A tert-butyl group contains how many equivalent protons?

• A. 3
• B. 6
• C. 9
• D. 12

Answer: (C)

Equivalent protons and molecular symmetry determine how many distinct proton environments exist. A tert-butyl group is C(CH3)3, with a central carbon bonded to three methyl groups. The three methyl groups are related by a threefold rotation symmetry; swapping the CH3 groups leaves the molecule unchanged, so all three methyl groups occupy the same chemical environment. Within each CH3, the three protons are chemically equivalent as well, and across the three methyl groups they’re averaged to one environment by rapid conformational averaging. Therefore all nine hydrogen atoms of the tert-butyl group are equivalent, giving a single 1H NMR signal with integration of 9. This is why the number of equivalent protons is nine.