are held together are shown in perspective as situated in a plane perpendicular to that of the paper. In fig. 25 a simpler arrangement is adopted.
The possibility of isomerism depends on the similarity or variety of the atoms or groups which saturate the four free valencies.
Of such types as :
a — C — a a — C — a a — C — a
II II II
a — C — a , a— G — b , b — C — b
��there is only one form possible, ever, possible for the types,
�Isomeric forms are, how-
�a—G—b
II a—G—b
and for
a—G—b
II a — C — c
�and and
�a—G—b
II b—G—a
a—G—b
Jl c — C — a
��The two atoms or groups a may be both on the same side, or they may be on different sides of the plane in which the two carbon atoms and the two bonds joining them lie. In the first case we have the cis or malenoid form, and in the second case the trans or fumaroid form. Examples of this class of isomerism are very numerous.
Fumaric acid,
H.C.C0 2 H
��Maleic acid,
��HOsC.C.H
��H.C.C0 2 H
II H.C.C0 2 H
Orotonic and isocrotonic acids,
H.C.CH3 CH3.C.H
II and ||
H.C.C0 2 H H.C.C0 2 H
�� �
