Virtual Lessons

Lesson 2 of 4

Geometric Isomerism in Alkenes

This virtual lesson explains geometric isomerism in alkenes, including how restricted rotation about the carbon-carbon double bond gives rise to distinct isomers with different physical and chemical properties.

The lesson opens by revisiting the nature of the pi bond and why it prevents free rotation about the C=C double bond. This restricted rotation is the fundamental reason geometric isomers can exist, as groups attached to each carbon are locked in position relative to one another.

The conditions required for geometric isomerism are then established: each carbon of the double bond must carry two different substituents. Where this condition is met, two distinct arrangements are possible, giving rise to cis and trans isomers, or more precisely described using the E/Z naming system.

The E/Z system is then applied using Cahn-Ingold-Prelog priority rules, which rank substituents based on atomic number. Where the two higher priority groups are on the same side of the double bond the isomer is assigned the Z configuration; where they are on opposite sides it is assigned the E configuration.

This lesson builds directly on the bonding concepts introduced previously and develops the ability to identify, name, and draw geometric isomers, a skill that appears regularly in examination questions.