For the corresponding meta- and or£/&o-derivatives the effect becomes more complicated owing to the asymmetry of the carbon atoms to which the C0 2 H groups are linked.
The derivatives of dihydro- and tetrahydrobenzene are still more complicated. As an example, let us take dihydroterephthalic acid. The position of the double bonds determines four cases of isomerism :
CO,H CO,H
��i
��c
/s /\
HC CH H 2 C CH
c c
��H CO,H CO,H
��C0 2 H H C0 2 H
��C C
/\ /\
H 2 C CH HC CH
II II
��H 2 C CH HC CH
V V
1 y\
C0 2 H H C0 2 H
The first of these four formulae contains an asymmetric carbon atom, and so we conclude that optically active isomers of it exist. The last formula is a cis -modification, and so a corresponding trans- form must also exist.
In reality, Baeyer has isolated 5 dihydroterephthalic
acids.
Stebeo-chemistry of Nitrogen. — Cases of isomerism are known for some substances containing nitrogen, and
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