The
Official Volatile Oils. BP 1958
Compiled by Ivor Hughes
Part 2 of 2.
OIL OF ROSEMARY
Geographical Source: S. France, and the islands off the Dalmatian coast.
Preparation: By distillation in steam.
Constituents: Borneol, 8 -18 %, official requirement being not less than 9-0 per cent w/w. Bornyl Acetate, 1-6 %, the official requirement being not less than 2-0 per cent w/w. Cineole, limited by an official test to an apparent content of 33-0 % Subsidiary constituents are the terpenes pinene and camphene, and camphor.
Adulteration: Both French and Dalmatian oil normally satisfy the official requirements. The chief substitute is an impure or commercial oil produced in large amounts in Spain by distilling together rosemary, spike lavender, and a high proportion of sage�spike lavender yielding an oil rich in borneol, and sage yielding an oil with an ester content of 6-20 per cent. Hence Spanish oil of Rosemary contains alcohols (chiefly borneol) and esters, and usually fulfils the official requirements. The optical rotation of oil of rosemary is officially - 5� to + 10�, that of oil of sage + 70� to + 24�, hence Spanish oil of rosemary usually has an optical rotation higher than that of genuine oil; this, with other factors, serves to detect substitution. Oil of camphor has also been reported as an adulterant; it is detected by the limit test for cineole.
OIL OF SANDALWOOD
(East-Indian Sandalwood Oil)Botanical Source: The heartwood of Santalum album Linn. Fam.: Santalaceae.
Geographical Source: India (principally Mysore).
Preparation: By slow distillation in steam or with water, the process taking a considerable time for completion owing to the density of the wood; yield 2-5 per cent.
Constituents: Santalol sesquiterpene alcohols, with other alcohols totaling 92-98 per cent, the B.P.C. requirement being not less than 90 per cent w/w. Subsidiary constituents are the terpenes, santalene, and esters, chiefly santalyl acetate, about 2 per cent.
Adulteration: The standard of not less than 90 per cent of alcohols calculated as santalol ensures rejection of grossly adulterated oil. The principal adulterant is castor oil, which is not easily detected in small proportions, as its solubility in alcohol and its viscosity are similar to those of sandalwood oil.
Other adulterants: sometimes, oil of cedar wood, oil of copaiba, benzyl alcohol, and glyceryl acetate.
4. Oils containing Aldehydes.
|
Name |
Aldehyde |
Av. Content |
Official Requirements |
|
Oleum Cinnamomi |
Cinnamic aldehyde |
58 � 70% |
55 � 70% w/w BP1953 |
|
Oleum Limonis |
Citral |
4 � 5.5% |
4% w/w BP 1953 |
|
Oleum AmygdalaeVolatile Purificata |
Benzaldehyde |
95 � 98% |
95% w/w or more. BP 1958 |
Principle of Estimation
C9H15CHO + H2NOH,
HC1 --� C9H15CHNOH + HC1 + H2O
Citral
Hydroxylamine Citraldoxim
hydrochloride
A known weight of oil is shaken in a stoppered tube with a known excess of an alcoholic solution of hydroxylamine hydrochloride which has been rendered first yellow
* to methyl orange. The above reaction occurs partially with liberation of hydrochloric acid, and change in colour to red. Semi-normal solution of potassium hydroxide is added to change the colour to yellow. Upon further shaking, more aldehyde combines and the red colour re-appears and is again discharged. This shaking and neutralization is repeated until the yellow colour of the lower layer remains unchanged after 2 minutes of vigorous shaking, followed by separation of the oil, indicating that all the aldehyde has combined. The total volume of semi-normal solution of potassium hydroxide needed for the above processes is noted and gives an approximate value for the aldehyde. The test is repeated using as the standard for the end-point the reaction mixture obtained previously, to which a defined excess of semi-normal solution of potassium hydroxide has been added. The result is calculated from the second determination (vide B.P., Appendix XI).* The solution contains, as a result, a small proportion of free hydroxylamine. The latter is a weak base, the pH of the hydrochloride being 3-2. The orange colour range of methyl orange lies between pH 2-9-4-0, hence if a neutral solution of hydroxylamine hydrochloride were used in the test, adjustment to the particular shade of orange corresponding to pH 3-2 would be necessary. This is difficult, hence a constant error is introduced by starting with a solution of hydroxylamine hydrochloride at about pH 9-0 (the pale yellow of methyl orange) and finishing at the same point�in this way an accurate end-point is obtained without affecting the result.
