Sapling Week 5/6 #16

[Isabel_Eslabon_2G]

Hello, I am struggling with understanding this question:

A cross-link is an ionic, covalent, or hydrogen bond that links one polymer chain to another and reduces the flexibility of the polymer chains. Several possible hydrogen bond cross links are shown between the two polymer chains in the image.

Identify the hydrogen bond crosslink between the two polymer chains.
(I made the specified bond colored red)

A) ...C=O—O=C...
B) ...C—H—H—N...
C) ...C—H—H—C...
D) ...C=O—H—N...
E) ...N—H—H—C...
F) ...C=O—H—C...

I don't know if the above answers represent the problem correctly because I can't figure out how to add an image, but the problem is in Sapling.

The answer is D, but I don't understand why. Thank you in advance for your help.

[Pranav Daggubati 3C]

The key thing to understand here is that a hydrogen bond can only occur between partially positive hydrogen and a partially negative anything else. This means that the hydrogen should be bonded to an electronegative atom (N, O, F) and the other atom should be bonded to a less electronegative atom (C, H, etc.). The only answer choice that has this is D

[JonathanSung_2G]

Carbon is not as electronegative as nitrogen, which means that when hydrogen is bonded to carbon, there will not be a significant positive charge that would be used during bonding. Hydrogen bondings require a slight negative charge and a slight positive charge in order to form, and in this case for D, the negative charge comes from the oxygen and the positive charge comes from that hydrogen bonded to nitrogen.