Cis- Trans isomers.

https://www.youtube.com/watch?v=UYdwwtTjBi0

What are stereoisomers

They are molecules with the same structural formula but the atoms are arranged differently in space.

Whats a key idea about covalent bonds

1,2-dibromoethane is being shown

A key idea we must understand is that single covalent bonds are fully rotational, so all of these diagrams show the same molecule.

What is key idea about double covalent bonds

They cannot fully rotate like alkanes, they have restricted movement, so they have 2 different forms.

These are called geometrical isomers of 1,2-dibromoethene.

The left hand is called cis-1,2-dibromoethene

the right hand molecule is trans-1,2-dibromoethene.

What are the two conditions that alkenes must meet to have cis / trans isomers

The carbon atom on the double bond must be attached to two different groups.

At least one of those groups must be the same on both carbon atoms on the double bond.

How do we choose what the cis isomer is

We look at the same which is the same on both sides of the double bond

Hydrogen atoms are on same side on left, so must be cis.