The furfural test for cannabis resin

Abstract

The test here described is a beautiful colour reaction and very sensitive, given by the "cannabinol compounds'' (cannabinol and related compounds) of Cannabis resin. I developed it about 1940, in large part using observations concerning the "Acid Beam" test made by G. D. Williams. Although often used by me in the years since then, it has not previously been published, nor has anyone else published anything similar, so far as I am aware.

Details

Author: Charles C. FULTON,
Pages: 33 to 34
Creation Date: 1970/01/01

The furfural test for cannabis resin

Charles C. FULTON, Chemist, Office of the State Medical Examiner, Newark, U.S.A.

The test here described is a beautiful colour reaction and very sensitive, given by the "cannabinol compounds'' (cannabinol and related compounds) of Cannabis resin. I developed it about 1940, in large part using observations concerning the "Acid Beam" test made by G. D. Williams. Although often used by me in the years since then, it has not previously been published, nor has anyone else published anything similar, so far as I am aware.

To a very small residue of the petroleum ether extract of cannabis leaf material in a white porcelain dish, or the residue of an extract, decolourized with activated charcoal, made with some other solvent (e.g., isopro-panol), add the following: 3-4 ml ethyl alcohol, 0.2-0.3 ml 1% furfural in alcohol (solution made preferably with redistilled furfural, or at least not highly coloured), and 0.2-0.3 ml conced HCl. Set on the steam bath and evaporate to dryness. The residue is somewhat greenish. Pour on it a little H 2SO 4-EtOH solution (55:45, made with absolute alcohol). A very strong purplish red colour is produced.

The test may be obtained by adding a little carbohydrate, e.g., sucrose, instead of furfural - the carbohydrate being one that readily decomposes in the acid heating, yielding furfural (or hydroxymethyl-furfural).

Hemp contains such a natural carbohydrate and a decolourized methanol extract of the upper leaves will give the test without adding either carbohydrate or furfural.

The test obviously has a relation to the "Acid Beam", and in fact was developed from it. The Beam reagent is absolute alcohol saturated with dry gaseous HC1. If it is added to an evaporated, decolourized methanol extract of cannabis, more or less orange-brown colour is produced at first, then with the dish placed on the steam bath it soon becomes deep red. Evaporated to dryness a greenish residue is left. A second application of the reagent then produces an intense purple-red colour.

The original Acid Beam test, however, was made on the residue from a petroleum ether extract. In this solvent the resin is mainly soluble, but the natural carbohydrate is not. Nevertheless, with the extract from some varieties of cannabis a colour is obtained, but usually no more than pink to weak red, simply by application of the Beam reagent to the residue from a petroleum ether extract.

The Beam test may be made simply by shaking some of the reagent with a portion of the petroleum ether containing the extracted resin. The result is frequently negative or nearly so, even though the extract is from known Cannabis and cannabinol compounds are certainly present. However, if 2 or 3 tiny grains of sucrose are then added, an intense red colour develops.

What still needs to be explained is the pink to red colour often obtained with the Beam reagent and a simple petroleum ether extract, without any furfural or carbohydrate being added. Presumably it is brought about by a glucoside that is soluble in petroleum ether; very likely a glucoside of one (or of more than one) of the cannabinol compounds.

Interesting as this "glucosidal" Acid Beam test is, it is too undependable for general use in identifying cannabis, and the preparation of the original reagent is inconvenient. The Furfural test on the other hand is one of the easiest and best in the chemical identification of cannabis.