Consider an alcohol and an amine compound with roughly the same molar mass. If I understand correctly, the boiling point for the alcohol is greater for two reasons:

  • The $\ce{O-H}$ bond is more polar than $\ce{N-H}$ because oxygen is more electronegative than nitrogen.

  • The oxygen atom has two lone pairs of electrons to act as hydrogen acceptors, whereas the nitrogen has only one.

The question is, which one of these reasons (if they are valid) contributes more to the increase in boiling point?

  • 1
    $\begingroup$ Related to your 2nd point, the partially negative oxygen, making only two bonds with neighbors, spacially presents more of itself to neighbors possibly increasing the network of attractions. Also,the partial positives of the amine are distributed over three moities instead of two. This, combined with nitrogen's lesser electronegativity, would make them less positive than those attached to the oxygen. $\endgroup$
    – bpedit
    Oct 4 '16 at 13:39
  • 1
    $\begingroup$ Nitrogen has one lone pair but three hydrogens. Water has two lone pairs and two hydrogens. Thus water can form a better network of hydrogen bonded molecules. $\endgroup$
    – MaxW
    Oct 4 '16 at 15:24

Both reasons given are valid and contribut to alcohols having higher boiling points than amines, with one caveat. It is not stated whether the amine is primary, secondary or tertiary, and a tertiary amine has no $\ce{N-H}$ bond.

Although the $\ce{O-H}$ bond is more polar than the $\ce{N-H}$ bond and can therefore form somewhat stronger hydrogen bonds, the fact that an alcohol has two lone pairs is the main reason for the higher boiling point.

Even though a primary amine has two hydrogens that can act as hydrogen bond donors, their already weaker $\delta +$ is spread among two hydrogens, making those hydrogen bonds very weak, effectively limiting them to act as a single hydrogen bond donor.

So, in general, an amine can only participate as a single hydrogen bond acceptor and donor (except for tertiary amines), while an alcohol's two lone pairs and single hydrogen allow for a much stronger hydrogen bonding network.


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