How is ethanenitrile protonated?
The structure of ethanenitrile is:

$$\ce{H3C-C#N:}$$

Qualitatively, we can draw some conclusions about the molecular orbitals in the structure. If covalent bonds result in in the involved electrons being lower in energy, then which pair of electrons in the structure has the highest energy?

However, if you are looking at nitriles, you probably already know that both lone pairs and pi bonds are good proton acceptors.

Let's consider what the difference is if the lone pair is the base or if a pi bond is the base:


  Lone Pair


$$\ce{H3C-C#N + H+ -&gt; H3C-C#N+-H}$$


  Pi Bond


Which atom will the proton end up attached to? Which of the two following carbocations is more stable?

$$\ce{H3C-C#N + H+ -&gt; H3C-C+=NH}\ \  or\ \  \ce{H3C-CH=N+}$$

Now... what is the relationship between the two conjugate acids drawn?

$$\ce{H3C-C#N+-H}\ \ and\ \ \ce{H3C-C+=NH}$$

Where is the HOMO?