Question

If I have a diprotic acid that is +1 positively charged in its fully protonated state, I can figure out the apparent equilibrium constants by titration with base. The net charge will be neutral after losing one proton, and -1 after losing the second proton.

Examples would be the hydronium ion or the amino acid glycine at a pH < 2.

How could you determine whether the single-protonated state is a zwitterion (glycine in neutral aqueous solution) or a molecule without charged functional groups (glycine in neutral polar aprotic solvent, or water at neutral pH)?

Is it necessary to do an experiment, or is there a simple rule to figure out whether a zwitterion is expected?

Answer 1

To determine whether the single-protonated state of a diprotic acid is a zwitterion or a molecule without charged functional groups, consider the pH and properties of the solvent.

Step-by-step explanation:

Determining whether a single-protonated state is a zwitterion

To determine whether the single-protonated state of a diprotic acid is a zwitterion or a molecule without charged functional groups, you can consider the pH and the properties of the solvent.

In a neutral aqueous solution, the pH is around 7, which is above the pKa value of most amino acids. At this pH, the amino acid is deprotonated and exists as a zwitterion.

However, in a neutral polar aprotic solvent or water at neutral pH, the amino acid does not have charged functional groups and exists as a neutral molecule without a net charge.

In summary, the determination of whether a single-protonated state is a zwitterion or a neutral molecule depends on the pH and the properties of the solvent.