Ester Volatile Oils (Esters of Aliphatic Acids, Esters of Aromatic Acids, Esters containing Nitrogen)

2.6.5.8 Ester Volatile Oils

The ‘Ester Volatile Oils’ essentially attribute their flavouring characteristics, odour, aroma and specific perfume by virtue of the presence of a good number of naturally occurring esters, most common among which are the acetates of borneol, geraniol and terpineol. However, it is an age-old practice to allow the maturation or ageing of such ester-containing perfumes in order to enhance the process of esterification in situ thereby ultimately improving the overall aroma and bouquet of the volatile oil. Incidentally, there are certain exceptions, such as: the ‘Oil of Wintergreen’ which contains upto 99% of methyl salicylate (an ester). Classification The ester volatile oils may be classified conveniently into three categories as follows:

(i) Esters of Aliphatic Acids,

(ii) Esters of Aromatic Acids, and

(iii) Esters containing Nitrogen.

These different categories of ester volatile oils shall be discussed along with their typical examples as under:

2.6.5.8.1 Esters of Aliphatic Acids The typical examples of esters of the aliphatic acids are, namely: Geranyl acetate, Linalyl acetate.

A. Geranyl Acetate

Chemical Structure 3, 7-Dimethyl-2, 6-octadien-8-acetate; (C₁₂H₂₀O₂). It is an olefenic terpene acetate mostly found in a number of essential oils.

Occurrence The ester is very widely distributed in a variety of essential oils, such as: oil of citronella, petit grain, lemon-grass, coriander, lavender etc. It is also found in Satureja montana L. (Lamiaceae)-Winter Savory, White Thyme, Spanish Savory; and Tilia europaea L. (Tiliaceae)- Lime Tree (Europe), Linden Tree (America).

Isolation It may be obtained from the rich source of volatile oil containing greanyl acetate by fractional distillation under vacuum.

Characteristic Features It is a colourless liquid having a pleasant aromatic fragrance resembling to that of rose. It boils at 242-245°C with decomposition at atmospheric pressure.

Identification The ester on saponification with alcoholic KOH yields geraniol and acetic acid as the products of reaction. The former may be identified by examining its physical parameters, for instance: bp₇₅₇ 229-230°C; bp₁₂ 114-115°C; d²⁰₄ 0.8894; n²⁰_D 1.4766; uvmax 190-195 nm (ε 18000).

Uses

1. It is used abundantly in perfumery.

2. It is also employed in making cosmetics and various types of toilet soaps.

B. Linalyl Acetate

Synonym Bergamot.

Chemical Structure 3, 7-Dimethyl-1, 6-octadien-3-yl acetate; (C₁₂H₂₀O₂). It is also an olefinic terpene acetate and regarded as the most valuable constituent of bergamot and lavender oils.

Occurrence It is found in a number of volatile oils, namely: Lavandula angustifolia Mill.

(Lamiaceae)-Lavender, True or Common Lavender; Salvia selarea L. (Lamiaceae)-Clary, Cleareye, Muscatel Sage; Satureja montana L. (Lamiaceae)-Winter Savory, White Thyme, Spanish Savory; Thymus vulgaris L. (Lamiaceae)-Common Thyme; Tilia europaea L. (Tiliaceae)-Linden Tree (America), Lime Tree (Europe).

Isolation It may be obtained from lavender oil or bergamot oil by subjecting it to distillation under very high vacuum, because on distillation at atmospheric pressure or with steam distillation linalyl acetate gets hydrolysed rapidly and decomposed eventually.

Characteristic Features It is a colourless oily liquid having a very pleasant fruity odour of bergamot oil. Its physical properties are: d²⁰₄ 0.885; bp 220°C; n²⁰_D 1.4460. It is almost insoluble inwater but miscible freely with ether and alcohol.

Identification Linalyl acetate upon saponification with alcoholic KOH yields linalool (linalol) and acetic acid. The dl-form of linalool has a bp720 194-197°C.

Uses It is used extensively in perfumery.

2.6.5.8.2 Esters of Aromatic Acids The various esters that are associated with aromatic acids and found in volatile oils are: Benzyl benzoate, Cinnamyl Cinnamate; Methyl Salicylate.

A. Benzyl Benzoate

Synonyms Ascabin; Venzonate; Ascabiol.

Chemical Structure Benzoic acid phenyl methyl ester; (C₁₄H₁₂O₂).

