Exercise 6.21

[Barbara Soliman 1G]

Can someone explain this problem to me?

6.21 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.

Thanks!

[jennaj3k]

A common mistake with this problem is thinking that the oxygen is going to take on those protons.
Oxygen is more electronegative than Nitrogen so it's unlikely to share its electrons. If it did, then thymine would have a positive charge which isn't stable. Nitrogen is less electronegative and more likely to share its electrons, so its going to accept those protons.

[Ella Bogomilsky 2B]

The thymine is able to accept 2 protons because there are two nitrogen atoms that are likely to share their electrons. The amphiprotic areas would also be the nitro groups because they can either accept a proton (like in part a) or donate their proton.