Chapter 23 of 31 · 2965 words · ~15 min read

CHAPTER VII

THE VEGETABLE ALKALOIDS

Vegetable alkaloids may be classified in three groups: (1) derivatives of pyridine, _e.g._ atropine, conine; (2) derivatives of quinoline, _e.g._ cinchonine, narcotine; (3) substituted amines and amides. The majority of the vegetable alkaloids belong to the first two groups. They are for the most part solid, crystalline, and colourless; a few, such as conine, nicotine, and pilocarpin, are liquid. They combine with acids to form salts, and the salts are more soluble in water than the free alkaloid. Alkaloids possess certain properties in common, amongst which is that of being precipitated from their solutions by certain reagents, which are called alkaloidal grouping reagents. Some are precipitated by all the group reagents, others only by a few.

Alkaloidal Group Reagents

1. _Iodine dissolved in solution of Potassium Iodide_--_Wagner‘s reagent_.--Gives a reddish-brown precipitate with most alkaloids.

2. _Phosphomolybdic Acid_--_Sonnenschein‘s reagent_.--Made by dissolving phosphomolybdate of soda in water containing one-tenth its volume of strong nitric acid. It gives a yellow precipitate with most of the alkaloids; it also precipitates ammonium salts and ammonia derivatives, also salts of lead, silver, and mercury unless there be sufficient acid to keep them in solution.

3. _Potassio-mercuric Iodide_--_Mayer‘s reagent_.--Made by adding a solution of potassium iodide to one of mercuric chloride until the red precipitate first formed be just dissolved. This solution precipitates most of the alkaloids. The solution to be tested must contain acetic acid.

4. _Phosphotungstic Acid_--_Scheibler‘s reagent_.--This acts in a manner very similar to phosphomolybdic acid.

Methods for detecting Vegetable Alkaloids

There are several methods recommended for the isolation and detection of the vegetable alkaloids, and their separation from the contents of the stomach or from the membranes and tissues of the body. The process, however, most generally pursued is that of Stas, which may be briefly described as follows:

(_a_) The substance to be examined is mixed with twice its weight of absolute alcohol, to which from ten to thirty grains of tartaric or oxalic acid--preferably the former--have been added, and the mixture subjected to gentle heat in a flask, 70° to 75° C., or 158° to 167° F.

(_b_) If the membranes or organs have to be examined, they are finely divided, treated with absolute alcohol, squeezed, and again treated with fresh alcohol as in (_a_).

In either case, the mixture, when quite cold, is filtered, and the alcoholic solution is concentrated by evaporation, either _in vacuo_ or in a current of air not exceeding 95° F. or 35° C.

The liquid residue is now passed through a moistened filter, which separates the fat and other insoluble matters. The filtrate is evaporated to dryness over sulphuric acid or _in vacuo_, and the acid residue of this evaporation dissolved in the smallest possible quantity of distilled water. The acid liquid is then _gradually_ neutralised with the bicarbonate of potash or soda until effervescence ceases, and afterwards shaken in a flask with four or five times its bulk of pure ether, and allowed to settle. When the ether has become quite clear, a small portion of it is decanted into a small glass capsule, and allowed to spontaneously evaporate in a dry place. If during evaporation streaks of liquid appear on the side of the capsule, running together at the bottom, a liquid volatile alkaloid is probably present. If none of these manifestations occur, the alkaloid is in all probability solid and non-volatile.

