CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.1.12 Chirality

Chirality & Enantiomers

Chiral centres in non-cyclic molecules

  • A chiral centre in a molecule is a carbon atom that has four different atoms or group of atoms attached
  • This gives rise to two optical isomers which are also called enantiomers
  • The enantiomers are mirror images of each other and cannot be superimposed

 

An Introduction to AS Level Organic Chemistry Enantiomers and Chiral Centre, downloadable AS & A Level Chemistry revision notes

The presence of the chiral centre in the molecule allows two enantiomers to exist which are stereoisomers as the molecules have the same atoms bonded to each other, but they are differently arranged in space

  • When the molecule contains more than one chiral centre (asymmetric carbon) more than two optical isomers will be formed
    • If there are two chiral centres, each chiral centre will rotate the plane of polarised light clockwise and anticlockwise
    • There are four possible optical isomers

 

An Introduction to AS Level Organic Chemistry Multiple Chiral Centres, downloadable AS & A Level Chemistry revision notes

Each chiral centre gives rise to two optical isomers; therefore, the molecule has a total of four optical isomers

Chiral centres in cyclic molecules

  • To determine the chiral centre in a cyclic molecule, the carbon bonded to four different atoms or groups of atoms should be found
    • Eg. 1,2-aminocyclohexanol has two chiral centres so it can form four optical isomers

 

An Introduction to AS Level Organic Chemistry Chiral Centre Cyclic Molecules, downloadable AS & A Level Chemistry revision notes

To decide where the chiral centres are in a cyclic molecule, the carbon atoms bonded to four different atoms or atom groups should be found

Exam Tip

Use a molecular modelling kit and make the models of enantiomers to help you understand that the two molecules are non-superimposable and therefore non-identical.

Identifying Chirality & Geometrical Isomerism

Identify chirality

  • Identifying chiral centres in cyclic and non-cyclic compounds is very straightforward as it is the carbon with four different atoms or atom groups in a molecule
  • This gives rise to two optical isomers
  • When more than two chiral centres are present, more than two optical isomers exist
  • A molecule with three chiral centres will have six optical isomers
  • A molecule containing chiral centres is called a chiral molecule

Identifying geometrical isomers

  • Molecules with restricted rotation about the C-C bond can have geometrical isomers
  • This includes unsaturated and cyclic compounds
    • Eg. alkenes and cyclopentane
  • When the groups are positioned on the same side of the C-C double bond, the compound is a cis isomer
  • When the groups are positioned on opposite sides of the C-C double bond the compound is a trans isomer

Worked example: Drawing optical isomers

An Introduction to AS Level Organic Chemistry Worked example - Drawing optical isomers, downloadable AS & A Level Chemistry revision notes

An Introduction to AS Level Organic Chemistry Answers Worked Example - Drawing optical isomers, downloadable AS & A Level Chemistry revision notes

Worked example: Drawing geometrical isomers

An Introduction to AS Level Organic Chemistry Worked example - Drawing geometrical isomers, downloadable AS & A Level Chemistry revision notes

An Introduction to AS Level Organic Chemistry Answers Worked Example - Drawing geometrical isomers (1), downloadable AS & A Level Chemistry revision notes An Introduction to AS Level Organic Chemistry Answers Worked Example - Drawing geometrical isomers (2), downloadable AS & A Level Chemistry revision notes

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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