Test 2, Question 7A

[DTingey_1C]

On Test 2, question 7A asks to give the total number of bonding sites for Xanthin. Is one hydrogen bonding site normally one N, O, or F with a lone pair, or one H atom? Because I counted these on the Xanthin molecule and get 9, but my TA wrote 11 as the correct answer. I don't know, is it possible for one O atom to contain 2 hydrogen bonding sites because it has 2 lone pairs or something? Thanks.

[Ryan 1K]

Yes, you are correct. Each O atom counts as two H bonding sites due to the two lone electron pairs.

[DTingey_1C]

Yes, you are correct. Each O atom counts as two H bonding sites due to the two lone electron pairs.

Ok but when I counted each O as two bonding sites, then added the H and N bonding sites both as one bonding site each, I got 12 instead of the answer, 11.

[Ryan 1K]

The H bonded to the C atom on the right side of the molecule does not count as a hydrogen bonding site since the electronegativity difference between C and H is not enough for there to be a strong partial positive charge on the hydrogen. H bonding can only occur with F, O, or N atoms.