Occurrence It is contained in Peru and Tolu balsams. It is also found in a variety of volatile oils, such as: Cananga odorata (Lam.) Hook. f. & Thomas (Annonaceae)-Ylang-Ylang, Cananga; Cinnamonum verum J S Presl (Lauraceae)-Ceylon Cinnamon; Myroxylon balsamum Var. Pereirae (Royle) Harms. (Fabaceae)-Balsam of Peru; Peumus boldus Molina (Monimiaceae)-Baldo; Vanilla planifolia Andr. (Orchidaceae)-Vanilla.

Isolation Benzyl benzoate may be isolated by cooling the corresponding fraction to a very low temperature when it gets separated as a solid (mp 21°C). It may also be further recrystallized from chloroform or ether.

Characteristic Features It is an oily liquid or leaflets, having a faint, pleasant aromatic odour. It possesses a sharp burning taste. Its physical characteristics are: mp 21°C; d²⁰₄ 1.118; bp 323-324°C; bp₁₆ 189-191°C; bp_4.5 156°C, and n²¹_D 1.5681. It is sparingly volatile with steam. It is insoluble in water or glycerol, but miscible with alcohol, chloroform, ether and oils.

Identification The benzyl benzoate on saponification yields the two products of reaction i.e., benzoic acid and benzyl alcohol that may be identified by carrying out specific tests for these compounds.

Uses

1. It is used extensively as a diluent and solvent of solid aromatics e.g., artificial musk.

2. By virtue of its low volatility benzyl benzoate is employed as a fixative in perfume composition.

3. It is used as a solvent for cellulose acetate and nitrocellulose.

4. It serves as a substitute for camphor in celluloid and plastic pyroxylin compounds.

5. It is also employed in confectionery and chewing gum flavours.

B. Cinnamyl Cinnamate

Synonyms Cinnyl cinnamate; Styracin.

Chemical Structure 3-Phenyl-2-propenoic acid 3-phenyl-2-propenyl ester.

Occurrence It occurs in the buds of Populus balsamifer L., (Family: Salicaceae)-Goris; Lavanga scandens Buch-Ham., Lavangalata-Baslas; and Styrox Benzoin Dryander (Family: Styracaceae)-Benzoin, Sumatra Benzoin, Styrax.

Isolation It may be obtained from the volatile oil fraction by chilling it to low temperature and collecting the solids having mp 44°C.

Characteristic Features The characteristic features of the trans-trans-cinnamyl cinnamate are: mp 44°C; uvmax (95% ethanol): 216, 223 nm (log ε 3.45, 3.25). It is practically insoluble in water but sparingly soluble in cold ethanol and soluble in ether (1 g in 3 ml).

Identification On saponification with alcoholic KOH this ester gives rise to cinnamic acid and cinnamyl alcohol which may be further identified by performing their specific tests.

Uses

1. It is used in perfumery.

2. It is also employed in making toilet soaps etc.

C. Methyl Salicylate

Synonyms Winter green oil; Betula oil; Sweet birch oil; Teabery oil.

Chemical Structure 2-Hydroxybenzoic acid methyl ester; (C₈H₈O₃)

Occurrence It is largely found in a variety of medicinal plants, namely: Flowers of Acacia farnesiana (L.) Willd. (Family: Fabaceae)-Cassie, Huisache; Cananga odorata (Lam.) Hook. f. & Thoms (Family: Annonaceae)-Cananga, Ylang-Ylang; Leaves of Chenopodium ambrosioides L. (Family: Chenopodiaceae)-Wormseed; Erythroxylum coca Lam. (Family: Erythroxylaceae)-Coca; Flowerbuds of Filipendula ulmaria (L.) Maxim (Family: Rosaceae)-Meadowsweet, Queen of the Meadow; Twigs of Gaultheria procumbens L. (Family: Ericaceae)-Wintergreen, Teaberry, Boxberry; Bark of Betula lenta L. (Family: Betulaceae)-Sweet Birch. However, oil of wintergreen contains upto 99% methyl salicylate. It is pertinent to mention here that in several aromatic medicinal plants, for instance: Wintergreen, the active chemical constituent i.e., methyl salicylate does not occur as such, but is present in the form of a glucoside known as Gaultherin which upon enzymatic hydrolysis gives methyl salicylate and primeverose (glucoxylose) as shown under:

Isolation Methyl salicylate may be obtained from gaultherin by enzymatic hydrolysis and then subjecting the resulting products of reaction to very low temperature when the former gets solidified at – 8.6°C and hence may be separated easily.

