# Reason for triphenyl methane being less acidic than acetylene

This question came through answering a question that talked about the acidic order of four compounds.

Upon researching the actual $$\mathrm p K_\mathrm a$$ values I noticed that even though there was a resonance destabilization of electrons (albeit partial) in triphenyl methane, the $$\mathrm p K_\mathrm a$$ values of acetylene were less than that of triphenyl methane? I was unable to find a solid reason for why this is the case.

Does anyone in the community have the reason?

• Basically you are seeking confirmation for your answer there, which is unusual. Anyway, I just upvoted it because is it at least in the right direction. – Alchimista Apr 12 at 9:21
• @Alchimista yeah. I would convert my answer there into a community post if I can get a proper explanation for this one. Since the first part answers the actual question but not the question I pose. – Safdar Faisal Apr 12 at 9:22
• Triphenyl methane is some kind of propeller shaped, so it has less conjugation, making it very less acidic than what we except. – 0.00 May 26 at 14:32

As mentioned by @orthocresol, just comparing methane and acetylene shows a difference of $$\approx 20$$ $$\mathrm pK_\mathrm a$$ units and the amount of resonance required to overcome this difference is very high.
Comparing $$\mathrm p K_\mathrm a$$ of phenol($$10.0$$) and methanol($$15.5$$), we see that the presence of one resonating ring only produces a 5.5 $$\mathrm p K_\mathrm a$$ difference. Resonance in triphenyl system is effectively only one phenyl group at a time due to the steric effects due to the bulkiness of the phenyl groups.