This method for the quantitative estimation of cineol in Eucalyptus oils was first introduced by Schimmel and Co. in 1907 (semi-annual report for October). The principle is based on the fact that cineol forms an addition product with resorcinol, the compound being soluble in a 50 per cent. aqueous solution of that substance.

As first proposed the process. was faulty, as several other substances which occur in the various Eucalyptus oils are absorbed at the same time as the cineol.

Later, Wiegand and Lehmann brought forward a modified process (Chemiker Zeitung 32, 1908, 109), in which the oil was first fractionated, the portion which came over between 170-1900 C, being used for the absorption, and the result calculated for the original oil. (See also in this connection C. F. Bennett, "Perfumery and Essential Oil Record," Oct., 1912.)

With the heavy cineol Eucalyptus oils of the E. folybractea class, in which only a very small portion distils below 1700 C, this modified method may succeed in concentrating the cineol, but with the pinene-cineol oils in which a considerable fraction is obtained boiling below 170° C. a good deal of cineol comes over in that portion, and it does not seem possible; by direct distillation of oils of this class, to prevent much cineol distilling over with the pinene in this way.

With a large number of Eucalyptus oils in which cineol is only present in small amount and other constituents soluble in resorcinol not at all pronounced, the process acts very well, and cineol as low as 5 per cent. can be determined in this way. Such oils are those consisting largely of pinene, as E. dextropinea, E. lævopinea, etc, and many of the phellandrene-bearing oils, such as E. oreades, E. Delegatensis, E. stelluata, etc.

The method was also found useful for comparative purposes, as, for instance, in the investigation of the oils of E. Smithii from various forms of growth (Proc. Roy. Soc, N.S.W., Aug., 1915). Those results are also published in this work. The oil of this species contains a very small amount of substances other than cineol absorbable by resorcinol, and this is also the case with that of E. polybractea and a few others of the richer cineol class.

The resorcinol method may be considered a useful one for determining the cineol in a number of Eucalyptus oils, and may be satisfactorily employed for that purpose, but is more particularly applicable with those for which the phosphoric acid method is useless. Like all other known methods it is not of universal application, owing to the great diversity of constituents in Eucalyptus oils, and it thus becomes necessary to discriminate when choosing the method to be employed. The resorcinol process is also useful in assisting the investigation of undetermined Eucalyptus oils.

Unfortunately many constituents which occur in varying amounts in Eucalyptus oils are absorbed by 50 per cent, resorcinol, equally with cineol, and this is the case with the alcohols, as geraniol, terpineol, eudesmol, amyl, butyl, etc.; the aldehydes as aromadendral, citral, citronellal, butaldehyde, etc.; the lower esters as amyl-acetate, butyl-butyrate, etc.; piperitone and other bodies containing oxygen.

It is thus evident that the resorcinol method cannot be expected to give accurate results with all Eucalyptus oils, particularly those in which the constituents enumerated above occur in quantity, and it is difficult, if not impossible, with the oils of many Eucalyptus species to prepare a fraction in which the whole of the cineol is concentrated, and at the same time for it to be comparatively free from other absorbable constituents. If these are known, however, they can be separately determined, and allowed for, and in this way the cineol results might be made fairly accurate.

Illustrations of this procedure are given under E. dives, E. piperita, and other species in which the amount of piperitone in the fraction boiling below 1900 C. was separately determined and allowed for.

As illustrating the difficulties in this connection the following analyses are given, and the four oils investigated are quite representative of the cineol Eucalyptus oils at present found on the market. The data have been obtained from the rectified oils distilling below 1900 C, in order that the results with the rapid phosphoric acid and resorcinol methods might be compared, and the amount of absorbable constituents, other than cineol, indicated.

Ordinary constants for the crude oils employed are given, together with their cineol contents, calculated from the results obtained with the rectified oils. The alcoholic values, determined after acetylation, are calculated for C10H18O.

