For compound below compare the indicated protons and determine whether they are homotopic, enantiotopic or diastereotopic_ H H a. diastereotopic b. enantiotopic homotopic
The Correct Answer and Explanation is:
The correct answer is a. diastereotopic.
To determine the stereochemical relationship between the two indicated protons, we can use the substitution test or analyze the symmetry of the molecule.
1. The Substitution Test
This method involves replacing each proton in question with a hypothetical group, typically deuterium (D), and then comparing the relationship between the two resulting molecules.
- The compound shown is bicyclo[2.2.1]heptane, also known as norbornane. The two protons are on the C7 methylene bridge. Due to the rigid bicyclic structure, one proton is oriented towards the C2-C3 bridge (the syn position), and the other is oriented away from it (the anti position).
- If we replace the syn proton with deuterium, we create 7-syn-deuterionorbornane.
- If we replace the anti proton with deuterium, we create 7-anti-deuterionorbornane.
Now, we compare these two new molecules. They have the same connectivity but a different three-dimensional arrangement, so they are stereoisomers. They are not mirror images of each other; the mirror image of the syn isomer is another syn isomer (its enantiomer), not the anti isomer. Stereoisomers that are not mirror images are defined as diastereomers. Since replacing the original protons leads to the formation of diastereomers, the protons are diastereotopic.
2. The Symmetry Analysis
This method involves checking if the two protons can be interchanged by a symmetry operation.
- Homotopic protons can be interchanged by an axis of rotational symmetry (Cn).
- Enantiotopic protons can be interchanged by a plane of reflection (σ) or a center of inversion (i).
- Diastereotopic protons cannot be interchanged by any symmetry operation.
The norbornane molecule is achiral and does possess symmetry elements, including a C2 axis of rotation and two perpendicular planes of reflection. However, none of these symmetry elements can interchange the syn and anti protons on the C7 carbon. The C2 axis and both planes of symmetry pass through the C7 carbon atom, but they do not reflect or rotate one proton into the position of the other.
Because the two protons are locked in fixed, non-interchangeable positions with different spatial relationships to the rest of the molecule, they exist in different chemical environments. Therefore, they are diastereotopic. This means they are chemically distinct and would produce separate signals in an NMR spectrum.
