2.6.5.3.3 Aromatic Terpene Aldehyde
In comparison to aliphatic aldehydes, the aromatic
aldehydes invariably play a vital role in essential oils. It has been observed that a major portion of certain volatile oils mainly comprise of aromatic terpene aldehydes, such as : bitter almond and cassia. A variety of such aromatic aldehydes commonly found in essential oils are, namely: anisaldehyde, benzaldehyde, cinnamaldehyde, cuminaldehyde and salicyldehyde.
Examples: A few typical examples of aromatic terpene aldehyde are discussed below, for instance: Cumminaldehyde, Vanillin etc.
A. Cuminaldehyde
Chemical Structure 4(1-Methylethyl) benzaldehyde.
Occurrence It occurs as a constituent of essential oils present in eucalyptus, myrrh, cassia and cumin. Cumin mainly comprises of the dried ripe fruits of Cuminum cyminum Linn., (family: Umbelliferae).
Isolation The essential oil obtained from the dried ripe fruits of cumin ranges between 2-4%, the major constituents of which cuminaldehyde (35-60%). The aldehyde may be separated by forming its sodium bisulphite adduct and subsequently regenerating the desired product by treatment with alkaline solution carefully.
Characteristic Features It is a colourless to yellowish, oily liquid. It possesses a strong persistent odour, acrid and burning taste.
The physical characteristic of cuminaldehyde are as follows: d20 0.978; bp760 235-236°C; n20D 1.5301. It is practically insoluble in water, but freely soluble in ether and ethanol.
Identification It is identified by forming its thiosemicarbazone derivative (C11H15 N3S).
Uses It is extensively employed as an adjunct in perfumery.
B. Vanillin
Chemical Structure 4-Hydroxy-3-methoxybenzaldehyde (C8H8O3).
Occurrence It occurs naturally in vanilla (vanilla bean) especially found in cured, fully-grown unripe fruit (pods) of Vanilla planifolia Andrews, known in commerce as Bourbon or Mexican vanilla. It is also found in the vanilla pods of Vanilla tahitensis J.W. More, usually recognised in commerce as Tahiti vanilla. Both these species belong to natural order Orchidaceae. The term vanilla has been derived from the spanish word vania, meaning sheath like pod and illa, meaning small; planifolia is derived from the Latin word planus, meaning flat, and follium, meaning leaf; tahitensis refers to Tahiti its adopted home.
It also occurs in small quantities in a variety of essential oils e.g., clove oil; gums and oleoresins
e.g., benzoin, Peru balsam. Interestingly, plants do not contain vanillin as such, but they exist in the form of glycosides, which upon hydrolysis in the presence of enzymes release vanillin.
Isolation It may be accomplished by any one of the following five methods, namely:
(a) From Vanilla Pods: Vanillin is obtained by the extraction of powdered vanilla pods with ether, by at least three successive extractions, evaporating the combined ethereal fraction (crude vanillin). It is further purified by crystallization of the crude product from ethanol.
(b) From Lignin Waste: More conveniently, vanillin is obtained from the lignin waste, a byproduct in the manufacturer of paper pulp. Lignin is a complex polymeric natural material of woody plants and essentially has the following fragment structural unit:
Thus, lignin when subjected to oxidation yields vanillin in a large extent which ultimately has rendered this process an economically feasible and viable one.
(c) From Eugenol: It may also be prepared from eugenol which is a major constituent of clove oil as given below:
Eugenol on alkaline treatment undergoes intramolecular rearrangement to give rise to isoeugenol which on acetylation with acetic anhydride yields the corresponding acetate derivative. The resulting product on oxidation yields 4-acetoxy-3-methoxy benzaldehyde which upon treatment with HCl yields vanillin.
(d) From Guaiacol: It may be prepared on an industrial scale by the help of Reimer-Tiemann's reaction, whereby o-vanillin is obtained from guaiacol i.e., catechol methyl ether as stated below:
Guaiacol on treatment with sodium hydroxide in the presence of chloroform yields vanillin and o-vanillin.
(e) From Bisulphite Adduct: Vanillin may also be isolated from its bisulphite adduct. The ethereal solution containing vanillin is extracted completely with saturated aqueous sodium bisulphite solution. Usually specialized techniques are adopted to ensure pure isolates from its tautomers and closely related isomers.
Characteristic Features It consists of fine, white to slightly yellow, needle-shaped crystals having
an odour and taste quite resembling to that of vanilla fruits. It is usually affected by light. On prolonged heating at 105°C, it decomposes with the formation of non-volatile byproducts. It is soluble in hot water (1 g dissolves in 16 ml of water at 80°C), and in glycerol (~ 20 ml of glycerol per 1g of vanillin). It is freely soluble in ethanol, ether, chloroform, carbon disulphide, glacial acetic acid, pyridine, oils and aqueous solutions of alkali hydroxides.
It has the following physical parameters: d 1.056; mp 80-81°C; bp 285°C.
Identification It may be identified by preparing a number of derivatives, such as: semicarbazone (mp 230°C); dinitrohydrazone (mp 271°C); para-nitrophenylhydrazone (mp 227°C); benzoate (mp 75°C) and acetyl derivative (mp 77°C).
Uses
1. It is employed as a pharmaceutical aid (flavour).
2. It is extensively used as a flavouring agent in beverages, confectionery, foods and perfumery.
3. It is used in the manufacture of liqueurs.
4. It has been established that 1 part of vanillin equals 400 parts vanilla pods; and 2.5-3 parts equals 500 parts tincture vanilla.
5. It also finds its use as a reagent in analytical chemistry.
