Page:Outlines of Physical Chemistry - 1899.djvu/123

 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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