OIL OF CINNAMON
Preparation : By distillation in steam; yield about 0-5-1-0 per cent.
Constituents : Cinnamic Aldehyde, 58-76 per cent, the official requirement being 55-0-70-0 per cent w/w.
Eugenol, 4-8 per cent. Subsidiary constituents are the terpenes cymene and caryophyllene
Tests and Adulteration : The two most important chemical tests are �
1. Determination of aldehyde (vide B.P., Appendix XI).
2. Limit-test for Eugenol. This test is designed to detect adulteration of oil of cinnamon with cinnamon leaf oil or cassia oil: the test is �
0.l ml. dissolved in 10 ml. of alcohol (90 per cent) assumes a slight green, but not a deep brown or blue coloration, on the addition of 0-1 ml. of test-solution of ferric chloride. Eugenol alone, and also cinnamon leaf oil which is rich in it, gives a blue colour with ferric chloride; cassia oil and its principal constituent cinnamic aldehyde gives a brown colour; the official oil of cinnamon, which contains both, produces a pale green colour, the test being, in effect, a limit-test for eugenol.
The high price of the oil has led to considerable adulteration and
substitution, which chemical examination may fail to detect. The two
principal adulterants are �
(a) Artificial cinnamic aldehyde, to give the correct content of this.
(b) Cinnamon leaf oil, which consists largely of eugenol, and is therefore quite different in composition from the official oil prepared from the bark. The latter contains from 4 to 8 per cent only of eugenol, which, in the factitious oil, is supplied by a suitable proportion of cinnamon leaf oil.
(c) Genuine oil of cinnamon to modify the odour of the above two constituents, and thereby make the product simulate genuine oil.
This factitious oil answers the official quantitative test for cinnamic aldehyde, and also the above qualitative test for limit of eugenol.
2. Cassia oil, obtained from the bark of Cinnamomum Cassia Blume, which contains about 75�90 per cent of cinnamic aldehyde, but no eugenol, hence the colour produced with test-solution of ferric chloride is brown. The adulteration of oil of cinnamon with a large proportion of oil of cassia would be revealed by an abnormally high content of cinnamic aldehyde, and by the failure to give the characteristic pale green colour in the qualitative test. The addition of 20 per cent or less of oil of cassia cannot usually be detected by either test, as the cinnamic aldehyde is not necessarily outside the official range, and with the qualitative test the intensity of the green colour, though reduced, is not masked.
From the above it will be seen that a factitious oil, or genuine oil containing less than 20 per cent of oil of cassia may fulfill the official chemical requirements, and considerable reliance must be placed on odour. The official oil contains a small proportion of substances not present in either the factitious oil or oil of cassia, and these substances modify the odour of the cinnamic aldehyde and eugenol, rendering it distinctively delicate by comparison.
OIL OF LEMON
Preparation : By expression, yield about 0-8 gm. per lemon.
Constituents : Citral � 4-5-5-0 per cent, the official requirement being not less than 4-0 per cent of aldehydes calculated as citral. Traces of other aldehydes, e.g. octyl aldehyde, monyl aldehyde, and esters, e.g. geranyl and linalyl acetates, are also present, and no doubt slightly modify the odour. Subsidiary constituents are the terpenes d-limonene (citrene) 80 per cent, accompanied by a small proportion of l-limonene and sesquiterpenes. Machine-made oil is usually inferior in odour and taste and has a lower citral content and an appreciable proportion of resinous substances.
Tests : The principal test is the estimation of the aldehydes.
Mention has been made of terpeneless oils, of which terpeneless oil of lemon is the principal, being produced in large amounts. The terpenes separated in this process play an important part in the adulteration of oil of lemon. As obtained from lemons, the oil contains upwards of 5-5 %
* of aldehydes, and one method of adulteration commonly practiced is to dilute the oil to a 4 per cent content (the B.P. minimum) with these terpenes. The product still fulfils all the official requirements, and detection is impossible because the diluent is a normal constituent of oil of lemon.*
From winter-collected lemons; summer-collected lemons yield less.Another method of adulteration is the admixture of citral from other sources, e.g. oil of lemon grass with the above-mentioned lemon terpenes, to give a product containing 4 per cent of citral. An admixture of this factitious oil with genuine oil of lemon is very difficult to detect because the chemical and physical constants are normal, and odour is the sole method of detection � oil of lemon grass giving a distinctive odour to the admixture.
PURIFIED VOLATILE OIL OF BITTER ALMOND
Fam.: Rosaceae.
Geographical Source : Southern France, Sicily, and Northern Africa.
Preparation : This oil does not pre-exist in the seeds. It is prepared by crushing the seeds and freeing them from fixed oil, e.g. Sweet Oil of Almond, of which they contain about 40 per cent. The residual cake is re-crushed, mixed with water, enzyme action allowed to proceed for some hours at about 40� C., and the mixture then distilled. The interacting substances are the glycoside amygdaline and the enzyme emulsin, the latter causing hydrolysis of the former as follows �
Amygdalin + Water ��> Benzaldehyde + Hydrocyanic acid
+ Dextrose
(Emulsin)
Benzaldehyde and hydrocyanic acid are both volatile, and the oil which separates from the aqueous portion of the distillate consists chiefly of benzaldehyde, with about 2-4 per cent of hydrocyanic acid, of which part is free and part combined with the benzaldehyde in the form of an additive compound called benzaldehyde-cyanhydrin. The proportion of hydrocyanic acid present renders this oil
very poisonous, hence prior to sale for pharmaceutical or domestic purposes the acid is removed as follows �The oil is mixed with milk of lime, whereby the hydrocyanic acid, both
free and combined, is converted into calcium cyanide. Ferrous sulphate is
next added, and this converts the calcium cyanide into calcium ferrocyanide
� a compound not decomposed in the subsequent distillation. The mixture is
then distilled in a current of steam, and the oil which separates from the
aqueous portion of the distillate consists almost entirely of benzaldehyde,
and is known as Oleum Amygdalae Essentiale sine Acido Prussico. On account
of its high cost it has been largely replaced by synthetic benzaldehyde. The
yield is 0-5-1-0 per cent.
Constituents :
5. Oils containing Ketones.
|
Name |
Ketone |
Av. Content |
Official Requirement |
|
Oleum Cari |
Carvone |
50 � 60% |
53 -63% w/w B.P.C |
|
Oleum Anethi |
Carvone |
35 � 60% |
43 � 63% w/w B.P.C |
Principle of Estimation: Ketones, like aldehydes, combine with hydroxylamine, and form ketox-imes. The method of estimating ketones follows therefore, in outline, that described for aldehydes.
OIL OF CARAWAY
Preparation : By distillation in steam ; yield 4-6 per cent.
Constituents : Carvone, 50-60 percent, the B.P.C. standard being 53-0-63-0 per cent w/w. d-Limonene (carvene), constitutes the remainder of the oil.
OIL OF DILL
Preparation : By distillation in steam; yield 3-4 per cent.
Constituents : Carvone, 35-60 per cent, the official standard being 43-63 per cent w/w. d-Limonene is the principal terpene present, phellandrene and others being present in small proportion.
Adulteration : Two other varieties of oil of dill occur in commerce. One, oil of Indian dill, from Anethum Sowa Roxb. (Peucedanum Sowa Kurz), has a different composition containing dill-apiol. The sp. gr. of oil of Indian dill is 0-946�0970, whereas that of oil of dill is 0-900-0-915, hence the addition of a proportion of the former oil to the latter will raise the sp. gr. and give prima facie evidence of adulteration, probably with oil of Indian dill. The other variety is oil of dill of Spanish origin. This oil contains less carvone than the official oil, and substitution would be revealed by the lowered sp. gr., and by estimation of the carvone.
6. Oils containing Phenols
Principle of Estimation. Estimation is based on the fact that phenols combine with caustic alkalis to form water-soluble compounds. Hence the difference in volume between the oil used and that remaining uncombined represents the amount of phenol present in the portion tested. The estimation is carried out in a special flask, called a Hirschsohn or Cassia flask, which is stoppered and has a long neck graduated like a burette. The oil and alkali are shaken thoroughly at intervals for a prescribed period, and the uncombined oil is then raised to the graduated neck by the addition of more alkali. After standing for 24 hours or more for complete separation to ensue, the volume of uncombined oil is read off. The percentage v/v of phenols is calculated from the data obtained.
OIL OF CLOVE
Official
Source: Madagascar, Zanzibar, Pemba,
Penang.
Preparation : By distillation in steam; yield about 15 per cent.
The oil can be fractionated into the two main constituents, eugenol (sp. gr. 1-072) and caryophyllene (sp. gr. 0-9085). In commercial distillation, however, the whole of the oil sinks to the bottom of the receiver. The first runnings of oil distilled from the buds possess quite a distinct character and are used in the perfumery trade.
Constituents : Eugenol, 76-90 per cent, the official requirement being 85-0-90-0 per cent v/v, determined as described above.
The remainder of the oil consists almost entirely of the terpene caryophyllene. Other constituents include furfuraldehyde (the provable cause of darkening on storage), methylamylketone and acetyleugenol.
7. Oils containing Oxides
|
Name |
Main Constituent |
Av. Content |
Official Requirement |
|
Oleum Cajuputi |
Cineole |
45 � 55% |
50 � 65% w/w B.C.P. |
|
Oleum Eucalypti |
Cineole |
Upwards 80% |
Min 70% w/w |
Principle of Estimation. The estimation is based on the fact that cineole combines with o-cresol to form a solid compound melting at 55-2� C. The other constituents lower the melting point according to the proportion present.
A prescribed weight of dry oil and pure dry o-cresol are mixed in a test tube, the contents warmed gently to melt the mixture, and the freezing point determined under prescribed conditions. The freezing point is re-determined by re-melting and cooling until two consecutive concordant results are obtained. The proportion of cineole which this represents is then found from a table (B.P., Appendix XI).
OIL OF CAJUPUT
Geographical Source : Malay Archipelago (Molucca Islands).
Preparation : By distillation in steam, followed by rectification.
Constituents : Cineole, 45-55 per cent, the official requirement being 50-65 per cent w/w, determined by the process described above.
Subsidiary constituents are the terpene Z-pinene, and certain aldehydes.
OIL OF EUCALYPTUS
Geographical Source : Tasmania, Eastern Australia, and, to a small extent, Southern Europe.
Preparation : By distillation in steam, followed by rectification; yield 1-3 per cent. The foliage of the undergrowth produces the best oil. The Australian season for cutting the leaves and distilling extends from January to June.
Constituents: Cineole, 50-80 per cent, the official requirement being not less than 70-0 per cent w/w, determined as described above.
