A LevelChemistryOCRRevision Notes6. Organic Chemistry & AnalysisOptical IsomerismIdentification of Chiral Centres

Identification of Chiral Centres (OCR A Level Chemistry A): Revision Note

Exam code: H432

Richard Boole

Written by: Richard Boole

Reviewed by: Caroline Carroll

Updated on

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: displayed formulae of the four isomers of C4H9Br (1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane, 2-bromo-2-methylpropane) with each carbon checked; only 2-bromobutane has a carbon bonded to four different groups and is optically active

There are four structural isomers of C4H9Br:

  • 1-bromobutane, CH3CH2CH2CH2Br: no carbon has four different groups, not chiral

  • 2-bromobutane, CH3CH2CHBrCH3: C2 is bonded to H, CH3, Br and C2H5, so it is chiral and the isomer is optically active

  • 1-bromo-2-methylpropane, (CH3)2CHCH2Br: C2 has two CH3 groups, not chiral

  • 2-bromo-2-methylpropane, (CH3)3CBr: C2 has three CH3 groups, not chiral

Only 2-bromobutane contains a chiral carbon.

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:

Ball-and-stick model of glyceraldehyde with an asterisk marking the chiral carbon, which is bonded to H, OH, CH2OH and CHO

A 3D ball-and-stick model of glyceraldehyde

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

Displayed formula of glyceraldehyde, HOCH2-CH(OH)-CHO, with an asterisk marking the middle carbon as the chiral centre

A displayed formula for glyceraldehyde showing 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

Stepwise drawing of the two optical isomers of alanine as mirror images across a dashed vertical mirror line, using solid, dashed and wedge bonds to show the tetrahedral arrangement of H, CH3, COOH and NH2

Drawing optical isomers

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Author
Reviewer
Richard Boole

Author: Richard Boole

Expertise: Curriculum Expert

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.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.