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
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
- Start with a central chiral carbon
- Add 2 lines for the bonds that are in the plane
- Add one solid wedge for the bond that is coming forward, out of the plane
- Add one dashed wedge for the bond that is going backwards, out of the plane
- 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