# Why does acetonitrile have a larger dipole moment and boiling point than acetaldehyde?

Experimentally, acetonitrile has a larger dipole moment than acetaldehyde, but I've never understood why.

I always thought that the charge separation between carbon/oxygen is larger than that of carbon/nitrogen, and since the $\ce{C=O}$ bond should be longer than the $\ce{C#N}$ bond, the dipole moment $\mu = \delta d$ would be larger with both larger charge separation and larger distance.

I assume geometry might have something to do with it but I can't really explain such a large boiling point difference.

Data: Dipole moment values:

• acetonitrile: $\approx \pu{3.4D}-\pu{3.5D}$ (source)
• acetaldehyde: $\approx \pu{2.7D}$ (sources: 1, 2)
• Comments are not for extended discussion; this conversation has been moved to chat. – orthocresol Mar 27 '18 at 22:52