Chemical equivalence

Equivalent protons, also termed chemical equivalence, is the concept of a group of protons being in the exact same chemical environment. As they are indistinguishable, this leads to the protons giving a single signal together in proton NMR.

Example 1

In ethanol, the three protons in the methyl group are equivalent, because each of them are in the exact same chemical environment (remember that there's free rotation around the single (σ) bonds). The protons in the methyl group are NOT equivalent with the other protons in the molecule, as their chemical environment is different.

3 overall peaks, peak at 1.1 ppm triplet, peak at 2.7 ppm small singlet, peak at 3.7 ppm quartet, the axis ranges from 0-4 ppm and the structure of ethanol is at the top of the spectrum.

Figure 1. NMR spectrum and structure of ethanol.

Example 2

When considering equivalent vs. non-equivalent protons, it's important to pay attention to symmetry. Check out the structure of the compound in the spectrum below. Because there's symmetry around the -CH group, the two methyl (-CH₃) groups are in the exact same chemical environment, which means the combined 6 protons will give a combined signal in the spectrum (at 0.9 ppm). Note that this is due to the symmetry in the molecule, not because they are both methyl groups.

3 peaks overall, peak at 0.9 ppm doublet, 1.8 ppm multiplet, 3.2 ppm doublet, structure of 1-bromo-2-methylpropane in top left.

Figure 2. NMR spectrum and structure of 1-bromo-2-methylpropane.