Optical Isomerism Optical isomerism occurs in carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon). C H C H C H H H C H H H O H H A carbon atom that has four different groups attached is called a chiral (asymmetric) carbon atom These four groups are arranged tetrahedrally around the carbon. C H5C2 CH3 H OH C C2H5 H3C H OH This causes two different isomers that are not superimposable to be formed. They are mirror images Two compounds that are optical isomers of each other are called enantiomers. A mixture containing a 50/50 mixture of the two isomers (enantiomers) is described as being a racemate or racemic mixture. Many naturally occurring molecules contain chiral C atoms, but are usually found in nature as a pure enantiomer Optical isomers have similar physical and chemical properties, but they rotate plane polarised light in different directions. One enantiomer rotates it in one direction and the other enantiomer rotates it by the same amount in the opposite direction. Different systems of nomenclature are is existence for optical isomers. D/L or +/- are commonly used, but both have been superseded by the more useful and informative R/S system (this is not on the syllabus – for information only). One optical isomer will rotate light clockwise (+)(called dextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory). 7 -ve enantiomer Anticlockwise rotation +ve enantiomer clockwise rotation Racemate no rotation Priority group side 1 Priority group side 2 Z-1,2-dichloroethene E-1,2-dichloroethene Cl C C Cl H H Cl C C H H Cl A racemic mixture (a mixture of equal amounts of the two optical isomers) will not rotate plane-polarised light. N Goalby chemrevise.org C C C C C C C C O O N C C C C C O O N H H H H H H H H H H Chemical Reactions and Optical Isomers Formation of a racemate A racemate will be formed in a reaction mechanism when a trigonal planar reactant or intermediate is approached from both sides by an attacking species :CN NC: C CH3 O H C NC CH3 OH H C H3C CN OH H There is an equal chance of either enantiomer forming so a racemate forms. No optical activity is seen Major product s 90% Minor product 10% If the alkene is unsymmetrical, addition of hydrogen bromide can lead to isomeric products. A racemate can also be formed in the AS reaction of the electrophilic addition of HBr to an unsymmetrical alkene H2C CH CH2 CH3 H Br H3C C + CH2 CH3 H C + C CH2 CH3 H H H H C CH3 Br H CH2 CH3 C CH3 Br H CH2 H3C :Br – :Br – The bromide can attack this planar carbocation from both sides leading to a racemate CH2 CH2 CH2 CH3 Br δ + δ – Drug action and optical isomers Drug action may be determined by the stereochemistry of the molecule. Different optical isomers may have very different effects Chiral carbon Thalidomide One enantiomer of thalidomide causes birth defects in unborn children whilst the other had useful sedative problems. Unfortunately it was given in a racemic mixture when first used. CH C CH CH C CH CH2 CH CH3 H3C CH CH3 C OH. Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the HCN attacking species: results in the formation of a racemate Mechanism for the reaction (drawn the same for both enantiomers)
3.3.7 Optical isomerism (A-level only)
Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.
An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non superimposable mirror images and differ in their effect on plane polarised light.
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).
Students should be able to:
• draw the structural formulas and displayed formulas of enantiomers
• understand how racemic mixtures (racemates) are formed and why they are optically inactive.