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 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
Worked example
Determine which isomers of C4H9Br contain chiral carbons
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
