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Optical Isomerism



Optical isomerism

AQA Content

Draw the structural formulas and displayed formulas of enantiomers
Understand how racemic mixtures (racemates) are formed and why they are optically inactive.

Specification Notes

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-super-imposable mirror images and differ in their effect on plane polarised light.
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).


Optical isomerism
There are two types of stereoisomerism - molecules having the same structural formula but arranged differently: geometrical (E/Z) and optical. Optical isomerism is associated with chirality.

A chiral molecule is a molecule that has a chiral carbon or chiral centre, which is a carbon atom that has four different atoms or groups of atoms attached to it. The term "chiral" comes from the Greek word for "hand," which is fitting as these molecules have a 'handedness'. A common example of a chiral molecule is a simple sugar molecule, such as glucose or butan-2-ol...

Enantiomers of butan-2-ol
When a compound has a chiral centre, it exists as two optical isomers, also known as enantiomers. These enantiomers are non-superimposable mirror images of each other, much like your left and right hand. They have the same chemical formula, but their atoms are arranged in space differently, resulting in different properties.

One of the key differences between enantiomers is that they have different abilities to rotate plane polarised light. When unpolarised light is passed through a polariser, the light becomes polarised, as the waves will vibrate in one plane only. One enantiomer will rotate the plane of polarised light clockwise, while the other will rotate it anticlockwise. This difference in the rotation is known as optical activity and is what makes these isomers "optical" isomers.

To distinguish between enantiomers, (+) and (-) notation is used (other types of notation are also used). The rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance. This can be done by passing 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 clockwise or anticlockwise.


Structural Isomers: Post that looks at a number of different types of isomer
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