Characteristic Features It is a colourless, yellowish or reddish, oily liquid. Its odour and taste resembles to that of gaultheria. Its physical parameters are: mp – 8.6°C; bp 220-224°C; d²⁵₂₅ 1.184; d of the natural ester is ~ 1.180; and n²⁰_D 1.535-1.538. It is very sparingly soluble in water (1 g in 1500 ml), but freely soluble in chloroform and ether. It is, however, miscible with alcohol and glacial acetic acid.

Identifications

1. It develops a red-violet colouration on being treated with cold saturated aqueous solution of FeCl₃, that lasts for about 15 minutes.

2. Methyl salicylate readily forms a soluble ester-salt with a moderately concentrated aqueous solution of KOH as potassium methyl salicylate.

3. Upon saponification the ester yields salicylic acid (mp 158°C) and methanol respectively.

4. Methyl salicylate may also be identified by the formation of several derivatives as stated below:

(i) Methylo-acetoxy benzoate (mp 52-52.5°C)—with Acetic Anhydride,

(ii) Methyl-o-benzoxy benzoate (mp 92°C)—with Benzoyl Chloride,

(iii) o-Cabomethoxyphenyl-N-phenyl urethane—with Phenylisocyanate.

Uses

1. Methyl salicylate has local irritant, antirheumatic and antiseptic properties. It is an important ingredient of Iodex(R) ointment for relief of pain in several conditions like, pulled muscle, muscular pain, pain in joints, etc.

2. It also finds its extensive usage in a variety of products, such as: flavouring of food products, beverages, candies, confectionery, toothpastes, mouth washes, gargles, and pharmaceutical preparations.

3. It is also used in perfumery.

2.6.5.8.3 Esters Containing Nitrogen The specific example of an ester containing nitrogen is

Methyl Anthranilate which is present in several volatile oils. It is described below:

A. Methyl Anthranilate

Synonyms Neroli Oil (Artificial).

Chemical Structure 2-Aminobenzoic acid methyl ester, (C₈H₉NO₂).

17 ml of distilled water. The resulting methyl anthranilate sulphate thus obtained gets crystallized in the cold, which may be further purified by recrystallization from alcohol. Finally the pure desired ester is regenerated by treatment with dilute NaOH solution (2 N) carefully.

Characteristic Features It is a crystalline mass having a powerful pleasant taste. It has a peculiar odour that mostly resembles to orange blossoms and certain varieties of grape. It gives an inherent blue-violet fluorescence which is distinctly visible in any volatile oil containing it. Methyl anthranilate has the following physical parameters, namely: d 1.168; mp 24-25°C; bp₁₅ 135.5°C.

It is slightly soluble in water, but freely soluble in ethanol and ether.

Identification It may be identified by preparing its derivatives, such as: Picrate (mp 104°C); Benzoate (mp 100°C).

Uses

1. It is used frequently as a perfume for ointments.

2. It is also employed for the manufacture of synthetic perfumes.

Occurrence It occurs in a good number of medicinal herbs, for instance: Flowers of Robinia pseudoacacia L. (Family: Fabaceae)-Black Locust, False Acacia; Citrus sinensis (Linn.) Osbeck. (Family: Rutaceae)-Sweet Orange; Cananga odorata (Lam.) Hook. f. & Thoms. (Family:

Annonaceae)-Ylang-Ylang, Cananga; Jasminum officinale Linn. var grandiflorum Bailey., (Family: Oleaceae)-Jasmine. It is also found in bergamot, other essential oils and in grape juice. It is also obtained synthetically by carrying out the esterification of anthranilic acid with methanol in the presence of HCl.

Isolation Methyl anthranilate may be isolated from the essential oils very conveniently by shaking the volatile oil with cold dilute sulphuic acid (2 N) i.e., 1 ml of conc. H₂SO₄ dissolved slowly in

17 ml of distilled water. The resulting methyl anthranilate sulphate thus obtained gets crystallized in the cold, which may be further purified by recrystallization from alcohol. Finally the pure desired ester is regenerated by treatment with dilute NaOH solution (2 N) carefully.

Characteristic Features It is a crystalline mass having a powerful pleasant taste. It has a peculiar odour that mostly resembles to orange blossoms and certain varieties of grape. It gives an inherent blue-violet fluorescence which is distinctly visible in any volatile oil containing it. Methyl anthranilate has the following physical parameters, namely: d 1.168; mp 24-25°C; bp₁₅ 135.5°C.

It is slightly soluble in water, but freely soluble in ethanol and ether.

Identification It may be identified by preparing its derivatives, such as: Picrate (mp 104°C); Benzoate (mp 100°C).

Uses

1. It is used frequently as a perfume for ointments.

2. It is also employed for the manufacture of synthetic perfumes.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar