Menthol (U. S. P.)—Menthol.
Preparations: Menthol Plaster
Related entries: Mentha Piperita (U. S. P.)—Peppermint - Oleum Menthae Piperitae (U. S. P.)—Oil of Peppermint - Mentha Viridis (U. S. P.)—Spearmint - Oleum Menthae Viridis (U. S. P.)—Oil of Spearmint
FORMULA: C10H19OH. MOLECULAR WEIGHT: 155.66.
"A stearopten (having the character of a secondary alcohol), obtained from the official oil of peppermint (from Mentha piperita, Smith), or from Japanese or Chinese oil of peppermint (from Mentha arvensis, Linné, var. piperascens, Holmes, and Mentha canadensis, Linne, var. glabrata, Holmes. Nat. Ord.—Labiatae). Menthol should be kept in well-stoppered bottles, in a cool place"—(U. S. P.).
Source and Preparation.—Menthol is the solid constituent of oil of peppermint, to which its characteristic odor is due, and was formerly known as peppermint camphor. It occurs in largest quantity in the oils of peppermint known as the Chinese and Japanese oils. Our home-grown peppermint (Mentha piperita) yields about 20 per cent of an excellent article of menthol, of a pure mint odor, and known in trade as pip-menthol. It forms handsome, dry crystals, and was first prepared in this country by Mr. A. M. Todd, of Nottawa, Michigan. Analysis by the late Prof. Henry Trimble (Amer. Jour. Pharm., 1884, p. 405) shows it to conform to the above formula more closely than Japanese menthol, which still contained traces of liquid hydrocarbons and also showed a slightly reduced melting point on this account. According to Mr. Todd (Amer. Jour. Pharm., 1886, p. 159), menthol is obtained by subjecting the distilled oil of peppermint to a temperature of -22° C, (-7.6° F.), by the aid of a freezing mixture. The menthol crystallizes out in satiny crystals, and the mother liquor is removed while the low temperature is maintained. In rare instances, the distillate congeals at -4.4° C. (24° F.).
Description.—The U. S. P. describes menthol as forming "colorless, acicular or prismatic crystals, having a strong and pure odor of peppermint, and a warm, aromatic taste, followed by a sensation of cold when air is drawn into the mouth. Menthol is only slightly soluble in water, but imparts to the latter its odor and taste. It is freely soluble in alcohol, ether, chloroform, carbon disulphide, or glacial acetic acid. It melts at 43° C. (109.4° F.) to a colorless liquid, boils at 212° C. (413.6° F.), and volatilizes slowly at the ordinary temperature. When it is triturated with about an equal weight of camphor, thymol, or chloral hydrate, the mixture becomes liquid. Its alcoholic solution is neutral to litmus paper, and deviates polarized light to the left"—(U. S. P.). Chemically, menthol is a secondary alcohol of the formula C6H3(OH)CH3C3H7.H6, or hexa-hydro-oxy-para-cymene, yielding, upon oxidation, a mobile, dextro-rotatory fluid (menthone, C10H18O), which is a ketone, and bears the same relation to menthol as Japan camphor does to borneol (see Camphora). Reversedly, menthol may be obtained from menthone by reduction with metallic sodium. The hydrocarbon (menthene, C10H18) is obtained by heating menthol with zinc chloride, or other dehydrating agents.
Tests.—"If a little menthol be heated in all open capsule, on a water-bath, it should gradually volatilize without leaving any residue (absence of wax, paraffin, or inorganic substances). If a few crystals of menthol be dissolved in 1 Cc. of glacial acetic acid, and then 3 drops of sulphuric acid and 1 drop of nitric acid added, no green color should be produced (absence of thymol)"—(U. S. P.). Cones or pencils of menthol, which are so extensively sold as a remedy for headache, neuralgia, etc., are usually largely mixed with wax, paraffin, or other foreign admixture, the presence of which is recognized by the above test.
Mr. L. F. Kebler (Amer. Jour. Pharm., 1897, p. 189) recommends the quantitative determination of menthol by a modification of the process given by Prof. Power and Dr. Kleber in Pharm. Rundschau, 1894, p. 157. According to this process, the combined menthol—i.e., occurring in the form of esters—is first determined by saponification of the oil with a known amount of caustic soda solution. The total menthol is then determined by forming an acetyl ester of menthol, and saponifying the product by a known excess of caustic soda. The amount of free menthol is then represented by the difference between the total and the combined menthol. (For the details of this process, consult the interesting papers quoted.)
Action, Medical Uses, and Dosage.—Menthol possesses the general properties of peppermint oil somewhat intensified. It undoubtedly has active germicidal and pronounced tonic properties. It exerts considerable anaesthetic power over parts to which it is applied, and is frequently added to ether, chloroform, etc., when they are to be used in spray for the relief of local pains. Paralysis of the spinal nerve centers and nerves, involving a loss of both sensation and motion, has resulted from large doses of menthol. When applied in solid form to the skin or mucous tissues, it first produced a sharp, burning sensation, soon followed by a sense of coldness with decreased sensibility of the parts. Ɣ It forms a good application to itching surfaces, and in pruritis vulvae a weak solution in olive oil has given us excellent satisfaction. It is largely used for itching of the surface, cellular inflammations, and local pains—such as occur in burns, scalds, insect bites and stings, eczema, ringworm, urticaria, toothache, earache, neuralgia, sciatica, boils, carbuncles, etc. In boils of the external auditory canal, a 20 per cent solution in oil may be applied on cotton, renewing every 24 hours. A petrolatum ointment of menthol (20 per cent) has been recommended as a stimulating agent where there is a lack of secretion of cerumen. Ɣ Coryza and hay fever are frequently treated with the vapor or the direct application of a 10 to 20 per cent solution to the nostrils, and a solution of the latter strength has been recommended where thickening of the membranes give rise to obstruction or stenosis of the Eustachian tubes. Ozoena, catarrhal sore throat, and chronic inflammation of the pharynx are treated with the solution, while it has been used by atomization with success in pertussis, asthma, and bronchitis of a chronic type. Its vapor relieves irritable bronchial cough. Extraordinary claims regarding its effects in nebulized spray, have been advanced concerning its retarding influence and even arresting power over both pulmonary and laryngeal phthisis. These claims, however, require substantiation. Small doses sometimes allay vomiting, as of pregnancy, etc. The dose of menthol for internal use is from 1/8 to 2 grains in pill, capsule, trituration, or olive oil; for local uses, ointments, liniments, and solutions are employed in strength ranging from 5 to 20 per cent.
Specific Indications and Uses.—Topically, to pruritic surfaces.
Related Product.—MENTHIODOL. Heat carefully in a capsule menthol, 4 parts, and add iodol, in fine powder, 1 part, and triturate until a uniform mass is obtained. Mold into suitable pencils or cones. If too hard, soften by remelting with a small quantity of gum camphor. A local application for neuralgia.