AH Chemistry - Organic Chemistry and Instrumental Analysis - (c) - Stereo Chemistry

What are isomers?

Isomers are molecules that have the same molecular formula but different structural formulae.

When do structural isomers occur?

Structural isomers occur when the atoms are bonded together in a different order in each isomer.

When do stereoisomers occur?

Stereoisomers occur when the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer.

What are the two types of stereoisomer?

The two types of stereoisomer are:

• Geometric

• Optical

When can geometric isomers occur?

Geometric isomers can occur when there is a restricted rotation around a carbon-carbon double bond or a carbon-carbon single bond in a cyclic compound. They must have two different groups attached to each of the carbon atoms that make up the bond with restricted rotation.

What are the different ways geometric isomers can be labelled?

The different ways geometric isomers can be labelled as:

• Cis

• Trans

When can geometric isomers be labelled as cis?

Geometric isomers are labelled as cis when the substituent groups are on the same side of the bond with restricted rotation. add pic

When can geometric isomers be labelled as trans?

Geometric isomers are labelled as trans when the substituent groups are on different sides of the bond with restricted rotation. add pic.

What can geometric isomers have differences in?

Geometric isomers can have differences in:

• Physical properties, such as melting point and boiling point.

• Chemical properties.

When can optical isomers occur?

Optical isomers occur in compounds in which four different groups are arranged tetrahedrally around a central carbon atom (the chiral carbon or chiral centre). Optical isomers are asymmetric and are non-superimposable mirror images of each other. They can be described as enantiomers.

What properties are identical in optical isomers?

Optical isomers have identical:

• Physical properties, except for their effect on plane-polarised light.

• Chemical properties, except for when in a chiral environment such as that found in biological systems (only one optical isomer is usually present).

How are optical isomers optically active?

Optical isomers rotate plane-polarised light by the same amount but in opposite directions and so are optically active.

What happens when optical isomers are mixed in equal amounts?

When optical isomers are mixed in equal amounts, they are optically inactive because the rotational effect of the plane-polarised light cancels out. This is called a racemic mixture.