The "labile" citral dihydrodisulphonate of sodium results likewise when an aqueous solution of sodium sulphite is shaken with citral:
C9H16CHO + 2Na2S03 + 2H20 = C9H17(S03Na)2CHO +2NaOH.
In as much as the sodium hydroxide set free would regenerate the citral from the compound just formed, it must be combined with dilute acetic acid, sulphuric acid, or an acid salt such as sodium bisulphite or sodium bicarbonate. According to Tiemann,1) the following method may be followed: A solution of 350 g. of sodium sulphite, Na2S03 +7H20, in 1 liter of water, colored red with a little phenolphthalein solution, is shaken with 100 g. of pure citral. The strongly alkaline reaction produced is reduced from time to time by the gradual addition of a standardized sulphuric acid of about 20 p. c. strength. The solution should always be slightly alkaline as revealed by the red color of the indicator. Otherwise the normal sodium acid sulphite addition product of citral, which is difficultly soluble, is exclusively formed and separated.
Inasmuch as the derivatives of citral with hydroxylamine, phenylhydrazine, and ammonia are liquid, they cannot be utilized for the characterization of citral. When dehydrated with the aid of acetic acid anhydride, the oxime is converted into the nitrile of geranic acid. When acted upon by semicarbazide, citral yields several well crystallizable semicarbazones.2) Under certain conditions3) this mixture of semicarbazones can be resolved into two compounds melting at 164° and 171° respectively. Hence these can be utilized for the identification of citral. (See below.)
Moderate oxidation, e. g. with silver oxide in ammoniacal solution yields the liquid geranic acid C10H16O2,4) the odor of which resembles that of the higher fatty acids. More energetic oxidation with chromic acid mixture yields methylheptenone which, upon further oxidation with potassium permanganate and chromic acid mixture, breaks up into acetone and laevulinic acid.1) Because of these results the above-mentioned formula, corresponding to that for geraniol, has been assigned to citral. This formula agrees well with the properties of citral.
1) Berl. Berichte 31 (1898), 3317.
2) Wallach, Berl. Berichte 28 (1895), 1957; Tiemann and Semmler, ibidem 2133; Tiemann, Berl. Berichte 31 (1898), 821, 2315. 3) Tiemann, Berl. Berichte 31 (1898), 3331. 4) Semmler, Berl. Berichte 23 (1890), 3555; 24 (1891), 203.
When condensed with acetone, citral yields a ketone C13H20O, pseudoionone. When heated with dilute sulphuric acid, this ketone yields ionone which is isomeric with the irone of orris oil. (See under Ionone.)
Inasmuch as citral possesses a penetrating odor, this will as a rule indicate its presence in volatile oils. For its positive identification, the aldehyde is separated by means of its solid acid sulphite derivative. Regenerated, it is then converted into the a-citryl-ji-naphthocinchoninic acid, discovered by Doebner,2) by condensation with pyrotartaric acid and /i-naphthylamine. For the preparation of this compound, Doebner has given the following directions: 20 g. of pyrotartaric acid and 20 g. of citral (or of the corresponding oil) are dissolved in absolute alcohol. To this solution, 20 g. of B-naphthylamine, likewise dissolved in absolute alcohol, are added and the mixture heated for three hours on a waterbath, the flask being connected with a reflux condenser. Upon cooling, the citrylnaphthocinchoninic acid, which has separated in a crystalline form, is separated by filtration and purified by washing with ether. If the acid is very impure, it is dissolved in ammonia and precipitated from the filtered am-moniacal solution by means of acetic acid. The substance thus purified crystallizes from alcohol in yellow laminae. Doebner3) gives the melting point at 197°. However, it lies somewhat higher being found at 200° or even slightly above that temperature.
It should be remembered, however, that, in the presence of but small amounts of citral or other aldehydes, a-methyl-/i-naphtho-cinchonic acid is formed by the interaction of pyrotartaric acid and B-naphthylamine. This acid melts at 310° and is less soluble in alcohol than is citrylnaphthocinchoninic acid. Hence it remains in the residue when the crude naphthocinchoninic acid is extracted with hot alcohol.
1) Tiemann and Semmler, Berl. Berichte 26 (1893), 2718.
2) Berl. Berichte 87 (1894), 354, 2026.
3) Loc.cit; Berl. Berichte 31 (1898), 1891; Comp. ibidem 3197, 3327.
It should further be remembered that when other aldehydes are present the corresponding naphthocinchoninic acids are formed. Thus Doebner found not only citryl- but also citronellyl-B-naphthocinchoninic acid (m. p. 225°) in fractions of lemon oil.
As already pointed out, natural citral apparently consists of two stereoisomeric forms, which have been designated citral "a" and citral "b" by Tiemann. The semicarbazone melting at 164° corresponds to the former and can be prepared in accordance with the following directions:
To a solution of 5 parts of citral (or of the fraction to be tested) in 30 parts of glacial acetic acid, a solution of 4 parts of semi-carbazide hydrochloride in a little water is added. After standing for a short time, a considerable quantity of semicarbazone separates in needle-like crystals. After having been recrystallised two or three times from methyl alcohol, they melt at 164°. From the mother liquid of this semicarbazone, the semicarbazone corresponding to citral "b" and melting at 171° can be separated.1) Mixtures of both semicarbazones reveal melting points varying from 130 to 171°. As related compounds, the thiosemicarbazone, melting at 107 to 108°, and the semioxamazone, melting at 190 to 191°, should be mentioned.
Citrylidene cyanacetic acid, C9H15CH:C(CN)COOH, obtained by condensation of citral with cyanacetic acid, is another derivative melting at 122° that crystallizes well and hence can be used for the identification of citral. It can be prepared according to the following directions: To a solution of one molecule of cyanacetic acid in three times its weight of water, two molecules of sodium hydroxide (as 30 p. c. sodium hydroxide solution) and one molecule of citral are added. If the citral be pure, it will dissolve without turbidity upon shaking. If the solution be turbid it is shaken out with ether. From the clear solution the citrylidene cyanacetic acid is precipitated by means of acids. It separates either in crystalline form or as an oil that readily congeals. Recrystallized from the benzene solution to which ligroin has been added, it is obtained in short, yellow crystals.2)
1) Tiemann, Berl. Berichte 31 (1898), 3331; 32(1899), 115; 33(1900), 877. 2) Tiemann, Berl. Berichte 31 (1898), 3329.
In as much as citral "b" condenses more slowly than does citral "a", this method can be utilized for the separation of both modifications. Citrylidene cyanacetic acid "a" melts at 122°, the "b"-variety at 94 to 95V)
Still another solid condensation product of citral is Obtained with acetyl acetone. It is obtained by condensing 15,2 g. citral and 20 g. of acetyl acetone at room temperature with the aid of piperidine. After standing for three days the entire reaction mass congeals to a crystalline magma. Recrystallized from a mixture of alcohol, ether and ligroin, citrylidene bisacetyl-acetone is obtained in light yellow wart-like crystals that melt at 46 to 48°.2)
Tiemann3) recommended that the presence of citral be established by condensing it with acetone to pseudo ionone and by identifying this by means of its semicarbazone. This method, however, is more complicated than the preparation of citryl naphthocinchoninic acid which is, therefore, given the preference.
A method for effecting the separation of citral, citronellal, and methylheptenone is given under the two last-mentioned compounds.