CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
In aqueous solution at 25 °C, the amino acid glycine (NH₃⁺–CH₂–COO⁻) behaves as a diprotic species with two exchangeable protons: one on the amino group and one on the carboxyl group. When the pH of a glycine solution is adjusted to 6.06, the molecule is observed to exist predominantly in its zwitterionic form, NH₃⁺–CH₂–COO⁻. Additional experimental observations are summarized below.
• At pH = 2.0, glycine carries an average net charge of +1.0 per molecule.
• At pH = 9.5, glycine carries an average net charge of –1.0 per molecule.
• At pH = 6.06, glycine is electrically neutral on average.
Assume that only the two principal acid–base equilibria of glycine occur in the pH range studied. Ignore any side reactions.
(a) For each of the three stated pH values (2.0, 6.06, and 9.5), identify which of the three possible protonation states of glycine—fully protonated (NH₃⁺–CH₂–COOH), zwitterionic (NH₃⁺–CH₂–COO⁻), or fully deprotonated (NH₂–CH₂–COO⁻)—is most abundant. Justify your answers qualitatively.
(b) Explain why the average net charges of +1.0, 0, and –1.0 correspond to the dominance of the protonation states identified in part (a).
(c) Using the information above, derive approximate numerical bounds for pKa1 (loss of the carboxyl proton) and pKa2 (loss of the amino proton). Show how each bound is inferred from the data.
(d) Propose estimated values for pKa1 and pKa2 that are consistent with all three observations. Discuss how the zwitterionic dominance at pH = 6.06 constrains your estimates.