(Solved): Geometric (cis-trans) Isomers (ONLY FOR ALKENES) The structure of 2 -butene can be represented in t ...

Geometric (cis-trans) Isomers (ONLY FOR ALKENES) The structure of 2 -butene can be represented in two ways: Since a carbon-carbon double bond (or sigma + pi bond) is not free to rotate, these represent two unique geometric forms of 2 -butene. If a plane is drawn through form I above, both methyl groups are on the same side of the plane. In form Il, the methyl groups lie on opposite sides of the plane. These two froms are known as cis (from Latin "on this side") and trans (from Latin "across") isomers of 2-butene. This type of isomerism can occur around any \( C=C \) bond as long as two different groups are bonded to each carbon involved in the double bond. Geometric isomers are closely related but do have distinct properties. cis-2-butuene \( \left(b p=4^{\circ} \mathrm{C}\right) \) trans-2-butuene \( \left(b p=1^{5} \mathrm{C}\right) \) cis-1,2-dichloroethene \( \left(\mathrm{mp}=-81^{\circ} \mathrm{C} ; b p=60^{\circ} \mathrm{C}\right) \) trans-1,2dichlorosthene \( \left(\mathrm{mp}=-81^{\circ} \mathrm{C} ; b p=48^{\circ} \mathrm{C}\right) \)