Homotopic diastereotopic and enantiotopic.

What is homotopic ?

Two atoms or groups in a molecule are homotopic  if the separate replacement of each  group or atom  by some other group or atom produce the same molecule.

For example, the  α-axial , β-axial and β-equitorial, α-equitorial H-atoms in chair conformation of cyclohexane are homotopic . All the hydrogen atoms in methane are alike, so they are also called homotopic atoms.

                     Chair conformation of cyclo-hexane in 3D form .

                                                                              

What is diastereotopic ?

Two atoms or groups in a molecule are diastereotopic if replacement of each in turn by some other groups leads to a pair of diastereomers.

For example, bromination of 3-methyl pentanoic acid gives a pair of diastereomers.

The two α-H in 3-methyl pentanoic acid are diastereotopic.

What is enantiotopic ?

Two atoms or groups in a molecule are enantiotopic  if replacement of each in turn by some other group leads to a pair of enantiomers.

When optically active compounds are prepared by synthetic methods, the usual results is a racemic modification.

For example, bromination of propionic acid gives DL α-bromopropionic acid.

Although, the two H-atoms in propionic acid are alike, yet replacement of one or other hydrogen does not produce same molecule, a pair of enantiomers is produced. This two H-atoms are therefore said to be enantiotopic.

Addition of HCN to acetaldehyde is also an example of enantiotopic.

Why meso tartaric acid is optically inactive?

Meso tartaric acid contains two chiral carbon atoms, yet  it is optically inactive. This is due to the fact that meso tartaric acid has a plane of symmetry.

Practice summary:

What is homotopic ? Give example .

What is diastereotopic ? Give example .

What is enantiotopic ? Give example .

Click here: Racemic modification and resolution.