Replacement Test Definition:

A test for determining the relationship between two protons. The compound is drawn two times, each time replacing one of the protons with deuterium. If the two compounds are identical, the protons are homotopic. If the two compounds are enantiomers, the protons are enantiotopic. If the two compounds are diastereomers, the protons are diastereotopic.

Replacement Test Explained:

To determine that the molecule has been rotated specifically, when the two protons on the alpha carbon of ethanol are not interchangeable by rotational symmetry. These protons are therefore not homotopic. Nevertheless, they can be interchanged by reflection symmetry.