AQA A Level Chemistry

Revision Notes

7.1.2 Identifying Chiral Centres

Identifying Chiral Centres

  • 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 whether a carbon has more than one of any atom/group
  • 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

Worked Example

Determine which isomers of C4H9Br contain chiral carbons

Answer

Worked Example answer, downloadable AS & A Level Chemistry revision notes

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:

Glyceraldehyde 3D, downloadable AS & A Level Chemistry revision notes

A 3D drawing space filling model of glyceraldehyde

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

Glyceraldehyde Displayed formula, downloadable AS & A Level Chemistry revision notes

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, downloadable AS & A Level Chemistry revision notes

Drawing optical isomers

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