#"Cis"# and #"trans"# isomers are compounds with identical formulae AND IDENTICAL connectivity, BUT DIFFERENT GEOMETRIES. I can best illustrate this by consideration of a pair of simple geometric isomers: #"cis-2-butene"# and #trans"-2-butene"#: #H_3C-CH=CH-CH_3#.
Both isomers have the SAME CONNECTIVITY. #C1# is connected to #C2# is connected to #C3# is connected #C4# for both isomers. However, the double bond between #C2# and #C3# opens the possibility of geometric isomerism. #C1# and #C4# may be on the same side of the double bond, the #cis# isomer, or on opposite sides of the double bond, the #trans# isomer.
While these species are geometric isomers, they do NOT have different colours. In fact, most organic compounds are clear colourless liquids (or gases.) The isomers of 2-butene should have different properties, however. The #trans# isomer has a boiling point of #1# #""^@C#, and the #cis# isomer has a boiling point of #4# #""^@C#. (I would check on the boiling points of these isomers, because I am not sure of the source. Normally, the #cis# isomer is more volatile than the #trans# isomer.)
Disubstituted ring systems, for instance, #"1,3-dimethyl-cyclohexane"#, also allows the possibility of geometric isomerism. How?
