Question

1.draw a geometric isomer of butene.

2.draw a geometric isomer of 2-butene.

Answer 1

Geometric Isomerism

Geometric isomerism occurs when substances have the same molecular formula, but a different arrangement of their atoms in space. There are three ways that this can happen:

• where there is a C=C bond in the molecule;
• where a molecule has rings; or
• where there is a >C=N bond.

An interesting example of geometric isomerism caused by rings is found in sugars such as glucose, fructose, mannose and galactose.

Below are models of the geometric isomers of 1- butene and 2-butene. 1-Butene does not form geometric isomers, even though it has a C=C bond, because one of the double-bonded carbon atoms has two identical groups on it (hydrogen atoms in this case). 2-Butene does form geometric isomers because each double-bonded carbon atom has two different groups on it. The cis- and trans- prefixes are used to differentiate the positions of the functional groups.

cis-1-butene	trans-2-butene	Where like groups are on the same side of the double bond, we call it a cis isomer; where they are on opposite sides we call it atrans isomer.
1-butene	Although 1-butene contains a C=C bond, it does not form geometrical isomers.