Di-derivatives x x x p v as $ v as s Here we have assumed the substituent groups to be alike; when they are unlike, a greater number of isomers is possible.
The next members are the -isomers anthracene and phenanthrene, C14H,0, formed from three benzene nuclei.
This tetrahedral configuration is based on the existence of only one methylene dichloride, two being necessary if the carbon valencies were directed from the centre of a plane square to its corners, and on the existence of two optical isomers of the formula C. A.
Thus in the tri-substitution derivatives six isomers, and no more, are possible when two of the substituents are alike; for instance, six diaminobenzoic acids, C 6 H 3 (NH 2) 2 000H, are known; when all are unlike ten isomers are possible; thus, ten oxytoluic acids, C 6 H 3 -CH 3.
In the case of tetra-substituted compounds, thirty isomers are possible when all the groups are different.