Page:Outlines of Physical Chemistry - 1899.djvu/115

 groups reduces the number of possible isomers. Still, for the formula

CH 2 (OH).(GH(OH)) 4 .CH 2 (OH)

we can expect ten optical isomers, two of which are inactive by intramolecular compensation, and four racemic modifications.

The structure of these substances has been elucidated by E. Fischer and his students.

Bemarks : 1. Thanks to the work of Guye, Franlcland, and a few other chemists, we are now beginning to learn the laws which regulate the rotatory power.

In the first place, it would seem that, so long as the order of the size of substituting groups attached to the asymmetric carbon atom of a substance remains unchanged, so long does the direction of the rotation remain the same. This can be seen from tartaric acid ; the dextro-acid gives neutral esters (diethyl tartrate, for example) which are also dextro-rotatory. The diacetyl derivative of this acid, however, is laevo-rotatory ; but here the order of the size of the groups is modified, for the group O.CO.CH 3 (originally OH) is greater than the group C0 2 H.

This rale, however, is not quite general, an exception to it being found in the etherification of laevo- amyl alcohol. In this case there can be no doubt that it is the heaviest group which is made heavier (CH 2 OH becomes CH 2 OB), and therefore the order is not changed ; yet the rotation of the ether is in the opposite direction to that of the original alcohol. From this we see that, besides their relative size, the specific influence of the groups must be taken into account.

When we examine an homologous series derived from the same active nucleus, the regularities become more apparent if we consider the molecular rotation. For the esters of lsevo-menthol Tchugaeff has found the following numbers :

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