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Problem 6.21

Posted: Sun Jul 28, 2019 9:01 pm
by Leah farhadi 1F
12.127 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.

Why can't the H+ protons be accepted by the lone pairs on the Oxygen atoms, why only can the nitrogen atoms accept the H+ protons?

Re: Problem 6.21

Posted: Sun Jul 28, 2019 11:05 pm
by Chem_Mod
If the oxygen atoms were to donate their lone pairs to H+, this would result in O atoms with positive formal charge (three bonds and 2 lone pair electrons for a valence of 5, 6-5= +1). Oxygen is more electronegative than nitrogen (which also gained a positive formal charge upon accepting the protons), thus making it much more unfavorable for the oxygen atoms to donate their lone pairs than the nitrogen atoms.