Page:Outlines of Physical Chemistry - 1899.djvu/113

 acetaldehyde gives a-allylpyridine, which, reduced with metallic sodium, gives inactive coniine.

8. If to a saturated solution of inactive-coniine acid tartrate a crystal of dextro-coniine acid tartrate be added, the dextro- salt crystallises out, leaving the salt of Icevo- coniine in the mother liquor.

Many other examples might be cited, such as valerianic acid, amyl alcohol, asparagine, &c.

Substances with Two Asymmetric Carbon Atoms. — Four modifications are possible, namely, a dextro- and a laevo-, an inactive (which cannot be resolved), and a racemic form.

Tartaric Acid :

co 2 h— Ghoh— Ghoh— co 2 h.

Our knowledge of the optical isomers of this acid is chiefly due to Pasteur's researches.

We know the following four varieties :

1. The dextro-rotatory acid. This is the ordinary tartaric acid obtained from cream of tartar. It separates without water of crystallisation in monoclinic prisms. It is dextro-rotatory in aqueous solution. Its salts frequently have small hemihedral faces on their crystals.

2. The racemic acid which crystallises with one molecule of water. It can be obtained by dissolving an equimole- cular mixture of the dextro- and laevo- acids. To prepare the racemic acid the dextro-acid is heated with about a tenth of its weight of water to 175°C. Inactive and racemic acids are both formed, and can be separated by crystallisation.

.The racemic acid crystallises with one molecule of water in efflorescent rhombic crystals. It is less soluble than the dextro-acid and forms salts similar to those of the dextro-acid but without any hemihedral facets.

The racemic acid can be separated into the two active forms in the following manner. A solution of the acid sodium salt is saturated with ammonia and the solution is

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