Acidity of Benzenepropanenitrile [closed]

Question

Which is the more acidic hydrogen in the parent chain of this compound? I am confused while considering the various electronic effects.

Answer 1

I would have called it 3-phenylpropanenitrile, but let's get to the real chemistry here.

Clearly the candidates for most acidic hydrogen atom are those on the saturated carbon atoms between the phenyl ring and nitrile (CN) group. If you choose to remove a proton from the carbon next to the phenyl ring, the negative charge can be dispersed into the phenyl ring via resonance. If instead you choose to remove a proton from the carbon next to the nitrile group, the negative charge can be delocalized into the nitrile group.

You were probably taught by rote whether the nitrile group or the phenyl ring is better at taking up the negative charge, but there is a better way to find out. This table gives the $\pu{pK_a}$ values of some simple compounds corresponding to different environments for removing a proton -- the lower the number, the stronger the acid and the better the conjugate base is for taking up the negative charge.

We find that acetonitrile (proton next to a nitrile group) gives 25 whereas toluene (proton nect to a phenyl ring) gives 41. Though neither of these is appreciably acidic in water, acetonitrile could be much more so in some other solvents.

So the hydrogens next to the nitrile group of your given compound, 3-phenylpropanenitrile, would be the most acidic.