Textbook Question 6.21

[Erika Patel 3I]

Given the thymine structure, we were asked to determine how many protons this base could accept. I understood that each nitrogen could accept one H+ ion, however I am not sure why the two oxygen atoms in the structure cannot accept protons since they have electron pairs available for bonding.

[Alejandro_Franco_3B]

If oxygen accepted a proton it would a positive formal charge, and it can't have a positive formal charge because it would be unstable. Oxygen is more electronegative than Nitrogen thus it is better for Nitrogen to have a positive formal change.