Identifying Chiral Centres (AQA A Level Chemistry)

• Identifying chiral centres in a molecule takes a bit of practice whether it is from a formula, a 2D drawing or a 3D drawing
• To be successful you need to differentiate the carbon atoms and determine one of the following:
  • Whether a particular carbon is bonded to four different atoms or groups of atoms and therefore is chiral
  • Whether a particular carbon is bonded to two of the same atoms or groups of atoms and therefore cannot be chiral

• If you are given a molecular formula you need to either draw the molecule as a condensed structural formula or a displayed formula so you can see all the bonds and groups

Answer

Recognising chiral centres in 3D drawings

• You should be able to spot chiral centres in 3D drawings
• Chiral centres are marked with an asterisk (*)
• For example, glyceraldehyde contains a chiral centre on the middle carbon:

A 3D drawing space filling model of glyceraldehyde

• You can show the chiral carbon in a displayed formula like this

A displayed formula for glyceraldehyde showing the the chiral centre

Drawing optical isomers

• You need to use stereochemical drawing conventions to represent optical isomers
• In the convention:
  • a solid line is a bond in the same plane as the paper
  • a dotted line is a bond receding behind the plane of the paper(this can also be hatched or shaded wedges)
  • a solid wedge is a bond coming out of the paper

• For example, suppose you are asked to draw the optical isomers of the amino acid alanine, CH₃CH(NH₂)COOH
  • Start by drawing a vertical dotted line to represent a line of symmetry in the centre of your page
  • Next draw the chiral carbon with four bonds in a tetrahedral arrangement
  • Make sure two bond lie in the plane of the paper, one comes out and one recedes
  • Add the four groups, but be careful to show the mirror image sequence of atoms

Drawing optical isomers