The aqueous distillate obtained as a byproduct in the preparation of volatile oils occasionally contains free fatty acids such as acetic, propionic, butyric or valeric acids. Like the methyl and ethyl alcohol obtained in like manner, these acids are in all probability decomposition products of esters contained in the parts of plants subjected to distillation. In as much as the lower fatty acids remain dissolved in the aqueous distillates they are readily overlooked. In certain cases the amounts are not inappreciable. Thus e. g. in a distillation of 40 kg. of seeds of Heracleum giganteum 120 g. of oil were obtained and 30 g. of acetic acid were separated from the aqueous distillate.1)
Although as a rule the percentage of acid in an oil is relatively low, there are a few oils consisting principally of acids. By way of example, orris oil contains 85 p. c. of myristic acid, the oil from Polygonum Persicaria consists largely of a mixture of volatile fatty acids,2) in Mexican valerian oil 89 p. c. of valeric acid have been found,3) and the oil of Morinda citrifolia contains as much as 90 p. c. of acid.4).
In the compilation of acids which follows, no differentiation is made between free acids and combined acids. This failure is due to two reasons. First, accurate statements are mostly wanting on this point. And, secondly, it may well be assumed that in most instances the presence of free acid is due to decomposition during the process of distillation. Upon saponification of the oils, the acids are obtained in the form of their salts.
Formic acid, H-COOH, has been found in turpentine oil (?), in thuja oil, the oils of Ceylon cardamoms, ylang-ylang, nutmeg, California laurel, in the oil from the fruits of Pittosporum undulatum, in the oils of myrrh and carrot, in the oil from the wood of Goupia tomentosa, in the oils of American pennyroyal, valerian, Achillea nobilis, and in the aqueous distillates of Pinus Sabiniana, Eucalyptus Globulus, AEthusa Cynapium, Micro-meria Chamissonis, Lippia scaberrima and Arnica montana.
1) Zincke, Liebig's Annalen 152 (1869), 21.
2) Horst, Chem. Ztg. 25 (1901), 1055.
3) Report of Schimmel & Co. April 1897, 44.
4) van Romburgh, Koninklijke Akademie van Wetenschappen te Amsterdam 1909, 17; Report of Schimmel & Co. October 1909, 78.
Formic acid is characterized by its great capacity for reduction. For purposes of identification, its solution, neutralized with alkali, is heated with mercuric chloride, where upon a precipitate of mercurous chloride and mercury results. The silver salt results upon the addition of silver nitrate to a solution of the formate. Upon heating it is decomposed and thus distinguished from acetic acid.
Acetic acid, CH3-COOH, is the most common of the acids found in volatile oils. An enumeration of its occurrences would include almost all of the volatile oils. Its esters are mostly characterized by an especially agreable odor. Hence they find manifold application in perfumery. This is especially true of linalyl and geranyl acetates. A few oils consist largely of acetic esters, e. g. the oils of lavender, bergamot, petitgrain, Siberian pine needle oil, etc.
For the purpose of identifying acetic acid, silver nitrate is added to the neutralized aqueous solution. The silver salt thus obtained is analysed. As a rule this method is generally applicable for the identification of most fatty acids.
Of the remaining fatty acids, the following have been found in volatile oils:
Propionic acid, C,H.-COOH, in the oils of chamomile, male fern (?), pastinaca and lavender.
n-Butyric acid, C3H7-COOH, in the oils of male fern, Polygonum Persicaria, nutmeg, Eucalyptus Globulus, niaouli, AEthusa Cynapium, Heracleum Sphondylium, H. giganteum, lavender, valerian, American pennyroyal, and in the aqueous distillate of Lippia scaberrima and Micromeria Chamissonis.
Isovaleric acid (isopropyl acetic acid), C4H9COOH, has been found, in the oils of male fern (?), cypress, citronella, laurel leaves, in the oil from the fruits of Pittosporum undulatum, in the oils of geranium (?), niaouli, Eucalyptus goniocalyx, E. palu-dosa, E. saligna, and senega root, in the oil from the wood of Goupia tomentosa, and in the oils of lavender (?), American peppermint, valerian, Mexican valerian, kesso root and wormwood.
Methyl ethyl acetic acid in the oils of champaca, angelica root and coffee.
Caproic acid, C5H11COOH, in the oils of male fern (?), Juniperus phoenicea, lemongrass, palmarosa, laurel leaves (?), Heracleum Sphondylium, pastinaca, in the oil from the wood of Goupia tomentosa, and in the oils of lavender and of the fruits of Morinda citrifolia.
