How to Recognize Chemical Equivalency and Symmetry in Hydrogens - dummies

How to Recognize Chemical Equivalency and Symmetry in Hydrogens

By Arthur Winter

Every hydrogen in a molecule that’s in a unique chemical neighborhood will show up as a peak on a nuclear magnetic resonance (NMR) spectrum. Two (or more) hydrogens that have equivalent chemical neighborhoods, though, will be represented by just a single peak. Such hydrogens that are in identical chemical environments are said to be chemically equivalent.

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For example, methanol (shown here) has four hydrogens in it, so you might expect it to show four peaks in the 1H NMR spectrum, one for each hydrogen. But it only shows two peaks. This is because there are only two different kinds of hydrogens in methanol. All the hydrogens in the methyl group (CH3) are in identical chemical environments — all three are attached to a carbon that’s bonded to two other hydrogens and an alcohol group — so each of these hydrogens sees the same chemical neighborhood. Therefore, all three hydrogens have the same resonance frequency and show up as a single peak. The second peak comes from the H on the alcohol (OH) group, which is in a different chemical environment.

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Butane (shown here) is another example of a molecule that has such chemical equivalency. Ten hydrogens are on the molecule, but, remembering the chemical equivalency of methanol, you might expect to see four peaks, one for each of the hydrogen sets attached to the carbons. But the NMR spectrum of butane has only two peaks. This is accounted for by a further element of equivalency that comes from the symmetry of the molecule. Because the right-hand side of the molecule is exactly equal to the left-hand side, the two CH3 groups on the ends are in identical chemical environments, as are the two CH2 groups in the middle. Because both CH3 groups are in identical chemical environments, and both CH2 groups are in identical chemical environments, you see only two peaks in the NMR spectrum.

Being able to recognize which hydrogens are chemically equivalent is extremely important. Generally, hydrogens attached to the same carbon are chemically equivalent. In addition, symmetry in the molecule can contribute further to chemical equivalency. You should practice looking at organic molecules for chemical equivalency so that you can determine how many peaks its 1H NMR will have.

Often, drawing out all the hydrogens on the molecule can help you see the different chemical environments each hydrogen experiences. Try this for the molecules shown here.

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