Tests : Limit of Phellandrene. There are many species of Eucalyptus, most of which yield an essential oil. The principal species used are E. polybractea and E. Smithii. Both yield an oil rich in cineole, averaging about 80 per cent in E. polybractea. Official standardization at not less than 70 per cent w/w of cineole has reduced admixture of oils rich in cineole with others containing but little. Certain of the latter, e.g. the oils yielded by E. Amygdalina, E. Baileyana, are not only poor in cineole but contain a considerable proportion of the sesquiterpene phellandrene, which is objectionable for medicinal use, and is therefore limited in the official specification. The limit-test is based on the combination of phellandrene with nitrous acid to form a crystalline compound insoluble in petroleum spirit. The test is carried out in the following manner.
"Mix 1 ml. of oil, 2 ml. of glacial acetic acid, 5 ml. of light petroleum, and 2 ml. of saturated solution of sodium nitrite, and shake gently." The formation of a crystalline precipitate in the upper layer indicates the presence of oils containing an undue proportion of phellandrene.
Limit of Aldehydes. Eucalyptus maculata var. citriodora Hook, yields an oil containing 84-90 per cent of the aldehyde citronellal. The latter has a lemon-like odour and, the above oil is therefore known as "lemon-scented" eucalyptus oil. The official limit test for aldehydes limits the use of this oil, and others containing citronellal or other aldehydes, for the purpose of diluting oils rich in cineole to the official standard. The test is carried out as for aldehydes in volatile oils.
8. Oils containing Peroxides
Principle of Estimation : Estimation is based on the fact that peroxides liberate iodine quantitatively from an acidified solution of potassium iodide, and the iodine so formed can be ascertained by titration with standard solution of sodium thiosulphate. In the case of ascaridole, the estimation is complicated by a number of factors and the official details (B.P. 1953) must be rigidly followed in order to obtain concordant results. The latter are calculated from an empirical factor based on the results of a large number of determinations of pure ascaridole. (T. T. Cocking and F. C. Hymans, Analyst, 1930, 55, 180.)
OIL OF CHENOPODIUM
Botanical Source : The fresh flowering and fruiting plants, excluding roots, of Chenopodium ambrosioides Luin var. anthelminticum Gray. Pam.: Chenopodiaceae.
Geographical Source : U.S.A., Central America, and West Indies.
Preparation : By distillation in steam. The process must be carried out rapidly, using special plant, since ascaridole is decomposed by slow distillation.
Constituents : Ascaridole, not less than 65-0 per cent w/w. (B.P. 1953.) Subsidiary constituents are the terpenes, cymene and terpinene. Due to the fact that ascaridole is a peroxide, the oil is a powerful oxidizing agent and therefore reacts, often very violently, with reducing agents. The oil explodes on heating.
9. Oils with Miscellaneous Constituents.
OIL OF NUTMEG
Preparation : By distillation in steam, broken and damaged nutmegs being used; yield 8-15 per cent.
Constituents : d-Camphene, up to 80 per cent, and other terpenes, including d-pinene and dipentene.
Alcohols* including linalol, terpineol, geraniol, and borneol, 6 per cent.
Phenols including safrole, eugenol, and iso-eugenol, 0-8 per cent.
Myristicin (a methoxy derivative of safrole), 4 per cent, responsible for the toxic effects of large doses of the oil.
* According to Power and Salway.
Official Fractions of Volatile Oils.
CAMPHOR
Natural Camphor.
Official Source : The tree, Cinnamomum Camphora (Linn.) Nees and
Eberm. Fam.: Lauraceae.
Geographical Source : Formosa (produces about 75 per cent of the
world's requirements). China (provinces on the straits of Formosa)
producing the remainder.
