The two protons here are equivalent and will give one NMR signal. You can see this by flipping the molecule 180o which produces the same molecule:
You can also visualize the symmetry plane reflecting protons a and b:
The following molecule does not have a plane of symmetry. However, protons a and b are exchangeable through a C2 (180o) symmetry axis and therefore, one NMR signal is expected too:
In both molecules, the protons were cis to a bromine and overall in the same environment.
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There is a trait for equivalent protons in alkenes which shows that the two protons must be cis to the same group:
In the last molecule, proton a is cis to the methoxy group while proton b is cis to the bromine which puts them in different environment and therefore, two NMR signals will be observed.
The non-equivalence of these two protons is also proved by the absence of symmetry elements between them.
The equivalent and non-equivalent protons are classified more rigorously as homotopic, enantiotopic, diastereotopic and constitutionally heterotopic. We will talk about these definitions and the methods for determining them in the next post and before doing that, here are somePractice problems determining the number of signals in MNR spectroscopy.
How many signals would you expect to see in the 1H NMR spectrum of each of the following compounds?
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