----------------------------------------+----------------------------------- _The Alkaloid is Volatile_. | _The Alkaloid is Non-Volatile_. ----------------------------------------+----------------------------------- To the original mixture in a flask | To the original mixture in a add a moderate quantity of a strong | flask add strong caustic potash or solution of caustic potash or soda, | soda solution, and agitate with mixed with ether; agitate, and allow | successive portions of pure ether the mixture to settle. Pour off | allowing it to completely settle the ethereal solution, and re-shake | each time. The ethereal solutions, residue with a fresh quantity of | being mixed, are evaporated, leaving ether; decant, and mix both solutions. | the alkaloid in an impure state. The ethereal solution is | To purify it, the solid residue left now shaken with a mixture of four | on evaporation is treated with a parts of water and one of sulphuric | small quantity of dilute sulphuric acid, which withdraws the alkaloid | acid, which dissolves the alkaloid, from its solution, leaving any fatty | leaving any fatty impurities behind. matter dissolved in the ether. The | The acid liquid is evaporated acid solution is now mixed with | to three-quarters of its bulk over strong potash or soda solution in | strong sulphuric acid, and then a excess,[20] agitated with ether, the | saturated solution of carbonate of ether poured off, and then evaporated | potash or soda added. The absolute at as low a temperature as | alcohol will then dissolve out the possible,[21] leaving the pure alkaloid| pure alkaloid, giving it, on with all its characteristic chemical | in the crystalline form, and in and physical properties. | evaporation, a state to show its | characteristic reactions. ----------------------------------------+-----------------------------------

[20] The sulphates of alkaloids are insoluble in ether; hence they must be decomposed by an alkali.

[21] The temperature should be low, or the greater part of the conine will be evaporated with the ether.

If morphine has to be sought for, the liquid should be shaken with ether immediately after being neutralised with carbonate of sodium, and the ether poured off as quickly as possible; for if the alkaloid have time to separate in the crystalline form, scarcely any of it is dissolved by the ether (Otto).

The method of Stas is based upon the fact that the salts of the alkaloids, as a class, are soluble in water and alcohol, but are insoluble in ether; and that these salts when in solution are readily decomposed by the mineral alkalies with the elimination of the alkaloids, which, in their free and uncombined state, are more or less readily soluble in ether.

Otto‘s Method.--Otto‘s modification of Stas‘s process is simpler, and at the same time equally accurate. Instead of numerous treatments and evaporations which have to be gone through in the original process, Otto converts the alkaloid into a salt, such as the sulphate, by the addition of acid, and after solution in a small quantity of water, agitates with successive quantities of ether, which remove all foreign fatty matters, leaving the solution of the alkaloid comparatively pure, and from which the alkaloid may be obtained in a state of great purity, by first rendering the solution alkaline and then using ether to dissolve the alkaloid.

R. Wagner‘s Method.--The presence of alkaloids in organic liquids--strychnia in beer, for example--may, according to R. Wagner (_Zeitschr. Anal. Chem._, vol. iv. p. 387), be detected by mixing the liquid, diluted with two vols. water (½ to 1 litre), with about 5 c.c. of a solution of iodine in potassium iodide (12.7 grains iodine to the litre) and a few drops of sulphuric acid. The precipitate separated from the supernatant liquid is dissolved in a dilute solution of sodium hyposulphite, and again precipitated by means of the iodine solution. If this new precipitate be now dissolved in aqueous sulphurous acid, the solution will leave, on evaporation, the pure sulphate of the base.

Dragendorff‘s Method.--This is intended for the purpose of separating alkaloids from each other when more than one are in aqueous solution, by using different solvents in sequence. Some solvents take up certain alkaloids to the exclusion of others. The process consists of extracting the aqueous acid solution of the alkaloids successively with petroleum spirit, benzene, chloroform, and amyl-alcohol, then alkalising it and repeating with the same solvents.

1. From the _acid_ solution _benzene_ removes caffeine, colchicine, santonin, digitalin, cantharidin. _Chloroform_ removes papaverine, colchicine, narceine, picrotoxin.

2. From the _alkaline_ solution _petroleum ether_ removes strychnine, brucine, aconitine, veratrine, conine, nicotine, lobeline, emetine, and aniline. _Benzene_ removes atropine, hyoscyamine, physostigmine, codeine, narcotine, and further quantities of strychnine, brucine, aconitine, veratrine, and emetine. _Chloroform_ removes morphine, narceine, papaverine, strychnine, and brucine. _Amyl-alcohol_ removes morphine, solanine, and narceine.