Camphor
production is a large industry in these areas; the quantity used in
medicine, although important, represents only a small fraction of the
output, a considerably larger quantity being used in the production of
celluloid. In Formosa replanting has been carried out to ensure continuous
and increasing supplies. In China, replanting has been neglected, with
consequent diminution of yield. Most parts of the tree contain
oil-cells in which volatile oil is secreted, averaging 3 to 6 per cent. This
oil contains 10-50 per cent of camphor, the highest proportion being found
in the oil contained in the older parts of the tree, e.g. the root and
trunk. It has been stated (T. Yahagi, Jap. J. Ghem., 1928, 3, 109) that the
formation of camphor is due to the action of an enzyme, possibly a
peroxidase, on certain cell-constituents, and that enzyme activity is
greatest in the actively-growing parts, notably in the layer of wood tissue
just within the cambium. Hence each zone of wood, as formed, becomes
rich in camphor which remains there as growth proceeds ; consequently, the
woody tissue of root and trunk from mature trees becomes the principal
camphor-containing part of the tree, and in Formosa distillation is mainly
from the wood of trees 40 to 50 years old. The average yield of camphor is
about 5 kilogram. per tree.
COMPOSITION OF NATURAL OIL OF CAMPHOR
The natural oil obtained from the camphor tree contains a large number
of compounds. This oil may be separated by fractional distillation into
three main fractions;
1. A Light Fraction. This is collected up to about 200� C. It consists chiefly of terpenes together with cineole. This oil is known as light oil of camphor and sometimes by the misleading name of essential oil of camphor.
2. A Middle Fraction. This is collected from about 200 to 230� C. It consists chiefly of camphor.
3. A Heavy Fraction. This is collected from about 230� C. It contains a high proportion of safrol. Upon refrigeration, safrole crystallizes out, the other substances remaining liquid. This liquid is drained off and the crystalline mass pressed. The latter is then allowed to liquefy, and purified by re-refrigeration. The product is almost pure safrole. The latter has almost entirely replaced Oil of Sassafras, and is also used for the manufacture of heliotropin, a substance having the odour of heliotrope and used in perfumery. The natural oil usually contains sufficient camphor to render it semi-solid at ordinary temperatures. When pressed, the liquid portion can be expelled leaving behind a solid mass consisting of crude camphor.
Preparation : The trunk and branches of the felled tree are cut into chips which are placed on the perforated false bottom of a still, connected by a pipe to a water-cooled receiver. Beneath the false bottom water is heated, or steam passed in, whereupon the oil is volatilized, passing with the steam into the condenser. If the whole of the oil is collected without interruption, a semi-solid product is obtained, from which the liquid portion is removed by draining and pressing, leaving behind crude camphor. Alternatively, the receiver may be changed when most of the terpenes have distilled over, and the crude camphor collected separately. These processes are carried out in the area of collection, and the crude camphor is then taken to refineries for purification. In one process of purification, the crude camphor is mixed with lime (to absorb the water present) and sand, and then sublimed in large vats, each fitted with a cone-shaped lid, provided with a flat, partial diaphragm. Upon heating, the sublimate of pure camphor collects on the lid in blocks, which are then cut for export into slabs weighing about 2 lb. In the other purification process the still is connected to very large cooled receivers. The hot camphor vapour condenses in the enclosed atmosphere in the form of small detached crystals (camphor flowers), which collect on the floor of the receiver.
Synthetic Camphor.
Camphor may be synthesized from pinene, the terpene which forms the bulk
of oil of turpentine. The pinene is converted through bornyl chloride to the
alcohol borneol and this is oxidized to the ketone camphor. Synthetic
camphor is optically inactive, being a mixture of equal amounts of
the laevo- and dextro-compounds; natural camphor consists only of the latter
and is therefore optically active. Pure synthetic camphor is identical in
appearance with natural camphor but is more waxy to the touch. Moreover the
synthetic material is liable to contain as impurity a small proportion of
iso-borneol due to the last stage in the synthesis not going to completion
and the difficulty of removing all unchanged iso-borneol on account of the
similarity in certain properties.
MENTHOL
Geographical Source : Brazil, Japan, principally the northern island, Hokkaido. China produces a relatively small amount.
The official oil of peppermint, from M. piperita grown in the U.S.A. and England is but little used for the preparation of menthol. The latter is obtained from Japanese peppermint oil yielded by M. arvensis Linn. var. piperascens Holmes, Chinese peppermint oil obtained from M. arvensis Linn. var. glabrata, and Brazilian peppermint oil. The herbs yield about 0-3 per cent of volatile oil of which the Japanese is the richest in menthol; it contains 70 � 90 per cent, the oil being a crystalline mass at room temperature due to crystallization of the menthol.
