Optical Isomerism

• A type of Stereoisomerism.
• Stereoisomers have same structural formula, but are arranged differently in space.

• Optical isomers have a chiral carbon atom.
• A chiral (or asymmetric) carbon atom is one that has four different groups attached to it.
• The chiral carbon is usually represented with an asterisk.

• Chiral molecules cannot be superimposed on their mirror image.

• The two non-superimposable mirror images are optical isomers or enantiomers.

• The enantiomers are mirror images.

• They are Non-Superimposable.

• If molecules can be superimposed, they are achiral- and there is no optical isomerism.

1. Locate chiral centre- look for C with 4 different groups attached.

2. Draw one enantiomer in a tetrahedral shape- put chiral carbon atom at centre and the 4 different groups in a tetrahedral shape around it (don't draw out full structure of each shape, just use structural formulas to make less complicated).

3. Draw mirror image of enantiomer- Put in a mirror line next to the enantiomer then draw mirror image of enantiomer on the other side of it.

• Optical isomers are optically active- they rotate plane polarised light.
• Isomers differ in their reaction to plane-polarised light
• Plane polarised light vibrates in one direction only
• One isomer rotates light to the right, the other to the left
• Rotation of light is measured using a polarimeter
• Rotation is measured by observing the polarised light coming out towards the observer.
• One enantiomer rotates clockwise (+), and one anticlockwise (-).

• A racemate (or racemic mixture) contains equal quantities of each enantiomer or an optically active compound.
• racemates do not show any optical activity- The 2 enantiomers cancel each other's rotating effect.
• Chemists often react 2 achiral things together and get a racemic mixture of a chiral product. This is because when two molecules react, there is normally an equal chance of forming each of the enantiomers.