Question

If the imaginary replacement of either of two protons forms enantiomers, then those protons are said to be enantiotopic.The NMR is not a chiral probe, and it cannot distinguish between enantiotopic protons. They are seen to be “equivalent by NMR”.

1. Use the imaginary replacement technique to show that the two allylic protons (those on) of allyl bromide are enantiotopic.
2. Similarly, show that the two H^Cprotons in cyclobutanol are enantiotopic.
3. What other protons in cyclobutanol are enantiotopic?

Short answer

a.

b.

The products formed are enantiomers and hence, H_Cprotons are enantiotopic.

c.The H_d protons are enantiotopic.

Step-by-step solution

Step-1. Explanation of part (a):

a.

Allylic protons of allyl bromide are enantiotopic in nature. By replacing the protons H_A and H_Bwith some atom “Z”, we get two products and these products are enantiomers of each other. Enantiomers are non-superimposable mirror images of each other and enantiotopic protons are not distinguishable by NMR.

Step-2. Explanation of part (b)

b.

Cyclobutanol has protons H_C and H_dand both are enantiotopic protons. On replacing the protons H_c with some atom “Z”, we get the two products which are enantiomers of each other as they are non-superimposable mirror images of each other.

Step-3: Explanation of part  (c):

c.

The H_d protons are also enantiotopic in cyclobutanol.