Question about acidity

[rakuenso]

 

is the question, I'm wagering my $ on c)

 

Because:

 

CH3CH3 doesn't want to lose any H+ because carbocations are highly unfavorable, which would mean its most likely the weakest acid

 

Molecule III is the strongest acid because its postively charged Oxygen is highly unfavorable, and if it loses an H it would become highly stable. Thus making it the strongest (though not necessarily strong in a technical sense) acid.

 

I'm also guessing that Molecule II is the second strongest because the OH group can lose an H and become resonance stabilized, making its conjugate base very weak. Even though the O has a negative charge, its also highly electronegative, so i guess its OK. Thus implying that the acid form is stronger.

 

Water is just water, i'm not really sure how to explain it.. but it was the only one left. It also has a pretty high pKa (~15.74)...

[budullewraagh]

you're right with the answer, but a few corrections:

 

"carbocations"

it would actually yield a carbanion, which is even less favorable

 

"Molecule III is the strongest acid because its postively charged Oxygen is highly unfavorable, and if it loses an H it would become highly stable"

 

actually, molecule III is reasonably stable, but certainly acidic. this sort of thing is what you get when you put a pure acid in an alcohol. in such conditions it's highly susceptible to nucleophilic attack and elimination reactions

 

"OH group can lose an H and become resonance stabilized,"

interestingly, it's actually resonance stabilized already. and the fact that the carbon is SP^2 helps

[rakuenso]

shoot you're right about the carboanion, oopsies.

 

so for mol III., the carbocation is responsible for the high suspectibility to nucleophilic attack/eliminations?

 

as for the last comment, if its already resonance stabilized, then how does the SP^2 contribute to its acidity?

[budullewraagh]

mol III doesn't have a carbocation. it does have a protonated hydroxyl group, which is an excellent leaving group. the hydroxyl itself is a terrible leaving group, but the OH2+ can easily come off as water. the fact that it's on a primary carbon makes it more susceptible to nucleophilic attack than elimination.

 

by carbon i meant the oxygen:\

the fact that the oxygen is SP^2 helps because the O-H bond is weaker

[rakuenso]

 

Using the same logic as hydroxyls having a terrible leaving group, (why is it a bad leaving group? if the H+ left, then oxygen would be O-, but i read somewhere that electronegative atoms didn't really mind having a negative charge)

 

Mol 3 would be the strongest acid... then how do you determine whether mol II or I is stronger?