# Why is C4 of butynone electrophilic?

A question asks whether C4 in 3-butyne-2-one can act as a nucleophile, electrophile, or acid (it can be more than one). The answer is that it can be both electrophilic and acidic.

The acidity makes sense, it's a terminal alkyne, sp orbitals stabilize the negative charge on the conjugate base.

The electrophilicity I don't quite understand. I can write a resonance structure that puts some positive charge on C4, analogous to an alpha-beta unsaturated carbonyl compound, but it looks bizarre. I know ketenes are a real thing, but is this resonance structure a real contributor? Is there another explanation for the electrophilicity?

Google searches for alkynone, ynones, etc. yielded mostly peer-reviewed literature that's above my understanding.

Yes, that second resonance structure, the one on the right, is a significant contributor to the overall description of the molecule. That resonance looks a little different because the p orbitals on the alpha and beta carbons are on $\ce{sp}$ hybridized carbons, rather than the $\ce{sp^2}$ hybridized carbon more commonly seen in enones. As this second resonance structure explains, alpha-beta unsaturated enones and ynones are both subject to attack by nucleophiles at the electrophilic beta carbon.