Many volatile oils contain as important constituents alcohols varying in composition, for instance, borneol, geraniol, terpineol, linalool, thujylalcohol, menthol, citronellol and santalol. For the quantitative estimation, their behavior toward acetic acid anhydride, with which they form acetic esters when heated, may be used. Taking geranyl acetate as an illustration, their formation takes place according to the equation:
C10 H17 OH + (CH3 CO)2 O == CH3 C02 C10 H17 + CH3 C02 H.
For the quantitative acetylation1) 10 cc. of the oil, mixed with an equal volume of acetic acid anhydride and about 2 g. of dry sodium acetate, are boiled uniformly for 1 hour in a small flask provided with a condensing tube which is ground into the neck of the flask and some small fragments of tiling to prevent bumping (fig. 74). After cooling, some water is added to the contents of the flask and then heated for 1/4 hour on a water bath, to decompose the excess of the acetic acid anhydride. The oil is then separated in a separating funnel, and washed with water, or better still with sodium chloride solution, until the reaction is neutral.2)
Of the acetylized oil, dried with anhydrous sodium sulphate, 1,5 to 2,0 g. are saponified with 20 cc. of half-normal potassium hydroxide solution according to the method described on p. 572, any free acid present having previously been carefully neutralized. The percentage of alcohol corresponding to the ester value found, and referring to the original non acetylized oil, can be looked up in the table appended to this chapter, viz., table I1), p. 618. These values are derived with the aid of the following formula:
1) Report of Schimmel & Co. October 1894, 62.
2) One can also proceed to neutralize the free acid with sodium carbonate solution and then to wash with water until the reaction is neutral. In this case, however, one occasionally encounters the difficulty of removing the soda solution from the oil, a difficulty that is obviated when proceeding as indicated above.
m = the molecular weight of the respective alcohol, a = the number of cc. of half-normal potassium hydroxide solution s = the number of grams of acetylized oil used. [used,
Attention should, however, be directed to the circumstance that this formula does not take into consideration that a part of the alcohol may be contained as ester in the original oil. A small percentage of ester in the original oil may be disregarded since the error caused thereby is small. If, however, the ester content is large, or if more accurate results are desired, this error should be eliminated. For this purpose the amount of free alcohol is computed from the difference between the ester number of the original oil and the ester number of the acetylized oil (table I, p. 618, alcohol in original oil). To this is added the amount of esterified alcohol computed from the ester number. The sum is that of the percentage of total alcohol.
By way of illustration. A peppermint oil has yielded 26,43 as ester value before and 173,57 as ester value after acetylation. In order to ascertain the amount of total menthol present one proceeds as follows:
173,57 - 26,43= 147,14 = 46,07 p. c. of free menthol;
26,43 == 7,36 p. c. of ester menthol; hence total menthol = 46,07 + 7,36 = 53,43 p. c.
Without this recalculation the amount of total menthol, as computed from the ester value after acetylation, would be 55,59 p. c.
') When 1,50 g. of acetylized oil are used the alcohol content can be ascertained directly with the aid of table II on p. 632 from the number of cc. of half-normal potassium hydroxide solution employed.
The values obtainable from the ester content can also be expressed by means of the following formulas:
In the above formulas m stands for the molecular weight of the respective alcohol, a1 for the number of cc. of half-normal potassium hydroxide solution consumed by s grams of the original oil, a2 for the number of cc. of half-normal potassium hydroxide solution consumed by s grams of acetylized oil.
Hence in both instances the amounts of potassium hydroxide solution pertain to equal amounts of oil.
For purely practical reasons the statements found in the second volume as to the total alcohol are made throughout without reference to the ester content of the original oils. In other words, they are the values derived directly from the ester values of the acetylized oil. For practical purposes this will suffice in general, as the differences are not appreciable. In the case of geranium oil, however, appreciable differences occur. This is due not only to the presence of a higher ester content, but also to the fact that the esterified alcohol of the original oil is combined with tiglic acid, whereas the formula considers only acetic acid. In this instance, therefore, the amount of originally esterified alcohol and the amount of free alcohol receive separate consideration; also the total amount of alcohol derived from the two is recorded. But here also the apparent alcohol content derived from the acetylation value (ester value after acetylation) is also recorded.