E, Z isomerism
We solve our difficulty by using the E,Z system.
Think of 'Z'as meaning 'ze zame zide' ;
These two compounds are
isomers, with three different groups attached to the C=C. The -COOH group has the highest priority so
Angelic acid is (Z)-2-methyl-2-butenoic acid.
and tiglic acid is (E)-2-methyl-2-butenoic acid.
Here is another example of
isomers where three different groups are attached to the central
C=C.
In the red box: ........................... In the blue box:
This example is a little
different because there are four different groups attached to the
central C=C.
Considering the position of
the chlorine atoms in these two molecules, we give these names:
On the left: (E)-1,3-dichloro-2-butene.
.................On the right: (Z)-1,3-dichloro-2-butene.
These two
compounds are cis- and trans- isomers of 2-butene.
They are easily recognized.
But how should we name these two compounds?
There is a difficulty as there are three different groups attached to the C=C.
We look at the (groups of) atoms attached to the C=C.
the two H atoms are on the same side of the molecule.
(Z)-1-chloro-2-butene............................ (E)-1-chloro-2-butene.
