100 OUTLINES OF PHYSICAL CHEMISTRY
In his researches on the carbohydrates, E. Fischer has on more than one occasion made use of this mode of resolving the racemic modifications.
��APPENDIX
��In order to complete our study of the stereo-chemistry of carbon, it is necessary to say a word on a kind of isomerism peculiar to substances containing one or more double linkages in the molecule.
In the stereo-chemical interpretation of the behaviour of substances towards polarised light, which we have just discussed, it has been assumed that two carbon atoms united together by a single bond are at liberty to execute rotatory movements round their common axis. 1 If" this were not the case, if there were a rigid union between the two carbon atoms, then we ought to find a very much greater number of isomers than really exist.
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But this free rotation is no longer possible when two neighbouring carbon atoms become united together by two bonds. And in this case we meet with isomers the interpreta- tion of which requires further stereo-chemical considerations.
In Fig. 24 the two bonds by which the carbon atoms
1 According to Wislicenus^ the atoms or groups carried by the two carbon atoms exercise a directive action on each other, and the carton atoms revolve until the system takes up the most stable con- figuration.
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