Preparation : The oil is steam-distilled from the cut herb after it has partially withered a condition found by experience to produce the highest yield. Upon freezing the oil, rather more than half of the menthol separates as crystals, and these are drained from the liquid portion of the oil, and then re-crystallized from alcohol.
The separated liquid portion is known as Japanese dementholized peppermint oil, and still contains 40-54 per cent of menthol, and 5-17 per cent of esters of menthol. By boiling with solution of sodium hydroxide these esters may be decomposed to form menthol, and, upon further freezing, the saponified oil yields a second crop of menthol crystals. However, most of the oil is exported without further treatment, to be used in the manufacture of confectionery and cordials.
Synthetic Menthol.
The molecule of menthol contains three asymmetric carbon atoms and
consequently various isomeric forms are possible. Natural menthol is
laevo-rotatary and the Pharmacopoeia also recognizes synthetic menthol in
its laevo and racemic forms.
THYMOL
Natural Thymol.
Official Sources : The volatile oils of the following plants :
|
Plant Name |
Part |
Source |
Oil Yeild |
Thymol content |
|
Trachyspermum Ammi * Linn Sprague. Fam. Umbelliferae |
Fruits |
India, Seychelles, Montserrat |
3-4% |
45-55% |
|
Monarda punctata Linn. Fam. Labiatae |
Leaves |
U.S.A., Montserrat |
0-3-0-4% |
60-70% |
|
Thymus vulgaris Linn. Fam. Labiatae |
Flowering plant |
Spain and France |
0-4-0-5% |
30-40% |
* Yielding Indian Ajowan Seed, formerly important but now little cultivated.
It has also been prepared from the stem and leaves of Ocimum gratissimum (Fam. Labiatae), grown in India, Ceylon and Java. This source yields about 0-5 per cent of oil containing about 55 per cent of thymol.
Preparation : After separation by steam-distillation, the oil is subjected to fractional distillation in order to separate the bulk of the hydrocarbons (low boiling-point fraction), and thereby raise the content of thymol. The hydrocarbon-free oil is then shaken with solution of sodium hydroxide, with which the thymol combines to form a water-soluble compound called sodium thymate � analogous to sodium phenate. Upon standing, the mixture resolves into two layers, an oily layer now free from thymol, and an aqueous layer containing sodium thymate. The latter is separated and acidified with hydrochloric acid, which decomposes the sodium thymate, re-forming thymol. The latter is practically insoluble in water and separates as an oily liquid � the presence of impurities preventing crystallization. Crystallization is induced by sowing a few crystals of thymol in the liquid. The product thereby obtained is next dissolved in the minimum of alcohol, the solution decolorized by filtration through animal charcoal, and after dilution with water it is set aside, whereupon thymol crystallizes out in the form of large rhombic prisms.
Synthetic Thymol.
Thymol is chemically 3-methyl-6-isopropylphenol and is prepared by the
oxidation of piperitone, derived from Javan Citronella Oil or from
Australian Eucalyptus dives oil. Other
synthetic methods start from p-cymene (derived from turpentine) or m-cresol
(a coal tar product) which is condensed with iso-propyl alcohol or propylene
in the presence of phosphoric acid.
EUCALYPTOL
Official Source : As for Oil of Eucalyptus,
Preparation : Preparation is usually based on the fact that at low temperatures eucalyptol combines with phosphoric acid to form a solid additive compound from which the non-eucalyptol fluid portion of the oil can be almost completely separated by pressure. Subsequent treatment of the solid compound with warm water resolves it into phosphoric acid and eucalyptol, the latter separating as an oily layer. The latter is then purified.
The Pharmacopoeia requires eucalyptol to have a freezing-point not lower than 0�.
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