20.3.1 Stereoisomers

• Isomers are compounds that have the same molecular formula but a different arrangement of atoms
• Isomers can be grouped into various categories, as shown:

Flow chart of the various isomers with points to help identify them

• At Standard Level, we encountered three types of structural isomers:
  • Functional group isomers, e.g. propanal and propanone
  • Position isomers, e.g. propan-1-ol and propan-2-ol
  • Branch-chain isomers, e.g. butane and methylpropane

• If the atoms within an isomer are arranged in the same order then we are dealing with stereoisomers
  • Stereoisomers can be conformational or configurational

Conformational Isomers

• Conformational isomers, or conformers, occur due to free rotation about a single σ-bond and can be described as:
  • Staggered
  • Eclipsed

• One of the simplest examples of conformational isomerism is ethane, CH₃CH₃

Three-dimensional structure of ethane identifying the bond for conformational isomerism

• By looking along the C-C bond highlighted in the diagram we can draw the two Newman projections, staggered and eclipsed

The staggered and eclipsed conformers of ethane

• The staggered conformer has angles between hydrogen atoms on adjacent carbons of 60^o, as shown
  • It is also more stable / lower energy than the eclipsed conformer because the C-H bonds are as far apart as possible to minimise the repulsion between the electrons in the C-H bonds

• The eclipsed conformer has angles between hydrogen atoms on adjacent carbons of 0^o, this is not shown in the diagrams so that the conformation can be seen
  • The eclipsed conformer is less stable / higher energy due to the repulsion between the electrons in the C-H bonds that are closer together

• The free rotation that causes these conformers means that it is easy to interconvert from one conformer to the other and back
  • This is also the reason that it is almost impossible to isolate a single conformer

Conformational Isomerism in Cyclic Structures

• Conformational isomerism can also be seen in cyclic structures
• A common example of this is cyclohexane, C₆H₁₂
  • Cyclohexane isomers exist in boat and chair forms:

Skeletal structures showing the boat and chair forms of cyclohexane

• The boat form is less stable / higher energy as there are four eclipsed bonds causing strain on the overall structure
  • There is also repulsion of the hydrogen atoms on the end of the boat structure

• It is possible to "flip" between the boat and chair forms which explains the difficulty in isolating just one of the forms
  • During the interconversions, it also possible to get other structures commonly called the half chair and the twisted boat

Configurational Isomers

• Interconversion of configurational isomers can only occur by breaking bonds or rearranging stereocentres
• Configurational isomers can be divided into:
  • cis / trans isomers and E / Z isomers
  • optical isomers