Theoretical organic chemistry / by Julius B. Cohen.

  • Cohen, Julius B. (Julius Berend), 1859-1935.
Date:
1942
Catalogue details

Licence: In copyright

Credit: Theoretical organic chemistry / by Julius B. Cohen. Source: Wellcome Collection.

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    CHAPTER I PURIFICATION OF SOLIDS AND LIQUIDS Before it is possible to determine the constitution of an organic substance, it is first necessary to be assured that it consists of one individual, or in other words, that it is a pure substance. It is then analysed .qualitatively and quantitatively, the weight of its molecule is determined, and finally its chemical behaviour is studied. Crystallisation.—If the substance under investigation is a solid or mixture of solids, it may be purified by cvy stalk sati on. The majority of organic substances can be obtained in the crystalline form by employing a suitable solvent, or mixture of solvents. A suitable solvent is one which dissolves much more of the substance when hot than cold, so that the hot saturated solution deposits a quantity of the solid on cooling. The usual solvents are water, boiling-point (b.p.) ioo°; methyl alcohol, b.p. 66°; ethyl alcohol, b.p. 78°; ether, b.p. 350; acetone, b.p. 56°; chloroform, b.p. 6i°; benzene, b.p. 8o°; petroleum spirit, b.p. 7° — 9° 1 ethyl acetate, b.p. 770; acetic acid, b.p. 1190, etc. It is sometimes convenient to use two miscible solvents, one of which dissolves the substance readily, and the other only slightly. Expt. 1.—Dissolve about 2 grams of acetanilide in 10 c.c. of absolute alcohol. No separation takes place on cooling. Add to the hot alcoholic solution 20 c.c. of hot water. On cooling crystals of acetanilide separate and fill the liquid. If more than one substance is present, one of the substances may be soluble and the others insoluble in the solvent. Separation is then partially effected by filtration. If they all dissolve, as more frequently happens, it is unlikely that they will be equally soluble, and consequently the first crystals which separate from the hot saturated liquid will represent the least soluble portion. If the mother-liquors are now concentrated by evaporation, a second