucalyptus citriodora oil and citral from Litsea cubeba oil. These compounds are used as such or serve as starting materials for the synthesis of derivatives, which are also used as flavor and fragrance substances. However, the importance of some of these oils has decreased substantially because of the development of selective synthetic processes for their components. 

Thujone was discovered by E. Schweizer. C. VolckeP discovered carvone in carraway seed oil, and cineol in wormseed oil. Carvone was shown to be a ketone related to limonene by H. Goldschmidt and R. Ziirrer. Linalool was discovered by distilling coriander oil by A. Kawalier and prepared from linaloe wood (licari kamali) by H. Morin.Geraniol was discovered by O. Jacobson in oil of palma rosa. F. W. Semmler " showed that it is an olefinic terpene with an open carbon chain. Menthone was obtained from oil of peppermint by R. W. Atkinson and H. Yoshida. (Menthol was known much earlier and was analysed by Dumas in 1832.) G. Bouchardat and R. Voiry obtained terpineol from essential oils. Citronellal (an aldehyde) was discovered by F. D. Dodge.2 ... 

Isopulegol, Cj Hj O, does not appear to exist in essential oils, but it results from the action of acids on citronellal. The last-named body, for example, when boiled with acetic anhydride yields isopulegyl acetate, from which the alcohol is obtained by hydrolysis. 

Citronellal which was originally termed citronellone, is one of the characteristic constituents of citronella oil, in which it occurs to a considerable extent. It can be prepared by shaking the essential oil with a hot solution of sodium bisulphite, and decomposing the resulting bisulphite compound by means of sodium carbonate and distilling the citronellal in a current of steam. Citronellal is optically active, and it is probable that the specimens isolated from natural sources are mixtures of the two optically active isomers, so that the actual theoretical rotation is not known with certainty. 

Production. Citronellal is still isolated from essential oils in considerable quantities it is also produced synthetically. 

With fractional distillation, single aroma chemicals can also be obtained, in more or less highly purified form, from essential oils. These are designated as isolates. In the days when synthetic organic chemistry was less versatile and powerful than it is today, this was the only way to obtain many aroma chemicals. Even today it is in some cases, such as citronellal or cedrol, the most economical way. 

Olefine Terpenes.— The olefine terpenes and the alcohols and aldehydes derived from them are found in several essential oils, e.g.y oil of geranium contains geraniol and citronellol (p. 167), and oil of lemony citral and citronellal (p. 170). 

Eucalyptus citriodora Hook f (Lemon gum) The oil of the fresh leaves of E. citriodora (2.2-8.3%), a tree introduced into Kenya from Australia (68), has been well researched in Kenya. Results indicate that there are two chemical varieties that were introduced. The chief essential oil constituents of one variety are citronellal (65-88%), citronellol (2-25%) and isopulegol (2-19%). In constrast, the second variety has as its major essential oil consitutents citronellol (32-52%), citronellal (38-59%) and 1,8-cineole (2-19%) (68,69). The essential oil is regarded as a perfumery oil. This plant is the second most commercially important Eucalyptus species in the essential oil trade and is only second to E. smithii Baker (69). The aromatic oil is used in soaps, creams and lotions due to its desirable odor properties, and as with many other essential oils is also used extensively in many technical preparations to mask other industrial malodors. A highly active mosquito repellent compound, p-menthane 3,8-diol has been isolated from the waste water of distillation of E. citriodora. There are now many commercial mosquito repellents based on this waste water (70-73). 

C12H22O2, Mr 198.30, hpioi.3 kPa 240 °C, cf° 0.8901, rv 1.4515, occurs in many essential oils either as one of its optical isomers or as the racemate. The odor of racemic citronellyl acetate differs little from that of the optical isomers. ( )-Citronel-lyl acetate is a liquid with a fresh-fruity rose odor. It is often used as a fragrance, for example, for rose, lavender, and geranium notes as well as for eau de cologne with citrus nuances. Since it is relatively stable to alkali, it can be used in soaps and detergents. Citrus flavors acquire specific character through the addition of citronellyl acetate it is also used to round off other fruit flavors. 

Definition Essential oil from citronella grass, Cymbopogon nardus, contg. citronellal, geraniol, citronellol... 

Plants especially from the Lamiaceae family can inhibit the growth of several weeds by releasing phytotoxic monoterpenes (a-pinene, P pinene, camphene, limonene, a-phellandrene, p-cymene, 1,8-cineole, borneol, pulegone, and camphor) (Angelini et al., 2003). The herbicide effect of 1,4-cineole and 1,8-cineole is also described by Dayan et al. (2012). Plants that are exposed to essential oils often metabolize them, and when citral was added geraniol, nerol and their acids appeared. When citronellal metabolization was tested, citronellol and citronellic acid were formed, and with pulegone (iso)-menthone, isopulegol and menthofuran were found (Dudai et al., 2000). 

Numerous components are offered on the market like citral from Litsea cubeba oil, geraniol from palmarosa oil, linalool from ho oil, coriander oil or lavandin, pinenes from different Pinus species, citronellal from Corymbia citriodora, cedrol from cedarwood, or even the santalols from different sandalwood species. All these are added to nish essential oils. As already mentioned before, synthetic chemicals can no more be applied as enantiomeric separation is a state of the art today and will convict the matter of fact of adulteration. By using synthesized, correct chiral compounds, the detection is hardly to be recognized but with NMR method, but this is an expensive analysis. 

A typical example is the hydroformylation of linalool, a monoterpenoid allyhc alcohol with a pleasant lily odor. It is a key building block for the synthesis of various vitamins and fragrances, such as vitamin A, vitamin E, citral, citronel-lol, and phnol (see Scheme 6.44). Linalool is found in the essential oils of several plants, for example, Brazihan rosewood and Chinese Ho leaf oils, but mostly it is produced industrially from a-pinene or from the petrochemically derived source 6-methylhept-5-en-2-one (Hoflimann-La Roche Process) [144]. 

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