Methyl heptenone 164

Of greater interest than the saturated ketones already enumerated, is the unsaturated methyl heptenone, C8H140. It occurs as constituent of several volatile oils and has been obtained as decomposition product of related compounds. As companion to the closely related linalol, geraniol and citral, it occurs in Mexican lignaloe oil, citronella oil and lemongrass oil. It also occurs in lemon oil, palmarosa oil and in the oil of Barosma pulchella. Apparently it owes its existance to the decomposition of the above-mentioned compounds. This decomposition can likewise be brought about artificially by oxidation. From the fractions boiling between 160 and 180° it can readily be isolated by means of its acid sulphite addition product.

As "Abbau" product it was first obtained in the dry distillation of cineolic acid anhydride.1) It was then observed during the saponification of geranic acid nitrile,2) and as oxidation product of citral.3) It also results from the latter when treated with alkalies.4) Synthetically it has been obtained from amylene bromide and acetyl acetone,5) also by the interaction of the iodide of acetopropylalcohol, acetone and zinc dust.6)

It is a colorless, mobile liquid, with a penetrating odor reminding of amyl acetate, and is optically active. The physical constants as recorded do not agree any too well. For the'ketone obtained from cineolic anhydride, Wallach records the following:

B. p. 173 to 174°; d20o 0,8530; nD20o 1,44003.7)

For natural methyl heptenone Tiemann and Kruger determined :

B. p. 170 to 171° (760 mm.); d.20o 0,8499; nD 1,4380.8)

1) Wallach, Liebig's Annalen 258 (1890), 323.

2) Tiemann and Semmler, Berl. Berichte 26 (1893), 2721.

3) Ibidem 27i9.

4) Verley, Bull. Soc. chim. III. 17 (1897), 175.

5) Barbier and Bouveault, Compt. rend. 122 (1896), 1422. 6) Verley, Bull. Soc. chim. HI. 17 (1897), 191.

7) Liebig's Annalen 258 (1890), 325.

8) Berl. Berichte 28 (1895), 2123.

For the "Abbau" product of citral Verley records: B.p. 168°, 84° (56 mm.); d14. 0,910 (!); nD310 1,437.1)

For the methyl heptenone isolated from lemongrass oil and regenerated from the acid sulphite addition product, Schimmel & Co. observed:

B.p. 173° (758 mm.); d15o 0,855; nD20o1,43805 ;2) and for a product obtained by boiling citral with potassium carbonate solution:

B.p. 173 to 174°; d15o 0,8656.

When reduced in alcoholic solution with sodium, methyl heptenone is reduced to the secondary alcohol methyl heptenol, C8H160.3) This alcohol is likewise found among the decomposition products of geraniol and the saponification products of geranic acid nitrile. Methyl heptenone combines with acid sulphites to crystalline addition products, with hydroxylamine and phenylhydrazine to liquid condensation products, and with semi-carbazide to a crystalline semicarbazone which can be utilized for the identification of the ketone. When oxidized, methyl hep-tenone breaks down into acetone and laevulinic acid, C5H8034) in accordance with the above formula. Dehydrating agents, such as zinc chloride, convert it into dihydro-m-xylene, C8H12.5)

Methyl heptenone is readily recognized by its characteristic odor reminding of amylacetate. For its identification it is converted into the semicarbazone. As in the case of citral, this appears to be a mixture of isomers. However, a product with a constant melting point is obtained when prepared according to the directions of Tiemann and Kruger.6) According to these investigators, a solution of 12 g. of semicarbazide hydrochloride and 15 g. sodium acetate in 20 cc. of water is added to a mixture of 12 g. methyl heptenone and 20 cc. of glacial acetic acid, and the mixed solutions set aside for half an hour. Upon the addition of water the semicarbazone separates as oil which soon congeals to a crystals which, when recrystallized from dilute alcohol, melt at 136 to 138°.

1) Verley, /oc. cit. 176.

2) Report of Schimmel &Co. April 1899, 65.

3) Wallach, Liebig's Annalen 275 (1893), 171.

4) Tiemann and Semmler, Berl. Berichte 28 (1895), 2128. 5) Wallach, Liebig's Annalen 258 (1890), 326.

6) Berl. Berichte 28 (1895), 2124.

A compound well suited for identification is obtained when methyl heptenone is treated with bromine in the presence of soda lye.1) Hypobromous acid is added and bromine simultaneously substituted with the formation of the well crystallized derivative C8 H12 Br30 • OH. For its preparation 3 g of ketone are shaken with a solution of 3 g of sodium hydroxide and 12 g of bromine in 100 to 120 cc. of water. The reaction product separates as a heavy oil which soon congeals. It is taken up with ether, the ethereal solution shaken out with dilute soda solution. The residue left upon the evaporation of the ether is recrystallized from ligroin to which some animal charcoal has been added. The melting point of the pure, white crystals lies between 98 and 99°. Upon keeping they gradually decompose.

A method for the identification of methyl heptenone in the presence of citronellal and citral has been worked out by Tiemann.2) It is based on the fact that methyl heptenone does not react with either a dilute or a concentrated solution of sodium sulphite and sodium acid carbonate, whereas the two aldehydes can be successively removed by shaking the oily mixture with the respective solutions of these salts.