For the amino acid shown, pK_a1 (COOH) = 2.34 and pK_a2 (NH3+) = 9.69. Which of Structures A to D is the predominant species at pH 12?

JEE Main levelSingle correct optionDifficulty 2 of 3bm255

For the amino acid shown, $p K_{a 1}$ (COOH) = 2.34 and $p K_{a 2}$ ( $NH X_{3} X^{+}$ ) = 9.69. Which of Structures A to D is the predominant species at pH 12?

Above

p K_{a 2}

both groups are deprotonated: at pH 12 (more than 2 units above 9.69) the amino group is

- NH X_{2}

and the carboxyl group is

- COO -

. The species is the anion, with net charge -1. The cation exists only below pH 2.34 and the zwitterion between 2.34 and 9.69.