The practical methods of organic chemistry / by Ludwig Gattermann ; translated by William B. Schober.

  • Ludwig Gattermann
Date:
1901
Catalogue details

Licence: In copyright

Credit: The practical methods of organic chemistry / by Ludwig Gattermann ; translated by William B. Schober. Source: Wellcome Collection.

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    takes place. The tubes are now cooled by contact with cold water, and an observation will show in which tube crystals have separated in the largest quantity. At times crystallisation does not occur on mere cooling; in this case the walls of the vessel are rubbed with a sharp-angled glass rod, or the solution is “seeded,” i.e. a small crystal of the crude product is placed in the solution; by this means, crystallisation is frequently induced. If the individual solvents of Class I. are shown to be unsuitable, experiments are made with the mixtures, — Class II. Compounds which are easily soluble in alcohol or glacial acetic acid, and which consequently do not separate out on cooling, are, as a rule, difficultly soluble in water. In order to determine whether a separation of crystals will take place on cooling, the hot solutions in the pure solvents are treated with more or less water, according to the conditions. Substances easily soluble in ether, benzene, toluene, etc., often dissolve in ligroi'n with difficulty. Hence mixtures of these solvents can be frequently utilised with ad- vantage, in the manner just described. If these experiments have shown several solvents to be suitable, the portions under examina- tion are again heated until solution takes place, and this time are allowed to cool slowly. That solvent from which the best crystals separate in the largest quantity is selected for the crystal- lisation of the entire quantity of the substance. If a substance is easily soluble in all solvents, recourse must be had to crystallisa- tion by evaporation, i.e. by allowing the different solutions to stand some time in watch-glasses. That solvent from which crystals separate out first is the most suitable. Frequently a compound dissolves in a solvent only on heating and yet does not crystallise out again on cooling; compounds of this class are said to be “sluggish” (trage). In this case, the solution may be allowed to stand for some time, if necessary over night, in a cool place. If a compound is very difficultly soluble, solvents with high boiling- points are used, as toluene, xylene, nitrobenzene, aniline, phenol, and others. The crystals obtained in these preliminary experi- ments, especially if they are of easily soluble substances, are pre- served, so that if from the main mass of the substance no crystals
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