The Stas process cannot be recommended for the detection of opium in organic liquids, for two reasons. Firstly, that it altogether fails to indicate the presence of meconic acid; and, secondly, because morphine is almost insoluble in ether. Dragendorff recommends the use of _benzole_ for separating the alkaloids, but in this substance morphia is nearly insoluble. It is, however, applicable to strychnine, aconitine, conine, and atropine; but for the two last, on account of their volatility, ether is preferable.

Rodger‘s and Girdwood‘s Method.--Extraction with dilute hydrochloric acid and the use of chloroform instead of ether. Chloroform is a much better solvent of most alkaloids than ether. Particularly useful for the isolation of strychnine and for most alkaloids, but there is a little danger of hydrolysis of the alkaloid in the use of a mineral acid, _e.g._ hyoscine.

Stevenson‘s Modification of the Otto-Stas Process.--The material to be examined, if solid, is finely divided, and digested for twenty-four hours with twice its weight of rectified spirit at 35° C.; if fluid, with twice its volume. The clear liquid is decanted and the residue again digested with fresh spirit; this is again decanted, and mixed with the first alcoholic solution. The residue is now digested with spirit faintly acidified with acetic acid; this is decanted, and the residue digested with two or three lots of unacidified alcohol. The alcoholic extracts obtained before acidification are mixed together and rapidly raised to 70° C. for a moment or two. They are quickly cooled and filtered, and the filter washed with spirit. The acidified extract and those after it are mixed and treated in the same way. The extracts are then separately evaporated at a temperature not above 35° C. to the consistency of a syrup, the excess of acid being neutralised with soda; these are extracted with absolute alcohol, and the extracts evaporated to a syrup as before. The syrupy extracts are now diluted with a small quantity of water, filtered, the filters washed with water, and the filtrates mixed. The liquid will contain the whole of the alkaloids, and will be free from albuminoids, which have been coagulated while the extracts were at 70° C. The liquid containing the alkaloids is extracted several times with washed ether, which removes fatty acids or oils, but does not remove alkaloidal salts. The ether should be washed with water to which a few drops of sulphuric acid has been added, and the water kept: this has to be done because some alkaloidal salts are slightly soluble in ether. The acid liquid and the acidified aqueous washings of the ether are mixed together, rendered alkaline with sodium carbonate, and exhausted firstly with a mixture of one volume of chloroform to three of ether, and lastly three or four times with ether alone.

The alkalisation with sodium carbonate liberates the alkaloids from their salts, and these are soluble in the chloroform-ether and ether. These ethereal extracts are then washed with water acidified with sulphuric acid, and water alone, and the washings mixed. The water acidulated with sulphuric acid converts them into sulphates, which are insoluble in the ether and chloroform, and are removed by the acidified water, while impurities are left behind. The mixed aqueous and acid extracts are again washed with ether, the ether removed, and the liquid re-alkalised with sodium carbonate and then re-extracted with chloroform-ether and ether.

The ethereal solutions are removed and are washed with water slightly alkalised with sodium carbonate. The ethereal solution is filtered through a dry filter, the filtrate evaporated to dryness first at 35° C. then at 100° C., and cooled over sulphuric acid. The residue is weighed and represents the weight of the alkaloids. A test quantity should be evaporated to see if there be any oily odorous residue, _i.e._ a volatile alkaloid, nicotine or conine. If so, the chloroform and ether extracts should be mixed with a little pure ether and strong hydrochloric acid; the alkaloids are thus changed into non-volatile hydrochlorides, which are left behind after evaporation of the chloroform and ether. Any alkaloid found should be converted into the hydrochloride, dissolved, and tested by special tests. Morphine cannot be extracted except in very minute amounts by this method. To obtain it, the first alkaline solution from which the other alkaloids have been removed should be extracted with acetic ether and ether, in which morphine is soluble.

[Illustration: Fig. 36.--Photo-micrograph of crystals of hydrochloride of morphine, × 50.

(R. J. M. Buchanan.)]

