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."
(a) Use the imaginary replacement technique to show that the two allylic protons (those on C3) of allyl bromide are enantiotopic.
(b) Similarly, show that the two Hc protons in cyclobutanol are enantiotopic.
(c) What other protons in cyclobutanol are enantiotopic?