Final answer:
The most acidic protons in malic acid's molecule are those attached to the carboxyl groups. This acidity is due to the resonance stabilization of the carboxylate ion formed when these hydrogen atoms dissociate.
Step-by-step explanation:
The order of proton acidity in malic acid, which is similar to other carboxylic acids, is determined by the ability of its hydrogen atoms to dissociate in water. Malic acid has two carboxyl groups (-COOH), and due to the resonance effect, the protons (hydrogen atoms) on these groups are more acidic compared to hydrogen atoms on alkanes.
Specifically, the hydrogen from the -COOH group is the most acidic, due to the formation of a resonance-stabilized carboxylate ion upon deprotonation, which is significantly more stable than the protonated form. Hence, in malic acid, the ordering starts with the hydrogen atoms on the carboxyl groups being the most acidic, followed by the hydrogen atoms attached to the carbon backbone, which are much less acidic.