Chirality (Cambridge (CIE) A Level Chemistry): Revision Note

Chirality & Enantiomers

Chiral centres in non-cyclic molecules

• A chiral centre in a molecule is a carbon atom that has four different atoms or groups 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

 Chiral centres in non-cyclic molecules 

• 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

Molecules with multiple chiral centres

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

  • E.g. 2-aminocyclohexanol has two chiral centres so it can form four optical isomers

Chiral centres in cyclic molecules

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

  • The maximum number of stereoisomers that a molecule can have is 2ⁿ, where n is the number of chiral centres

• So, a molecule with three chiral centres will have 2³ = eight 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

  • E.g. 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