All that would be needed to change cis-2-butene to trans-2-butene
is to break the double bond momentarily, twist the molecule around,
then reconnect the same bond. One cannot go from 2-butene to either
of the other structural isomers by breaking, twisting, and then
re-forming the same bond. Cis-trans isomerism is possible whenever
two different groups are connected to each end of a double bond
("cis-" means "on the same side," and "trans-"
means "across from").
All of the isomers of butene have similar chemical and physical
properties, and the distinction between geometrical and structural
isomers is less significant to their properties than is the actual
shape of the molecule.
1-Butene has a "swivel' about the bond from the CH3CH2-
to the C=C, and is a flexible molecule. It therefore packs clumsily
into a solid crystalline lattice, and melts at a low -1850C.
All of the other three isomers are rigid.
Trans-2-butene is a lumpy rod that packs well with its neighbors
in the solid and hence does not melt until -1060C.
The other two isomers are more irregular, but are similar in shape
and melt at an intermediate temperature within 10C
of each other.
Shape is less important in affecting boiling points because the
molecules in a liquid already move freely past one another, so all
four isomers boil within a 110C