Identification of Chiral Centres (OCR A Level Chemistry)

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Identification of 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

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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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.