As the pH of a solution containing a fully protonated amino acid is increased, which group is most likely to deprotonate first?

• A. Hydroxyl group
• B. Protonated amine
• C. Carboxylic acid next to Cl2
• D. Carboxylic acid not near Cl2

Answer: (C)

In a fully protonated amino acid, the groups that are protonated include the carboxylic acid and the amine group. As you increase the pH of this solution, the first group to lose a proton and deprotonate will typically be the most acidic functional group present.

In this case, the carboxylic acid group is generally more acidic than the amine group. The presence of an electron-withdrawing group, such as a chlorine atom attached to the carboxylic acid, can further stabilize the negative charge that forms when the carboxylic acid loses a proton. This stabilization occurs due to the inductive effect, which makes the carboxylic acid even more willing to lose a proton.

Therefore, as the pH is increased, the carboxylic acid group next to the electron-withdrawing chlorine (Cl2) is likely to deprotonate first due to its increased acidity compared to the other functional groups. As the pH continues to rise, the remaining groups may deprotonate, but the carboxylic acid adjacent to the Cl2 is specifically the first to do so.