optical isomerism/isomers

when does optical isomerism occur

When there is an asymmetrical carbon within an organic molecule known as a chiral centre

Optical isomerism

Type of stereoisomerism

Chiral centre

Carbon atom with four different groups bonded around it so there is no line of symmetry to the molecule

What does the presence of a chiral centre lead to

The presence of two possible isomers that are mirror images of each other (optical isomers)

What are the two different optical isomers called

Enantiomers

How are enantiomers unique

Their effect on plane polarised light - each enantiomer causes the rotation of plane polarised light by 90 degrees in opposite directions

What is a racemate/racemic mixture

A mixture that contains equal amounts of two optical isomers called enantionmers

Why is a racemic mixture optically inactive

The rotation occurs in equal but opposite directions so they cancel out

When are racemic mixtures formed in organic reactions

When a reaction produces two enantiomers in equal amounts

Example of when racemic mixtures are formed in organic reactions

nucleophilic addition to carbonyls

What is the usual situation when racemic mixtures are formed in organic reactions

usually when a molecule with a planar functional group is attatched from either side with equal probability

When does nucleophilic addition occur

Occurs when a nucleophile attacks a molecule containing a carbonyl group (C=O), the carbonyl group is polar meaning the carbon is S+ and attracts nucleophiles, oxygen is S-

Why does nucleophilic addition produce optical isomers

The carbonyl group (C=O) is flat and planar, this means the nucleophile can attack the carbonyl carbon from above the plane OR from below the plane (both attacks occur with equal likelihood), these two possibilities create two mirror image products, which are enantiomers (forming a racemic mixture)

Why does the nucleophilic attack from both sides equally

Because the planar sp² - hybridised carbonyl group does not block attack from either side

What is the final result of nucleophilic addition to a carbonyl

Two chiral products (optical isomers), made in equal amounts, that form a racemic mixture, which is optically inactive