§ 444. Properties of Atropine, C17H23NO3.—Atropine, hyoscyamine, and hyoscine have all the same formula, but differ in their molecular constitution. Atropine by hydrolysis, either by heating it with hydrochloric acid or baryta water, is decomposed into tropine and tropic acid:—
| C17H23NO3 | + | H2O | = | C8H15NO | + | C9H10O3. |
| Atropine. | Tropine. | Tropic acid. | ||||
On the other hand, by heating tropic acid and tropine together, atropine is regenerated. Hence it is proved by analysis and synthesis, that atropine is tropic acid-tropine, just as aconitine is benzoyl-aconine. Tropic acid has been produced synthetically by boiling β-chlorphenyl-propionic acid with potash, which at once shows its constitutional formula, viz.:—
Tropic acid has a melting-point of 117° to 118°. Tropine is a four-fold hydrated oxethyl-methyl-pyridine, and has the constitutional formula of C5H3(H4)(C2H4OH)N(CH3); hence the constitutional formula of atropine is—
Tropine is a white, crystalline, strongly alkaline mass, melting at 60°, and volatilising at 230° undecomposed. It is soluble in water, alcohol, and ether, and gives precipitates with tannic acid, iodised hydriodic acid, Mayer’s reagent, gold chloride, and mercuric chloride. Tropine gold chloride melts at 210° to 212°. Atropic acid (C9H8O2), melting-point 198° to 200°, and isatropic acid (C9H8O2), may also be obtained by the action of hydrochloric acid—the first, in radiating crystals, melting at 106°, and capable of distillation; the second, in thin rhombic plates, melting about 200°, and not volatile. Picric acid also gives a precipitate of beautiful plates. To obtain this the carbazotic acid must be in excess, and time must be given for the precipitate to form.
Atropine forms colourless crystals (mostly in groups or tufts of needles and prisms), which are heavier than water, and possess no smell, but an unpleasant, long-enduring, bitter taste. The experiments of E. Schmidt place the melting-point between 115° and 115·5°. It is said to sublime scantily in a crystalline form, but the writer has been unable to obtain any crystals by sublimation; faint mists collect on the upper disc, at about 123°, but they are perfectly amorphous.
Its reaction is alkaline; one part requires, of cold water, 300; of boiling, 58; of ether, 30; of benzene, 40; and of chloroform, 3 parts for solution. In alcohol and amyl alcohol it dissolves in almost every proportion. It turns the plane of polarisation weakly to the left.
§ 445. Tests.—Atropine mixed with nitric acid exhibits no change of colour. The same is the case with concentrated sulphuric acid in the cold; but on heating, there ensues the common browning, with development of a peculiar odour, likened by Gulielmo to orange flowers, by Dragendorff to the flowers of the Prunus padus, and by Otto to the Spiræa ulmaria—a sufficient evidence of the untrustworthiness of this as a distinctive test. The odour, indeed, with small quantities, is certainly not powerful, nor is it strongly suggestive of any of the plants mentioned. A far more intense odour is given off if a speck of atropine is evaporated to dryness with a few drops of strong solution of baryta, and heated strongly; the scent is decidedly analogous to that of hawthorn-blossom, and unmistakably agreeable.