Isomerism

Structural isomerism = molecules with the same molecular formula but with different structural arrangement of atoms

Stereoisomers = compounds with the same structural formula, but with a different arrangement of atoms in space

Can occur in three ways:

• the hydrocarbon chain can be branched or unbranched (eg butane and 2-methylpropane)

• a functional group can be in different positions along the main hydrocarbon chain (eg propan-1-ol and propan-2-ol)

• The functional groups may be different (eg propanal and propanone)

A molecule needs:

• a carbon-corbon double bond must be present (ALWAYS ALKENES)
• each carbon in the double bond must be attached to two different groups (eg H and Cl)

It is similar to cis-trans isomerism. Cis-trans isomerism is a special type of E/Z isomerism in which there is a non-hydrogen group and a hydrogen group on each C of C=C double bond.

In cis-trans isomerism:

• the cis isomer is the Z isomer ("zusammen" meaning together)
• the trans isomer is the E isomer ("entgegen" meaning opposite)

For example: C₂H₂Cl₂

has the isomers E-1,2-dichloroethene (trans - groups on opposite sides) and

Z-1,2-dichloroethene (cis - groups on the same side)