Why is C4 of butynone electrophilic?

Question

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.

Answer 1

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.

Resonance structures, like the one pictured below, explain the reactivity seen in enamines and yneamines (the triple-bond analogue); however, now the positive charge is on the nitrogen and a negative charge on the beta carbon. Therefore these nitrogen compounds are subject to attack by an electrophile at the nucleophilic beta carbon.