Chirality (CIE AS Chemistry)

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

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

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

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

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
  • 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

Worked example: Drawing geometrical isomers