Why does HCN boil at a higher temperature than NH3?

The boiling point of ammonia is −33 °C while that of $\ce{HCN}$ is 25 °C. In a recent AP (Advanced Placement) Chemistry test, a free response question asked why this is the case. Can someone shine light on this?


Based on Jan's answer: 


1. Although the $\ce{C-H}$ bond does not usually exhibit good hydrogen bond, $\ce{H-CN}$ is a special case in which the bond is polar enough to provide better hydrogen bonding than $\ce{NH3}$. I agree with Jan on this point.
2. However, Jan's explanation is for the polarity of the $\ce{H-CN}$ bond is based on $\mathrm pK\_\mathrm a$ (lower $\mathrm pK\_\mathrm a$ suggests higher acidity and good hydrogen bonding). As one user pointed out, $\mathrm pK\_\mathrm a$ is only defined in aqueous solution, so stating pK$\_a$ as an evidence is not suitable for the system I have which is a pure $\ce{HCN}$ liquid. In such system, $\ce{HCN}$ does not significantly dissociate. A suggestion was made on using proton affinity as the evidence for the *acidity* of $\ce{HCN}$.


I perceive that the answers (reasons and evidence) are mostly targeted at the *chemical process* of dissociation of $\ce{HCN}$. Should this question be addressed more in terms of a *physical process*? I mean there is little dissociation of pure-liquid $\ce{HCN}$. 


What I have been thinking about is a MO picture in which the cyano group withdraws electron density from the hydrogen atom, in a similar way to how acetate group withdraws electron from the hydrogen in acetic acid. What is your thought on this?




The enthalpy of vaporization of [$\ce{HCN}$](http://webbook.nist.gov/cgi/cbook.cgi?ID=C74908&Mask=4#Thermo-Phase) is higher than for [$\ce{NH3}$](http://webbook.nist.gov/cgi/cbook.cgi?ID=C7664417&Units=SI&Mask=4#Thermo-Phase), which suggests that $\ce{HCN}$ molecules interact more strongly than $\ce{NH3}$ molecules. $\ce{C-H}$ bonds are not usually considered good hydrogen bond donors, but $\ce{HCN}$ is unusual. For example $\ce{HCN}$ has a [$\mathrm pK\_\mathrm a$ value of 9.2](http://en.wikipedia.org/wiki/Hydrogen_cyanide), indicating that the $\ce{CN}$ group is electron withdrawing and that it is a reasonably good hydrogen (bond) donor. This is likely due to the electronegativity of nitrogen, and also the high "s-content" of the sp-hybridized $\ce{CH}$ bond, which keeps the electron pair close to the nucleus.


 14

Supporting Jan's answer, actually: Consider the points below from the book [Hydrogen Bonding: A Theoretical Perspective](https://books.google.com/books?id=xF8PV2GeT-AC&pg=PA102&lpg=PA102&dq=hydrogen+bonding+of+hcn+and+nh3&source=bl&ots=6XZhdYmZXA&sig=PpJ1aANkDc8Jec_G2uv7stWSbw0&hl=en&sa=X&ei=YFJgVY2CO5GZyQSv6IDIBQ&ved=0CCoQ6AEwAQ#v=onepage&q=hydrogen%20bonding%20of%20hcn%20and%20nh3&f=false), p. 102.


"Moreover, the hydrogen in HCN is acidic enough that the molecule may act as an effective proton donor... When paired with NH$\_3$, HCN acts as a proton donor..."   

This agrees with Jan's note, on the pK$\_a$ of HCN.


Your comment that "the hydrogen is electronically deficient, leading to an excellent electron acceptor" seems to be about hydrogen bonding, whereas there is evidence that HCN is more ionic in nature.


 8

Besides the existing answers, which focus on the acidity of HCN, note that HCN is also a considerably larger molecule than NH3. Thus, *even if the interactions between the molecules were qualitatively identical*, one would still expect a higher boiling point for HCN on the basis of the size difference (and resulting stronger dispersion interactions; see comments) alone.


For an illustrative example, we can look at [methylamine](//en.wikipedia.org/wiki/Methylamine), CH3NH2, which resembles ammonia in most respects, except for having one of its hydrogens replaced by a bulky methyl group, making it similar in size to HCN. Its boiling point is −6.6 °C, well above the −33 °C for NH3.


The remaining ~32 K difference between the boiling points of HCN and CH3NH2 is then presumably explained by the stronger acidity of the HCN hydrogen, and thus the stronger hydrogen bonding between HCN molecules than for NH3 and CH3NH2.


 7

Although $\ce{NH3}$ consist of hydrogen bonding $\ce{HCN}$ has a very stronger dipole-dipole interaction which makes its boiling point equivalent to alcohols.


See the diagram:


![enter image description here](https://i.stack.imgur.com/zRHOB.png)


 3

"Acidity" can't be responsible, since acidity refers to what happens in water solution. (I assume the user is referring to the $\mathrm{p}K\_\mathrm{a}$ data.) I don't know how great the tendency of $\ce{HCN}$ is to ionize in the pure liquid, but I doubt it's significant. But if you have data, please cite it. Even for water, $K\_\mathrm{w} = 10^{-14}$, its tendency to ionize does not contribute to its high boiling point.


 2

I think the crux of the question is not in hydrogen bonding. Recently, I did a question in the USA Chemistry Olympiad (Local Section) on boiling point of substances. I discovered that self-ionisation played a key role in determining boiling point and melting point. The answer to the question I attempted asking for the substance with the highest boiling point was pure sulfuric acid, which is apparently capable of self-ionisation to a large extent. 


The reason why self-ionisation is able to raise the boiling point of the substance is because of the ionic interactions in the liquid of the resultant ions produced by the process. This strong ionic interactions increases the boiling point of the liquid, like in the case of sulfuric acid. 


The same concept can be applied here. The hydrogen cyanide molecule being capable of self-ionisation, although not to such a significant extent, can produce ions in the liquid phase. This ionic interaction, although it may be very litte compared to sulfuric acid, does increase the boiling point of the substance. 


In comparison, ammonia's self-ionisation is probably negligible and thus, the strongest interactions are only hydrogen bonding.


This post is a bit late but I hope this provides a new perspective.


 1