The first step of a nitration reaction involves using a mixture of sulfuric acid and nitric acid to form an NO₂⁺ electrophile, as shown below. In the forward reaction of the equilibrium, which of the following molecules acts as a Brønsted-Lowry base?

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11ecba2d_1220_15ba_a3bf_119ed5736885_MD0008_00 According to the Brønsted-Lowry definitions, an acid is a chemical species that donates a proton (H⁺ ion), and a base is a chemical species that accepts a proton. The conjugate of an acid (or base) is the species that the acid (or base) becomes after losing (or gaining) one proton. A base is converted into its conjugate acid when the base accepts a proton. Likewise, an acid is converted into its conjugate base when the acid donates a proton. Therefore, an acid (or base) and its conjugate have chemical formulas that differ by only one proton. Whether a molecule functions as an acid or as a base sometimes depends on the relative acidity (acid strength) of the molecule. For example, H₂SO₄ and HNO₃ both usually function as strong acids (proton donors). However, if H₂SO₄ and HNO₃ are mixed together, H₂SO₄ is demonstrated to be a stronger acid than HNO₃ because the HNO₃ is induced to accept another proton to form H₂NO₃⁺. By the Brønsted-Lowry definition, a base is a proton acceptor. Therefore, in the forward reaction of the equilibrium, HNO₃ acts as a Brønsted-Lowry base by accepting a proton from H₂SO₄. (Choice A) H₂SO₄ functions as a Brønsted-Lowry acid (proton donor). (Choice C) HSO₄⁻ acts as a Brønsted-Lowry base, but it does so in the reverse reaction of the equilibrium. (Choice D) H₂NO₃⁺ is the conjugate acid of HNO₃. Educational objective: By the Brønsted-Lowry definitions, an acid is a chemical species that donates a proton, and a base is a chemical species that accepts a proton. The conjugate of an acid (or base) is the species that the acid (or base) becomes after losing (or gaining) one proton. An acid (or base) and its conjugate have chemical formulas that differ by only one proton.

The First Step of a Nitration Reaction Involves Using a Mixture