From the results here appended it is seen that the cineol in the oils of the E. cneorifolia and E. Australiana types cannot be satisfactorily determined quantitatively by the resorcinol method, although it might well be employed for the oils of the class to which E. polybractea and E. Smithii belong, and give results comparable with those obtained with phosphoric acid

Eucalyptus Polybractea. Wyalong, N.S.W

Crude oil.

Rectified portion boiling

below 190° C.

(91 per cent.)

Specific gravity at 150 C. ......

= 0.9201

= 0.9194

Optical rotation aD ........

+ 1.8°

+ 1.9°

Refractive index at 200 C..........

= 1.4595

= 1.4594

Cineol (rapid phosphoric acid method) .......

= 78 per cent.

= 86 per cent.

Cineol (resorcinol method) ........

= 82 per cent.

= 90 per cent.

Absorption with NaHSO3 .....

......

= 2 per cent.

Saponification number (1 1/2 hours hot) ...

......

= 4.06.

After acetylation (1 1/2 hours hot) ........

........

= 10.52.

= 1.8 per cent, free alcohol.

By allowing for the absorbable constituents and the free alcohol the cineol was practically the same both by the phosphoric and resorcinol methods.

Eucalyptus Smithii. Hill Top, N.S.W

Crude oil.

Rectified portion distilling below 190° C. (95 per cent.)

Specific gravity at 15° C.........

= 0.9292

= 0.9213

Optical rotation aD.........

+ 4.0°

+ 37°

Refractive index at 20° C..........

= 1.4618

= 1.4598

Cineol (rapid phosphoric acid method)........

= 81 per cent.

= 85 per cent.

Cineol (resorcinol method) ........

= 85.5 per cent.

= 90 per cent.

Absorption with NaHSO3..........

.......

= 2 per cent.

Saponification number (1 1/2 hours hot) .........

......

= 5.4

After acetylation (1 1/2 hours hot)............

.......

= 11.08

= 1.58 per cent, free alcohol

In this case, also, after allowing for the absorbable substances other than cineol, the results were practically the same by both methods.

Eucalyptus Australiana

Nerrigundah, N.S.W. (1st Hour Oil. Commercial sample.)

Crude oil.

Rectified portion distilling below 1900 C. (92 per cent.)

Specific gravity at 15° C .........

= 0.9198

= 0.9190

Optical rotation aD ..........

+ 2.8°

+ 270

Refractive index at 20° C......

= 1.4609

= 1.4602

Cineol (rapid phosphoric acid method).........

= 72 per cent.

= 78 per cent.

Cineol (resorcinol method).........

= 84 per cent.

= 91 per cent.

Absorption with NaHSO3 ................

...... .

= 3 per cent.

Saponification number (1 1/2 hours hot)...

........

= 2.97

After acetylation (1 1/2 hours hot) .........

..........

= 24.88

= 6.05 percent, free alcohol.

The resorcinol method was thus shown not to be suitable for cineol determination in the oil of this species. The alcohols present were largely terpineol and geraniol.

Eucalyptus Cneorifolia (a somewhat old sample). Kangaroo Island, South Australia

Crude oil.

Rectified portion distilling below 190o C. (91 per cent.)

Specific gravity at 15° C....................

= 0.9304

= 0.9198

Optical rotation aD .................

- 2.4°

-1.5°

Refractive index at 200 C..............

= 1.4677

= 1.4637

Cineol (rapid phosphoric acid method) .......

= 62 per cent.

= 68 per cent.

Cineol (resorcinol method) .........

= 83 per cent.

= 91 per cent.

Absorption with NaHSO3 ........

.......

= 6 per cent.

Saponification number (1 1/2 hours hot)...

.........

= 9.55

After acetylation (1 1/2 hours hot)........

..........

= 20.05

= 2.9 per cent. free alcohol.

It is thus evident that the resorcinol method is neither suitable for the quantitative estimation of cineol in the oil of this species, nor in oils of this class, and numerous instances of this will be found recorded in this work, under the several species.