Question regarding this type of problem

[DDS.230]

Hey guys,

For Organic Chemistry, there was a question that asks, "predict the predominant form of the following amino acid at pH 7. Then it gave the molecule, and pKa of the 3 functional groups, Amine, Thiol, and Carboxylic Acid. which was (8, 8.3, 3.1) respectively.

Would the predominant form just be the molecule with the Hydrogen on the Carboxylic acid deprotonated, since the pKa of the carboxylic is lower than the pH, therefore the deprotonated form is more abundant?

I thought this was the answer, but the answer is giving me a molecule with the carboxylic acid deprotonated AND the amine group protonated.

This is question #22 from the Ochem section in datQvault's free sample test.

I have no idea why they're protonating the amine group, and hopefully this is an error...

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[hurley]

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[DDS.230]

oh i get it. so we're basically comparing the acid and conjugate base, and so if the pka is lower than the pH, it should get deprotonated, but if it's higher, the H stays on.

Is that right?

 

[J777]

oh i get it. so we're basically comparing the acid and conjugate base, and so if the pka is lower than the pH, it should get deprotonated, but if it's higher, the H stays on.

Is that right?

For the purposes of the DAT, absolutely