Question

a) For the following compounds which pairs of hydrogen (H_a and H_b) are enantiotopic hydrogens?

b) which pairs are diastereotopic hydrogens?

Short answer

a) Structure A

b) Structure B, Structure C, Structure D

Step-by-step solution

Definition of enantiotopic and diastereotopic hydrogens

If carbon is attached to two protons and to two different groups, then the protons are said to be enantiotopic hydrogens. These two hydrogens are called enantiomers if deuterium is placed instead of these protons.

Diastereotopic hydrogens results in diastereomers when replacing two identical hydrogens with another group

Likely enantiotopic hydrogens

(a)

In diagram A, butane contains enantiotopic hydrogens. Replacing H_a and H_b proton of second carbon of butane creates a pair of enantiomers. Identical protons are attached to different groups in the same carbon so, these hydrogens are enantiotopic hydrogens.

Likely diastereotopic hydrogens

(a)

In diagram D, Propene contains diastereotopic hydrogens. Replacing H_a and H_b proton results in a pair of diastereomers. This can be represented as shown below.

Propene

b.

Bromocyclobutane

In diagram B, bromocyclobutane contains diastereotopic hydrogens.Replacing geminal protons that is adjacent to the chiral central results in diastereomers.

c.

1,1- methylcyclopentane

In diagram C, 1,1-methylcyclopentane contains diastereotopic hydrogens.When H_a and H_b protons are replaced by deuterium, it results in a pair of diasteromers.