Taylor‘s method for the extraction of morphine may be briefly described as follows:

The liquid--porter, &c.--to be examined is acidified with acetic acid; or, if a solid organ is to be tested, it must be cut into thin slices and placed in distilled water acidified in a similar way. In either case the liquid is digested for one or two hours at a gentle heat, and filtered. Acetate of lead is now added to the filtrate until no further precipitation occurs; the liquid is then boiled and filtered. The meconic acid remains on the filter as meconate of lead, while the filtrate contains the morphine as acetate. The liquid is freed from excess of lead by passing through it a current of sulphuretted hydrogen, filtered to remove the precipitated sulphide of lead, and the resulting liquid evaporated to an extract on a water bath, and treated with alcohol. The alcoholic solution on evaporation gives acetate of morphine, which may then be tested.

[Illustration: Fig. 37.--Photo-micrograph of meconic acid crystallised from aqueous solution, × 50. (R. J. M. Buchanan.)]

[Illustration: Fig. 38.--Photo-micrograph of meconic acid crystallised from an alcoholic solution, × 50. (R. J. M. Buchanan.)]

The meconate of lead which remains on the filter is decomposed by treating it with dilute sulphuric acid, and gently boiling the mixture. The filtered liquid should be neutralised before the tests for the presence of meconic acid are applied.

The reactions of both morphine and meconic acid are best seen from the following Table:--

The characteristic tests for morphine are its reactions with nitric acid, iodic acid and starch, and perchloride of iron. The reaction with the perchloride of iron is also characteristic of meconic acid. This last-mentioned test is a very conclusive one for meconic acid, when certain precautions are taken; for the property of striking a deep red with a persalt of iron is shared equally by sulphocyanides and alkaline acetates. The colour produced by sulphocyanic acid _is instantly bleached_ on the addition of _corrosive sublimate_. The question thus lies between acetic and meconic acid. To distinguish the one from the other, the solution to be tested should be boiled for a short time after the addition of a few drops of sulphuric acid. Any acetate present is decomposed and the acetic acid is expelled by the boiling; so that if, after allowing the solution to cool, it still gives the red colour with perchloride of iron, the reaction may be taken as conclusive of meconic acid. By these means morphine and meconic acid may be detected in porter and other liquids.

Table showing the Characters and Tests of the Following Poisons ------------------------------+------------------------------- Morphine. | Strychnine. ------------------------------+------------------------------- 1. Crystallises in colourless| 1. Crystallises in white transparent prisms, belonging| four-sided prisms, terminated to the trimetric | by four-sided pyramids. system. | | 2. Sulphuric acid and | 2. Treated with cold sulphuric bichromate of potash give | acid, no reaction; on a bright-green coloration. | the addition of a crystal of | potassium bichromate, an | intense purple colour is | produced, becoming crimson | and then light red. | 3. Strong colourless nitric | 3. Strong nitric acid usually acid, added freely to a cold | produces a yellow or solution, produces a deep | yellow-brown colour. orange-red coloration, not | changed by stannous chloride.| ------------------------------+----------------------------------- Brucine. | Narcotine. ------------------------------+--------------------------------- 1. Crystallises in oblique | 1. Crystallises in right rhomboidal prisms, sometimes | rhombic prisms, or in needles agglomerated mushroom-like | grouped in bundles. heads. | | 2. Sulphuric acid gives a | 2. Sulphuric acid a bright rich rose-pink tint; on the | sulphur-yellow colour, potassium addition of potassium | bichromate added a bichromate, none of the | green colour as with morphine, reactions of strychnine | but slower in production. are observed. | | 3. Strong nitric acid | 3. Strong nitric acid forms produces a blood-red | a colourless fluid, becoming colour, changed after | yellow on heating. warming and diluting with | distilled water to purple | by stannous chloride; | ammonium sulphide gives | a similar but less marked | reaction. Excess of | stannous chloride | discharges the blood-red | colour in the cold. | ------------------------------+---------------------------------