Enantiomers (HL) (DP IB Chemistry): Revision Note

Enantiomers

What are optical isomers?

• Optical isomers are chemicals that contain a chiral carbon or chiral centre 

  • A chiral carbon atom has four different atoms or groups of atoms attached to it

  • Chira comes from a Greek word meaning hand, so we talk about these molecules having a handedness

• The carbon atom is described as being asymmetric, i.e. there is no plane of symmetry in the molecule

• Compounds with one chiral centre (chiral molecules) exist as a pair of optical isomers, also known as enantiomers

  • Enantiomers are drawn using stereochemical formulae - see our revision note on Representing Formulas of Organic Compounds

• Just like the left hand cannot be superimposed on the right hand, enantiomers are non-superimposable

  • Enantiomers are mirror images of each other

How optical isomers form

How to draw optical isomers

• Optical isomers are drawn using stereochemical formulae to show the tetrahedral arrangement around the chiral carbon

  1. Start with a central chiral carbon

  2. Add 2 lines for the bonds that are in the plane

  3. Add a solid wedge for the bond projecting towards the viewer

  4. Add a dashed wedge for the bond going behind the plane (away from the viewer)

  5. Then draw a mirror image of this

Basic structure of all optical isomers

• The four different atoms or functional groups are added to each carbon on enantiomer 1

  • Enantiomer 2 then has the same atoms or functional groups added but in a way that forms the mirror image of enantiomer 1

Can optical isomers contain more than one chiral carbon?

• Optical isomers can contain one or more chiral carbons

  • Isomers with one chiral centre will form enantiomers or mirror images

• Diastereomers are compounds that contain more than one chiral centre

  • Diastereomers are not mirror images of each other because each chiral carbon has two isomers

  • This means that they have different physical and chemical properties

Diastereomers of 2-bromo-3-chlorobutane

2-bromo-3-chlorobutane exists as a diastereomer due to 2 chiral centres

Properties of optical isomers

Chemical properties

• Knowledge of the different chemical properties of enantiomers is limited to different behaviours in chiral environments.

• For example, optical isomers interact with biological sensors in different ways:

  • One enantiomer of carvone smells of spearmint

  • The other enantiomer of carvone smells of caraway

Optical isomers of carvone

Physical properties 

• Optical isomers have identical physical properties, with one exception:

• Isomers differ in their ability to rotate the plane of polarised light

  • This means that enantiomers are described as optically active

  • Diastereomers are not typically optically active 

How a polariser works

• The major difference between the two enantiomers is:

  • One enantiomer rotates plane polarised light in a clockwise direction and the other in an anticlockwise direction

  • A common way to differentiate the isomers is to use (+) and (-), but there are other systems using d and l, D and L, or R and S

• The rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance

  • For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance

  • Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise by a fixed number of degrees

How enantiomers interact with plane polarised light

What is a racemic mixture?

• A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer

  • One enantiomer rotates light clockwise, the other rotates light anticlockwise

• A racemic mixture is optically inactive as the enantiomers will cancel out each other's effect

  • This means that the plane of polarised light will not change

How a racemic mixture interacts with plane polarised light

Racemic mixtures and drugs

• In the pharmaceutical industry, it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug

• Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures

• Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer

• This separation process is very expensive and time-consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active

• For example, the pain reliever ibuprofen is sold as a racemic mixture

The structural formula of ibuprofen showing the chiral carbon