Citral from lemon grass oil

Natural aroma chemicals Isolation and purification via physical processes Citral from lemon grass oil, eugenol from cloves, menthol from Mentha species... 

Commercial production of vitamin A, as well as other carotenoids, typically proceeds via the key intermediate pseudoinone (48) [21], which is prepared by condensation of acetone with the natural product citral. This commercial procedure has the drawback of relying on the purification of citral from lemon grass oil. In an attempt to relieve this dependence on citral, Kimel and coworkers at Hoffmann-La Roche developed an alternative fuUy synthetic route to pseudoinone (48) involving multiple Carroll rearrangements (Scheme 8.18) [22]. 

STEAM DISTILLATION ISOLATION OF CITRAL FROM LEMON GRASS OIL... 

Steam Distillation of Citral from Lemon Grass Oil... 

Why was a steam distillation rather than a simple distillation performed in the isolation of citral from lemon grass oil ... 

The purest specimens which have been prepared of natural citral, either from lemon-grass oil, lemon oil, or the oil of Tetranthem citrata, have the following characters —... 

Racemic isophytol was synthesized from lemon grass oil, or totally from acetone. In the synthesis of isophytol from lemon grass oil (Fig. 9) its main constituent, citral, is condensed with acetone to give pseudoionone. Hydrogenation leads to hexahydropseudoionone. Lengthening of the chain by an isoprene unit is effected by condensation of this ketone with sodium acetylide in liquid ammonia followed by partial hydrogenation of the triple... 

The terpenes are sometimes used alone, sometimes together with a little citral obtained from lemon-grass oil. It is a common custom to export oil of lemons with a guaranteed citral-content, and to sell it upon that basis. In judging of the value of such a basis for the market value of the oil, the following points should be noted —... 

Methyl-heptenone, CgH, 0, occurs in various essential oils, especially lemon-grass oil, in which it is associated with, and difi cult to separate from, the aldehyde citral. 

The lipidic vitamins (ref.84) include vitamin A (32), a substance intrinsic to the physiology of vision, vitamin E (83), a natural protective antioxidant, and vitamins K, (84) with Kj (85), antihemorrhagic compounds, each of which is derivable from an initial natural product intermediate. Although traditionally a -ionone obtained from citral (a major constituent of lemon grass oil) was used for the synthesis of vitamin A, a synthetic source has now replaced this in a process which also gives /g-carotene. In one method the Cl4 aldehyde in that process is reacted with a C6 eneyne component and selective hydrogenation followed by dehydration and isomerisation affords the final product (ref.85). 

The two oils were formerly the main source of natural citral, obtained as a ca. 4 1 mixture of geranial and neral by distillation [463, 466a, 466b]. However, lemon-grass oil has declined in commercial importance due to the competitive synthesis of citral and isolation of natural citral from Litsea cubeba oil. Today, a few 100 t/ yr are still produced. In addition to being processed into citral, it is used to some extent for perfuming soap and household products. 

Apart from its pleasant aroma, citral is valued for its sedative, antiviral and antimicrobial properties. Citral-rich oils derived from lemon grass and lemon-scented tea tree were shown to inhibit Candida albicans at more than four times the rate (zone of inhibition) of tea tree oil (Williams and Home 1995). However, many aldehydes are irritants, causing skin sensitivity in some people thereby restricting their use in topical applications. 

The commercial importance of citral has stimulated an extensive search for its presence in nature. One source is the oil from the skins of lemons and oranges, although it is only a minor component of this oil. However, citral is the major constituent of the oil obtained from lemon grass, and in fact 75-85% of the crude oil derived from pressing lemon grass is this natural product. 

Citral is a chemically labile substance, and its isolation therefore presents a challenge. This task is simplified by the fact that citral, bp 229 °C (760 torr), is relatively volatile and has low solubility in water. These properties make it a suitable candidate for steam distillation, a technique that allows distillation of citral from crude lemon grass oil at a temperature below 100 °C, which is far below its normal boiling point. Neutral conditions are maintained in steam distillation, as is the partial exclusion of atmospheric oxygen, so the possibility of oxidation and/or polymerization of citral is minimized. 

The oil obtained from it differs from the Malabar lemon-grass oil by its lesser solubility in alcohol and the frequently lower citral-content, for which reason it is usually less valuable than the Malabar oil. 

The conclusions drawn by the Imperial Institute chemists do not seem to have any great amount of evidence to support them. The author observed, some years ago, that West Indian lemon-grass oil decreased considerably in O tral value by keeping, and also became far less soluble. The freshly distilled oil from Cymbopogon citratus distilled in the West Indies was soluble in 3 volumes of 70 per cent, alcohol when freshly distilled. Since then he has had the opportunity of examining numerous samples of Cochin oil which had been tested before shipment, and which on arrival in Europe had lost from 3 to 5 per cent, or even more of their citral contents. In the author s opinion, there is room for ths belief that the recently found insolubility of Cochin lemon-grass oil may be due to differences in the exact type of grass used, and also in the conditions of cultivation. 

PROP Main constituent is citral. From steam distillation of freshly cut and partially dried grasses of Cjmbopogon citratus (STAFF) znd Andropogon nardus var. ceriferus (Hack). Light-yellow to brown liquid Kght lemon odor. D 0.869-0.894, refr index 1.483. Sol in mineral oil, propylene glycol insol in water,... 

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