Ranking acidity

[circulus vitios]

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Explain why acidity trends from most to least acidic in: phenol, cyclopentanol, or acetone.

Phenol has 4 resonance structures but the negative charge on oxygen is the only significant structure. The other structures are unfavorable because they interrupt the aromaticity of the ring and place a negative charge on the carbons.

Cyclopetanol has no resonance structures, only a negative charge on the oxygen.

Acetone has two resonance structures. The favorable structure places a negative charge on the oxygen. The unfavorable structure places a negative charge on the terminal carbon. My book says "even though it has resonance, it is less acidic than cyclopentanol because some of the charge resides on the carbon."



All 3 of these molecules have a structure where the negative charge is on the oxygen. These are the most favorable.

The book acknowledges that phenol's resonance structures with a negative on the carbons interrupts the aromaticity of the ring. These structures are unfavorable but the resonance still makes phenol more acidic than cyclopentanol.

The book then says that acetone is less acidic than cyclopentanol because it places a negative on the terminal carbon, even though it has more resonance structures than cyclopentanol. This contradicts the reasoning for phenol being more acidic than cyclopentanol. Can someone this?

 

[cheesier]

A hydrogen bonded to an oxygen is always going to be more acidic than a hydrogen bonded to a carbon. Only after you put acetone in last place can you compare cyclopentanol and phenol with respect to resonance.

 

[circulus vitios]

A hydrogen bonded to an oxygen is always going to be more acidic than a hydrogen bonded to a carbon. No matter what, acetone will have a partial negative charge on carbon which is very unstable while cyclopentanol and phenol won't.

I didn't think of it like that. I wonder why they left it out.

Phenol does have a partial negative on carbon?

 

[cheesier]

I didn't think of it like that. I wonder why they left it out.

Phenol does have a partial negative on carbon?

I changed my post but you beat me to it lol. Anyway, you're right, phenol will have a partial negative charge on carbon, but it is a minor contributor since the oxygen can have all of the negative charge associated with it. First, you have to lose the hydrogen, and it comes off of an oxygen much easier than it comes off of a carbon, resonance notwithstanding.

BTW the picture you linked is for phenol with it hydroxyl group intact. The resonance structure that you have to think about when determining acid strength is the deprotonated form of the acid. If you look at deprotonated phenol, you can see that the oxygen will have a negative charge. If you look at deprotonated acetone, there's no way to draw it without carbon having charges.

 

[LoLCareerGoals]

I thought aromatic stuff wasn't on the MCAT?

 

[circulus vitios]

I thought aromatic stuff wasn't on the MCAT?

According to AAMC's latest topic list (link), electrophilic aromatic substitution reactions are not on the MCAT, although "effects of substituents on basicity of aromatic amines" are fair game. Given that, it's reasonable to assume that substituent effect on acidity of other simple aromatics might be testable material.