most acidic?

[premed8]

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Hi I have a question about acidity of these.. thanks for you kind explanations in advance.


Which of the following compounds is expected to be
most acidic?
A. CCl3COOH
B. CH3OH2+
C. NH4+

 

[Rabolisk]

Ch3oh2+ because it is a protonated form of a very weak base (alcohol)

 

[vsl5]

I would have said CCl3COOH because its a carboxylic acid and its conjugate base is stabilized by 3 electron withdrawing Cl atoms. Whats the answer given?

 

[crazyasian]

It should be A. because of the 3 EWG. Generally COOH acids are stronger acids then the other 2. The 3 EWG make it an even stronger acid. when comparing acidities, first think of the acidites of the class of molecules and if the acidities of the 2 classes are pretty even, then look at EWG and EDG, and resonance. EWG tend to make the molecule more acidic, and EDG make less acidic.

 

[Rabolisk]

You probably misread choice B, which is a protonated form of an alcohol, not an alcohol. I am confident that the answer is B, even after accounting for the three chlorines. Literature values for pKa of trichloroacetic acid have been reported to be between 0.5 and 1. PKa of water is 15.7, and that of hydronium ion is -1.7. Protonated ethanol will be close to -1.7, and I am sure, well less than 0.5.

 

[loveoforganic]

Agree with above poster, although I don't think this is a very reasonable question

 

[orgohacks]

Agree with above poster, although I don't think this is a very reasonable question

Agreed, I think CH3COOH would have been a far better choice of acid. We're talking about very subtle differences here.

 

[Salient]

The MCAT isn't going to give you widely different compounds like this unless their pKas are also widely different. I'd believe it if they used methanol, but not protonated methanol.

Agree that a more likely question is asking you to compare something like acetic acid with trichloroacetic acid.

 

[plzNOCarribbean]

A because even without the inductive effects of the 3 Cl groups, which significantly increase the acidity because they stabilize the conjugate base, a Carboxylic acid has a pKa of about 5, compared to that of an alcohol, which is around 16 i think. So clearly, A is the correct choice because of both pKa, resonance effects of the carbonyl group, and inductive effects of the electronegative substituents.

 

[donaldduck]

I would say the answer is A, after the hydrogen leaves the carboxyl group gains resonance, so it going to want to give away its proton easily, and not to mention, the chlorine groups provide further stabilization by the inductive effect. In the other choices loosing a hydrogen just causes it to loose its positive formal charge which isnt as great as a stabilizing feature when compared to the inductive effect and resonance.

 

[Rabolisk]

The answer is B not A. B is CH3OH2+ not CH3OH. You've essentially added a proton to an alcohol, a very very weak base that does not want that proton. Hence the reverse is a significantly favorable reaction resulting in a high acidity. It's sort of like comparing H3O+ and phosphoric acid (a weak acid with a relatively low pKa).