3.1 Stereochemistry: definition
Stereochemistry is the chemistry of molecules in three dimensions. A clear
understanding of stereochemistry is crucial for the study of complex molecules that are biologically important, e.g. proteins, carbohydrates and nucleic
acids, and also drug molecules, especially in relation to their behaviour and
pharmacological actions. Before we go into further detail, let us have a look at
different types of isomerism that may exist in organic molecules.
3.2 Isomerism
Compounds with the same molecular formula but different structures are
called isomers. For example, 1-butene and 2-butene have the same molecular formula, C 4 H 8 , but structurally they are different because of the
different positions of the double bond. There are two types of isomer:
constitutional isomers and stereoisomers.
1
2
3
4
1
2
3
4
1-Butene
2-Butene
3.2.1 Constitutional isomers
When two different compounds have the same molecular formula but differ
in the nature or sequence of bonding, they are called constitutional isomers.
For example, ethanol and dimethylether have same molecular formula,
C 2 H 6 O, but they differ in the sequence of bonding. Similarly, butane and
isobutane are two constitutional isomers. Constitutional isomers generally
have different physical and chemical properties.
OH
O
Butane
Isobutane
Ethanol
Dimethylether
3.2.2 Stereoisomers
Stereoisomers are compounds where the atoms are connected in the same
order but with different geometries, i.e. they differ in the three-dimensional
arrangements of groups or atoms in space. For example, in a-glucose and
b-glucose, the atoms are connected in the same order, but the three
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CH3 STEREOCHEMISTRY
Stereochemistry is the chemistry of molecules in three dimensions. A clear
understanding of stereochemistry is crucial for the study of complex molecules that are biologically important, e.g. proteins, carbohydrates and nucleic
acids, and also drug molecules, especially in relation to their behaviour and
pharmacological actions. Before we go into further detail, let us have a look at
different types of isomerism that may exist in organic molecules.
3.2 Isomerism
Compounds with the same molecular formula but different structures are
called isomers. For example, 1-butene and 2-butene have the same molecular formula, C 4 H 8 , but structurally they are different because of the
different positions of the double bond. There are two types of isomer:
constitutional isomers and stereoisomers.
1
2
3
4
1
2
3
4
1-Butene
2-Butene
3.2.1 Constitutional isomers
When two different compounds have the same molecular formula but differ
in the nature or sequence of bonding, they are called constitutional isomers.
For example, ethanol and dimethylether have same molecular formula,
C 2 H 6 O, but they differ in the sequence of bonding. Similarly, butane and
isobutane are two constitutional isomers. Constitutional isomers generally
have different physical and chemical properties.
OH
O
Butane
Isobutane
Ethanol
Dimethylether
3.2.2 Stereoisomers
Stereoisomers are compounds where the atoms are connected in the same
order but with different geometries, i.e. they differ in the three-dimensional
arrangements of groups or atoms in space. For example, in a-glucose and
b-glucose, the atoms are connected in the same order, but the three
36
CH3 STEREOCHEMISTRY