Enantiomers (HL) (HL IB Chemistry)

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

A molecule has a chiral centre when the carbon atom is bonded to four different atoms or group of atoms; this gives rises to enantiomers

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 one solid wedge for the bond that is coming forward, out of the plane
  4. Add one dashed wedge for the bond that is going backwards, out of the plane
  5. Then draw a mirror image of this

Basic structure of all optical isomers

All optical isomers exist in pairs and are represented by stereochemical formulae

• 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

The different optical isomers of carvone have distinctive smells

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

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

• 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

Each enantiomer rotates the plane of polarised light in a different direction

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 are optically inactive because the enantiomers cancel each other's ability to rotate 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

The chiral carbon of ibuprofen is responsible for the racemic mixture produced in the synthesis of the drug