Sapling 6.21

[Aaron Akhavan-Dis1B]

The two strands of the nucleic acid DNA are held together by hydrogen bonding between four organic bases. The structure of one of these bases, thymine, is shown below. (a) How many protons can this base accept? (b) Draw the structure of each conjugate acid that can be formed. (c) Mark with an asterisk any structure that can show amphiprotic behavior in aqueous solution.

a) Why do we only look at the Nitrogen for lone pairs rather than the oxygen or carbons in order to figure out how many protons can be accepted?
b)Is the reason why the Nitrogens are amphiprotic is that they have a lone pair as well as a Hydrogen bonded?

Just want to know how to approach this problem.

[Chem_Mod]

We look at nitrogens to see how many protons can be accepted because, for many bases, nitrogens are the ones that accept the proton. Carbon doesn't accept protons because they usually don't have lone pairs when they bond with other atoms, and in this case, oxygen is stable with two double bonds and two lone pairs so it doesn't have a tendency to accept protons.

The nitrogens for this molecule are amphiprotic for exactly the reason that you stated.

[Sophia Stewart 3F]

Both oxygen and nitrogen have a formal charge of zero, but available lone pairs here. Are you saying oxygen is more stable here because of the double bond, which is stronger?