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The image illustrates the concept of homotopic protons in organic chemistry.

Homotopic protons are chemically and magnetically equivalent. They can be interchanged by a symmetry operation (like rotation about an axis of symmetry) of the molecule. If replacing each of two protons with a different group (e.g., deuterium) leads to identical compounds, then the original protons are homotopic.

In the example of trans-1,2-Dichlorocyclopropane shown:

• The molecule possesses a C2 axis of symmetry. This axis passes through the midpoint of the bond between the two chlorine-bearing carbons and the third carbon (C3).

• Rotation of the molecule by 180 degrees about this C2 axis interchanges the two hydrogen atoms, H$^a$ and H$^b$, on the C3 carbon.

• Because H$^a$ and H$^b$ can be interchanged by a symmetry operation, they are considered homotopic.

• Implication: Homotopic protons are indistinguishable and will give a single signal in Nuclear Magnetic Resonance (NMR) spectroscopy.