Which compounds exhibit geometric isomerism? Draw and name...

Which compounds exhibit geometric isomerism? Draw and name the two isomers in each case:
$$
\begin{array}{ll}{\text { (a) propene }} & {\text { (b) } 3 \text { -hexene }} \\ {\text { (c) } 1,1 \text { -dichloroethene }} & {\text { (d) } 1,2 \text { -dichloroethene }}\end{array}
$$

Key Concepts

E/Z Nomenclature

This is an alternative system used for naming geometric isomers based on the Cahn-Ingold-Prelog priority rules. When the simple cis-trans notation is insufficient or ambiguous, the E (entgegen, meaning 'opposite') and Z (zusammen, meaning 'together') descriptors provide a clear method for distinguishing between isomers.

Substituent Requirements

For geometric isomerism to occur, each carbon atom involved in the double bond must have two different substituents. If one of the carbons bears two identical substituents, the molecule will not exhibit geometric isomerism. This requirement is critical in determining whether compounds like certain alkenes can exist as distinct isomers.

Alkene Stereochemistry

Alkenes have a double bond that is planar due to sp2 hybridization, which forces substituents to lie in the same plane. This planarity is crucial for geometric isomerism, as different spatial arrangements (such as cis versus trans) can occur when each carbon of the double bond has two different substituents.

Geometric Isomerism

This concept involves compounds that have the same connectivity of atoms but differ in the spatial arrangement of substituents around a rigid structure, such as a carbon–carbon double bond. The lack of free rotation about the double bond leads to fixed orientations, creating distinct isomers.

Cis-Trans Isomerism

A common type of geometric isomerism where the relative positions of substituents across a double bond are different: 'cis' denotes substituents on the same side, while 'trans' indicates that they are on opposite sides. This distinction is valid only when there are two different groups attached to each carbon of the double bond.

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