Citral1) is the only aliphatic aldehyde of the formula C10H16O that has thus far been isolated from volatile oils. It is also known as geranial, because of its close relation to geraniol, being the first oxidation product of the latter. Its occurrence in nature is rather common. It was first found by J. Bertram2) in the oil of Backhousia citriodora. In as much as it was shown to be identical with the lemon-scented constituent of oil of lemon, it was named citral. Larger amounts are contained in the lemon-grass oil as well as in the Backhousia oil just referred to. It occurs also in Java citronella oil, ginger oil, kobushi oil, sassafras leaf oil, Japanese cinnamon oil, the oils from the fruit, bark and leaves respectively of Tetranthera citrata, German rose oil, the oils from the leaves and twigs of the sweet orange tree and the lemon tree, cedro oil, West-Indian limette oil, mandarin oil, oil of sweet orange (?), Japanese pepper oil, may oil, oil of bay, oil of pimenta, the oils of Eucalyptus patentinervis, E. Staigeriana, E. vitrea (?), Leptospermum Liversidgei, verbena, Monarda citriodora, and melissa. In these oils citral occurs in two modifications, designated citral "a" and citral "b", which appear to be stereoisomeric.3) In practically all cases citral "a" predominates.
From all of the oils enumerated above, citral can be isolated by means of its crystalline addition product with bisulphite (see below). From this compound, after previous purification by washing with alcohol and ether, citral can be obtained in a pure state by decomposition with alkali carbonate.
1) Concerning the history of citral see Tiemann, Berl. Berichte 31 (1898), 3278.
2) Schimmel's Bericht October 1888, 17. 3) Tiemann, Berl. Berichte 33 (1900), 877.
To the extent of 30 to 40 p. c. yield citral can be obtained artificially by the oxidation of geraniol with chromic acid mixture.1) In like manner the tertiary alcohol linalool (likewise nerol) yields the same oxidation product, since the acid oxidizing agent first causes the rearrangement of linalool to geraniol. Synthetically, citral has been obtained by the distillation of the calcium salt of geranic acid with calcium formate.2)
Citral is a thin, light yellow liquid, which is optically inactive and which possesses a penetrating odor of lemon. Under atmospheric pressure it distils at 228 to 229° with slight decomposition. Its constants, as recorded by Tiemann and Semmler,3) are as follows:
B. p. 110 to 112° (12 mm.); 117 to 119° (20 mm.); 120 to 122° (23 mm.); d15o 0,8972; nD 1,4931; d22o 0,8844; nD 1,48611.
In addition Tiemann4) records the following constants:
For citral "a": B. p. 118 to 119° (20mm.); d20o 0,8898; nD 1,4891. For citral "b": B. p. 117 to 118° (20 mm.); d20o0,8888; nD 1,49001.
In the laboratory of Schimmel & Co. the following.observations have been made in connection with citral carefully purified through the bisulphite or the hydrosulphonic acid addition products:
For citral from lemongrass oil:
B. p. 110 to 111° (12 mm.); d16o0,893; nDl7o1,49015.5) For citral from lemon oil:
B. p. 92 to 93° (5 mm.); d15o 0,8926; nD20o1,48853. For citral from the oil of the fruits of Tetranthera citrata: d15o 0,8941; nD20o1,8767.
Commercial products prepared by Schimmel & Co. had the following properties: d15o 0,892 to 0,895; nD20o1,488 to 1,489; soluble in about 7 vols, of 60 p. c. alcohol.
1) Tiemann, Berl. Berichte 31 (1898), 3311.
2) Tiemann, Berl. Berichte 31 (1898), 827.
3) Berl. Berichte 26 (1893), 2709.
4) Berl. Berichte 32 (1899), 117, 120; 33 (1900), 880.
5) Report of Schimmel & Co. April 1899, 64.
As diolefinic aldehyde citral adds two molecules of bromine, but does not yield a solid bromide. Toward acids and acid reagents it is very susceptible, being altered materially by them. Like many of its derivatives, it can readily be converted into cyclic compounds. Dilute sulphuric acid and potassium acid sulphate act very energetically with the formation of cymene.1) Alkalies likewise attack citral. When boiled with potassium carbonate solution it is broken up into acetaldehyde and methyl-heptenone, C8H140.2) The same ketone results upon the oxidation of citral and is found accompanying the citral in lemongrass oil (see also under "Ketones").
Citral reveals all of the properties of an aldehyde. It reacts with the well-known reagents for aldehydes and when reduced with sodium amalgam in acetic acid solution it is converted into geraniol.3) Toward sodium acid sulphite solution it behaves in a peculiar manner.4) If the solution does not contain too large amounts of free sulphurous acid, the difficultly soluble, normal crystalline addition product, C9H15CH(OH)S03Na, is formed when the aldehyde is shaken with such a solution. From this compound sodium carbonate and sodium hydroxide regenerate the citral, but not quantitatively. If, however, this crystalline addition product is gently heated with an excess of bisulphite solution, it is dissolved with the formation of a "labile" dihydro-disulphonic acid derivative of citral, C9H17(S03Na)2CHO. From this the citral can be regenerated no longer by means of alkali carbonate, but by means of caustic alkali. If, however, the temperature is allowed to rise too high, the "labile" compound has been changed to a "stabile" dihydrodisulphonic acid derivative, which is no longer yields citral even when acted upon with caustic alkali. This same compound results likewise when the normal compound is suspended in water and subjected to the action of steam until it has gone into solution. If the solution of the "labile" citral dihydrosulphonate of sodium is shaken with citral, it is changed back to the citral hydromonosulphonate of sodium, C9H16(S03Na)CHO.
1) Semmler, Bed. Berichte 24 (1891), 204.
2) Verley, Bull. Soc. chim. III. 17 (1897), 175; Tiemann, Berl. Berichte 32 (1899), 107.
3) Tiemann, Berl. Berichte 31 (1898), 828.
4) Tiemann and Semmler, Berl. Berichte 26 (1893), 2708; Tiemann, Berl. Berichte 31 (1898), 3310.