Diastereotopic

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Like ligands in an organic molecule that are not equivalent (see equivalent ligands) under any condition are classified into two classes.

Diastereotopic Ligands

When the replacement test is applied to two like ligands in a molecule, if the resultant molecules are diastereomers, the two ligands are said to be diastereotopic.

eg:

Apply the replacement test to find if the two bromine atoms in 1 are diastereotopic.

Molecules 2 and 3 are diastereomers. Thus, the two bromine atoms in 1 are diastereotopic. Diastereomers have different chemical properties under all conditions. Since the replacement of Br (a) and of Br (b) in 1 leads to molecules that have different chemical properties under all conditions, Br (a) and Br (b) have different chemical properties under all conditions.

Constitutionally Heterotopic Ligands

When the replacement test is applied to two like ligands in a molecule, if the resultant molecules are constitutional isomers, the two ligands are said to be constitutionally heterotopic.

eg:

Apply the replacement test to find if the two methyl groups in 1 are constitutionally heterotopic.

Molecules 2 and 3 are constitutional isomers. Thus, the two methyl groups in 1 are constitutionally heterotopic. Constitutional isomers have different chemical properties under all conditions. Since the replacement of CH₃ (a) and of CH₃ (b) in 1 leads to molecules that have different chemical properties under all conditions, CH₃ (a) and CH₃ (b) have different chemical properties under all conditions.