BR O Chem Example 1.23

[Fing Fang Foom]

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Which of the following compounds has the HIGHEST boiling point?

A. (H3C)3N
B. (H3C)2NH
C. (H3C)3CH
D. H3COCH3

Choices C and D are easily eliminated but I struggled with A and B. Answer A has a lone pair that allows it to form a hydrogen bond and yet the choice is eliminated along with choice D because they have all their hydrogens on carbon. Amines can form hydrogen bonds with hydrofluoric acid for example.

The passage explanation states that both a lone pair and hydrogen bonded to a small highly electronegative atom (N, O, or F) are needed to form a hydrogen bond. It seems to me that the explanation is suggesting that both of these must be found on the same compound.

 

[ilovemcat]

No, that's actually not true. Choice A is not capable of Hydrogen Bonding by itself because it does NOT contain a hydrogen attached to a Fluorine, Oxygen, or Nitrogen. This is a common mistake people make. The hydrogen in choice A is attached to the carbon, not the nitrogen. Choice B however is capable of Hydrogen Bonding (the hydrogen IS attached to nitrogen). Usually, provided that the other choices aren't capable of hydrogen bonding (as is the case here), the molecule WITH hydrogen bonding has the highest boiling point and melting point. Choice C is non-polar (lowest boiling and melting point). Choice D is a polar molecule, as well as choice A. However, the answer is B because as mentioned earlier, it's the only molecule capable of hydrogen bonding.

The passage explanation states that both a lone pair and hydrogen bonded to a small highly electronegative atom (N, O, or F) are needed to form a hydrogen bond. It seems to me that the explanation is suggesting that both of these must be found on the same compound.

This is also a true statement. If a molecule contains F, O, or N, it not capable of Hydrogen Bonding with itself, BUT if it's added to another solvent that IS capable of hydrogen bonding, then it can hydrogen bond. The lone pair in choice A for example could hydrogen bond with the hydrogen attached to a fluorine, oxygen, or nitrogen.

 

[GreenRabbit]

The passage explanation states that both a lone pair and hydrogen bonded to a small highly electronegative atom (N, O, or F) are needed to form a hydrogen bond. It seems to me that the explanation is suggesting that both of these must be found on the same compound.

They don't have to be on the same compound, but because this question is asking for boiling point, it's talking about intermolecular forces in a pure substance, and so for this question you're looking for a molecule with both an H-bond donor and acceptor.