Isoheptoic acid, C6H13COOH, in American pennyroyal oil.
Caprylic acid, C7H15COOH, in the oils of wormseed (?), nutmeg, camphor, sweet orange, American pennyroyal, in the oil from the fruits of Morinda citrifolia and the oil of Artemisia Herba-alba.
Caprinic acid, C9 H19 • COOH, crystallizes in needles melting at 31° and has been found in lemongrass oil, kobuschi oil, hera-cleum oil, American pennyroyal oil, chamomile oil, and in the oil of Artemisia Herba-alba.
Laurie acid, CuH23COOH, crystallizes as needles which melt at 43,6°. It occurs in the oils from the laurel berry, the pichurim bean, Psoralea bituminosa (?), the wood of Goupia tomentosa and Heracleum.
Myristic acid, C13H27.COOH, crystallizes in laminae that melt at 53,8°. It occurs in the oils of nutmeg, Blumea bal-samifera and orris root.
Pittosporum undulatum, pimenta, the seeds of Monodora grandi-flora, celery seed, parsley seed, carrot, Micromeria Chamissonis, the stems and leaves of Tagetes patula, carline thistle, arnica flowers, wormwood, and Blumea balsamifera (?).
Stearic acid, C17 H35 • COOH, crystallizes in laminae that melt at 69,2° and occurs in cascarilla oil.
Unsaturated acids have been found in only a few instances.
Methacrylic acid, CH2:C(CH3)COOH, crystallizes in long prisms and melts at 15 to 16°, b. p. 160,5°. It probably occurs in Roman chamomile oil.
Angelic acid, CH3.CH:C(CH3)COOH, melts at 45 to 45,5° and boils at 185°. It has been found in the aqueous distillate of the oils of angelica root and Roman chamomile.
Tiglicacid, CH3CH:C(CH3)COOH, melts at 64,5°, boils at 198,5° and occurs in geranium oil.
Oleic acid, C8H17CH:CH(CH2)7COOH, crystallizes in needles that melt at 14° and boil at 223° (10 mm.). It occurs in orris oil and kobuschi oil.
Of hydroxy acids the following have been identified:
Hydroxymyristic acid, C14H2803, crystallizes in pearly laminae that melt at 51°. It occurs in sabadilla seed oil and in angelica seed oil.
Hydroxypentadecylic acid, C15H30O3, crystallizes in needles that melt at 84°. It occurs in angelica root oil and possibly in Japanese angelica oil.
A dibasic acid has been observed but once as a natural constituent of a volatile oil, viz.
Succinic acid, COOH • CH2 • CH2 • COOH, in the oil from the wood of Goupia tomentosa.
Of aliphatic acids there remain to be mentioned
Citronellic acid, C9H17COOH, which boils at 257° or 143,5° (5 mm.). It occurs in the oil of Barosma pulchellum.
Teresantalic acid, a reduced cyclic acid, C10H10O2, which melts at 157°, [a]D - 70°24' (in 25 p. c. solution) occurs in sandal wood.
Of aromatic acids the following should be mentioned:
Benzoic acid, C6H5-COOH, crystallizes in shiny laminae that melt at 121,4°, and boil at 249° (760 mm.). It occurs in the oils of vetiver, tuberose, hyacinth, champaca, ylang-ylang, cinnamon leaves, wild cherry bark, neroli, tolubalsam and cloves.
Phenylacetic acid, C6H5CH2.COOH, crystallizes in thin laminae that melt at 76,5° and boil at 265,5°. It occurs in neroli oil.
Salicylic acid, C6H4OHCOOH, m. p. 155 to 157°, occurs widely distributed as methyl ester (see p. 504). In addition it has been found in the oils of spicewood, ylang-ylang, Pittosporum undulatum, and American pennyroyal.
Anisic acid, p-methoxybenzoic acid, C6H4OCH3-COOH, melts at 184°, occurs in the Tahiti vanilla. It is also found in all oils which, like anise oil, contain much anethol, from which it results upon oxidation.
Veratric acid, (CH30)2C6H3COOH + H20, melts between 179,5 and 181°, and has been found in the oil of saba-dilla seed.
Methyl-p-cumaric acid, p-methoxycinnamic acid, CH30-C6H4CH:CHCOOH, melts at 171° and occurs in Kaempferia oil.