[Super Newb]

According to this website (https://quizlet.com/24520569/acidity-constants-pka-of-acids-flash-cards/), the pKa of Ethyloxonium and hydronium ion are -2.4 and -1.7 respectively. Based on the pKa values, we can hence suggest that ethyloxonium ion is a better acid as compare to the hydronium ion. This can lead us to conclude that water is a better base as compared to ethanol with regards to proton abstraction.

Now, I would like to find out why is it that ethyloxonium is actually more acidic than hydronium ion. I would very much assume that the methylene (CH2) that is connected to the Oxygen in the ethyloxonium to be able to stabilize the oxygen with formal positive charge via hyperconjugation, and hence reducing its acidity. However, the result is surprising and I would like to find out how can we explain the difference in the pKa value. Any input will be greatly appreciated.

I am also aware that in a typical acid, an electron donating group will destablize the right hand side of the equation as the oxygen is already electron rich (due to its formal negative charge) and any attempt to donate electron inductively will destabilize the negatively charge ion (conjugate base); hence, shifting the equilibrium to the left. But this explanation is certainly not applicable to that of the ethoxyoxonium.

Any input will be greatly appreciated.

[Babcock_Hall]

I am not sure what the correct answer is.  However, I would at least consider solvation effects.

[orgopete]

I'm with Babcock Hall on this one. I've had many discussion with Souro who advocates gas phase acidities. I understand why this could be good data, but I think it has a different set of issues, so we come back to why does this difference exist.

If we go back to some simple principles that are consistent with the question. If one compares ammonia, ethylamine, and diethylamine, one would have concluded that carbon is more electron donating than hydrogen. However, triethylamine does not follow that trend. Dimethyloxonium cation is a stronger acid yet. You may find various arguments, but I'm more inclined to think it is related to solvation properties (though I cannot quantitate the actual effect). I'm not bothered if they affect oxygen differently than nitrogen.