The inversion of pseudoionone to ionone may be brought about more particularly by concentrated sulphuric acid, phosphoric acid, formic acid, and oxalic acid, also by dilute mineral acids and solutions of acid alkali sulphates, even by neutral salts such as sodium acetate and magnesium sulphate in an autoclave. In the case of the concentrated acids, the action should be restricted to a lower temperature, also to a shorter time. In the case of dilute acids and other substances, prolonged heating is required. In all of these reactions a mixture of two isomers, the a- and B-ionone, is formed.

If in place of acetone its homologues be employed, the corresponding homologues of pseudoionone are formed, which can be inverted into the isomeric ionone derivatives as stated above.

Reduced pseudoionones and homologues thereof are obtained by the condensation of citronellal with acetone and its homologues. These in turn can be isomerised to the corresponding ionones and homologues by means of acids. Acetyl pseudoionones and acetyl ionones have also been prepared. As the most important representative of the cyclocitral series, ionone is of considerable scientific interest.

1) Berl. Berichte 26 (1893), 2691.

Pseudoionone. Although this ketone does not react immediately with sodium acid sulphite solution at ordinary temperature, it does combine with it upon prolonged heating to a hydrosulphonic acid derivative which is soluble.1) This compound can be utilized for the preparation of pure pseudoionone. The solution is first shaken repeatedly with ether or other immiscible solvent for the purpose of removing aldehyde resins or other impurities. The ketone is then regenerated with the aid of alkali at ordinary or lower temperature.

Pseudoionone is a light yellow, strongly refractive, somewhat viscid oil, the odor of which is little characteristic. Its constants are:

B. p. 143 to 145° (12 mm.); d20o 0,8980; nD 1,53346.

Of characteristic derivatives the p-bromphenylhydrazone, melting at 102 to 104°, may be mentioned.

Ionone. As already pointed out, the production of ionone results invariably in the formation of a mixture of two isomers. If concentrated sulphuric acid be used, or if dilute sulphuric acid be allowed to react for a longer period2), the resulting mixture consists largely of B-ionone. If, however, the inversion be brought about by concentrated phosphoric or formic acids, a-ionone results almost exclusively. Toward acid sulphite solution ionone behaves like pseudoionone. The resulting hydrosulphonic acid can be well utilized for the purification of ionone.

Freshly distilled ionone is a colorless oil with a decided odor of cedarwood. It is only in dilute solution, e. g. in alcohol, that its odor of violets, reminding at the same time somewhat of grape blossoms, becomes apparent. Noteworthy is also the property of ionone to benumb the sense of smell.

1) Tiemann, Berl. Berichte 31 (1898), 842.

2) Tiemann, Berl. Berichte 31 (1898), 868, 870.

Even the ionones purified through the hydrosulphonic addition products vary more or less in their a- and /i-ionone content according to the reagents employed in their isomerization. Hence their physical constants vary appreciably. For a pure ionone, consisting however of a mixture of the two isomers, Tiemann1) records the following constants:

B. p. 126 to 128° (10 mm.); d20o 0,9351; nD 1,507; whereas Schimmel & Co. observed the following on their own products:

B. p. 104 to 109° (4 to 5 mm.); d15. 0,9350 to 0,9403; d20o 0,9335; nD20o 1,50335 to 1,50510; soluble in 2,5 to 3,0 vols, of 70 percent, alcohol.

As to odor, the differences between a- and B-ionone are but slight, yet nevertheless recognizable by the expert. That of a-ionone is fresher and more fragrant than that of its isomer, which is rather strong and reminds more of the odor of the leaves of the violet. Hence the former is preferred.

To a-IONONE Tiemann2) assigns the following properties:

B. p. 123 to 124° (11 mm.); d20o0,932; nD 1,4980.

Chuit3) found:

B. p. 127,6° (12 mm.); d15o 0,9338; nD17.2o1,50001.

The p-bromphenylhydrazone, which is best suited for the characterization of a-ionone, melts at 142 to 143°. It is prepared in glacial acetic acid solution and recrystallized from dilute methyl alcohol or ligroin. The semicarbazone melts at 107 to 108°, respectively at 137 to 138°, the thiosemicarbazone at 121°, the oxime at 89 to 90°.

For pure -IONONE Tiemann4) determined the following constants:

B. p. 127 to 128,5° (10 mm.); d17o 0,946; nD17o1,521;

1) Berl. Berichte 31 (1898), 851.

2) Berl. Berichte 31 (1898), 876.

3) Rev. gen. de Chim. 6 (1903), 432; Chem. Zentralbl. 1904, I. 280.

4) Berl. Berichte 31 (1898), 871, 879.

Chuit1) the following: B. p. 134,6° (12 mm.); d15o 0,9488; nD17.5o 1,52008.

B-Ionone can best be identified by means of its semicarbazone2-) which melts at 148 to 149°. Its b-bromphenylhydrazone melts at 116 to 118°, its thiosemicarbazone at 158°, its hydrazone at 104 to 105°. The oxime is liquid.

Concentrated sulphuric acid inverts cr-ionone to B-ionone. Inversely, i-ionone can be changed, though more difficultly, to a-ionone by means of alcoholic potassa.

The physical constants of commercial ionone do not afford a definite clue as to its purity. In order to test such a preparation as to its purity, the by-products that result during its manufacture should be removed.

For this purpose the oil to be examined is boiled for 10 to 15 hours with thrice its weight of sodium acid sulphite solution in which the free sulphurous acid has been neutralized with dilute soda solution. The duration is dependent on the reaction of the ionone with the bisulphite. According to Schimmel & Co., the addition of alcohol suggested by Schmidt,3) is not necessary. In order to remove those sub-stances that have not combined with the acid sulphite, the solution after having been diluted with water is extracted four times with ether. If the ionone is comparatively pure, the addition of water will cause but a slight turbidity, if impure the separation of an oil. Should the separated portion smell of ionone, a second treatment with acid sulphite is necessary. The difference between the amount originally used and the amount extracted is computed as ionone. Some idea as to the relative proportions of or- and /i-ionone present can be obtained from the physical constants of the mixed ionones separated from the acid sulphite addition product by means of lye, also by means of the semicarbazone and p-bromphenyl-hydrazone.

1) Loc. cit.

2) For the purification and separation of the ionone semicarbazones compare Tiemann, Berl. Berichte 31 (1898), 875, 1736. 3) Zeitschr. f. angew. Chem. 13 (1900), 189.