Textbook Problem 6.21

[DArcy Perlman 1C]

In textbook problem 6.21, it asks how many protons can the base thymine accept. The lewis structure for thymine is shown below. In the answer key it states that only 2 protons can be added, one at each N. Since there are other atoms in the structure with lone pairs, why can only the nitrogen atoms accept protons?

[jennaj3k]

It usually makes sense for those oxygens to take on the protons, but because Oxygen is more electronegative than Nitrogen, it is unlikely to share electrons. If it did share electrons, then thymine would have a positive charge and be less stable. Nitrogen is less electronegative and more likely to share its electrons, so it's going to accept those protons.

[Libby Stafford 1L]

Exactly! If the oxygen atoms were charged (as they would be if they accepted a proton), this would make the compound more unstable, and so the protons will go for the more energetically favourable option of bonding to the nitrogen.