Aside from the aldehydes citral and citronellal, which are to be mentioned later in this chapter and which occur rather frequently, the aliphatic aldehydes constitute only minor constituents of the volatile oils. In as much, however, as the charac- teristic odor of some oils is dependent on the presence of small amounts of these aldehydes, they are, nevertheless, of great importance. Because of this property, such aldehydes as nonylic aldehyde and decylic aldehyde, play an important role in the production of synthetic oils.
Like the corresponding alcohols, the lower members of this series, which are probably formed during the process of steam distillation, are found principally in the aqueous distillate or in the first distillate obtained during the fractionation of the oil. Their presence is frequently revealed by their pungent odor. They are most readily isolated by means of bisulphite. Lower aldehydes are contained in the oils of ginger, kesso root, saba-dilla, Allium ursinum, milfoil, Eucalyptus Globulus. In the following account brief mention is made of the aldehydes thus far found in volatile oils.
Formaldehyde, H • CHO, has thus far been identified with certainty in apopin oil. For its identification, the liquid, in which its presence is suspected, is evaporated with ammonia on a water bath. If present, the characteristic crystals of hexamethylene tetramine are formed.
Acetaldehyde, CH3CHO, has been observed rather frequently, more particularly in seed oils. It has been found in the oils of orris, camphor, anise, caraway, rosemary (?) and peppermint, especially when working with larger amounts of these oils. It is identified by means of the color reaction1) with trimethyl-amine and sodium nitroprusside.
Butyric aldehyde, C3H7CHO, boils at 75° and presumably occurs in the oils of Eucalyptus Globulus and cajeput. It can be identified by means of its /Miitrophenylhydrazone'2) which melts at 91 to 92°.
Isovaleric aldehyde, C4H9CHO, can frequently be recognized by its disagreeable, irritating odor which causes coughing. It possibly occurs in the oils of cajeput, niaouli, Eucalyptus rostrata, E. Globulus, and cloves. In addition iso-valeric aldehyde has been found in American and French peppermint oil, in kesso oil (?), and in lavender oil (?). It can be characterized by its oxidation to /'sovaleric acid, Isovaleric aldehyde boils at 92°. Its thiosemicarbazone melts at 52 to 53V)
1) Rimini, Annali Farmacoterapia e Ch. 1898, 249; Chem. Zentralbl. 1898, II. 277.
2) Dakin, Journ. of Biol. Chem. 4 (1908), 235; Chem. Zentralbl. 1908, I. 1259.
Capronic aldehyde, C5H11CHO, boils at 128° and is very likely contained in the oil of Eucalyptus Globulus. The unpleasant, irritating odor of several eucalyptus oils is probably due to the presence of this aldehyde as well as that of butyric and valeric aldehydes.2) n-Octy/iC aldehyde, C7H15CHO, is possibly contained in lemon oil. It possesses a strong odor reminding of cenanthol. The following properties were ascertained by Schimmel & Co.:5) in connection with a preparation obtained by the oxidation of octylic alcohol and purified through the bisulphite compound:
B. p. 60 to 63° (10 mm.); d15o 0,827.
Semmler4) records the following constants:
B. p. 60 to 61° (9 mm.); d20o 0,8211; nD 1,41955; b. p. of the oxime 60°; m. p. of the semicarbazone 101°.
Octyl-B-naphthocinchoninic acid forms small, white crystals which melt at 234°. With phosphonium iodide, octylic aldehyde combines to a compound that melts at 115,5°.
n-Nonylic aldehyde, C8H17CHO, is a constituent of the oil of orris root, of Ceylon cinnamon oil, of German rose oil, of mandarin oil, and probably also of lemon oil. The following constants have been observed in connection with an aldehyde isolated from oil of rose:
B. p. 80 to 82° (13 mm.), d15o 0,8277; nD16o1,42452.5)
1) Neuberg and Neimann, Berl. Berichte 35 (1902), 2052.
2) Schimmel's Bericht April 1888, 18.
3) Report of Schimmel & Co. April 1899, 26.
4) Berl. Berichte 42 (1909), 1161.
5) Report of Schimmel & Co. October 1900, 54.
Upon oxidation it yields pelargonic acid which boils at 252 to 253°. The aldehyde is characterized by its oxime which melts at 69° and by its semicarbazone which melts at 100°.
n-Decylic aldehyde, C9H19-CHO, occurs in the oils of lemongrass, orris root, sweet orange, mandarin, neroli, cassie blossoms, and coriander. Furthermore, its presence in the leaf oil of Abies alba seems probable. Stephan1) records the following constants for a decylic aldehyde isolated from oil of sweet orange:
B. p. 207 to 209° (755 mm.) with slight decomposition, 93 to 94° (12 mm.); d15o 0,828; nD15o 1,42977.
For decylic aldehyde from lemongrass oil the following constants were observed:
B. p. 80 to 81° (6,5 mm.); d15o 0,8361.-)
Characteristic are the naphthocinchoninic derivative, obtained by condensation of the aldehyde with B-naphthylamine and pyrotartaric acid, and which melts at 237°; the oxime, which melts at 69°; the semicarbazone, which melts at 102°; and the /7-caprinic acid, which results upon oxidation and which melts at 30 to 31° and boils at 267 to 269° (753 mm).
Laurinic aldehyde, C11H23CHO, occurs in the leaf oil of Abies alba and possibly also in oil of rue. At ordinary temperature it is solid. Exposed to air it oxidizes to laurinic acid which melts at 43°. The semicarbazone melts at 101,5 to 102,5°.
Of much greater importance than the aldehydes thus far enumerated are the aliphatic terpene aldehydes citral, C10H16O, and citronellal, C10H18O; also a number of aromatic aldehydes. The former are especially noteworthy because of their relation to other volatile oil constituents, more particularly the alcohols. Of the latter, benzaldehyde and cinnamic aldehvde occur in considerable amounts in some oils.
1) Journ. f. prakt. Chem. II. 62 (1900), 525.
2) Report of Schimmel & Co. October 1905, 45.