[kauzs-17]

Hey guys, just started first semester of Orgo. Going pretty good so far, but I've got a question.

We learned that a more electronegative atom in an acid lowers the pKa (thus making it stronger) than does having a less electronegative atom. For example, NH₃ is weaker than HF, as Fluorine is more electronegative.

We also learned that larger atoms lower the pKa more than smaller atoms. Example: HF is weaker than HI.
So far, we have NH₃<HF<HI, so size trumps electronegativity when it comes to acidity.

However, why is this not the case when there is a substituent? In CH3COOH, if one of the hydrogens on the sp3 C is changed to a bromine, it is weaker than if it was changed to a fluorine (though bromine is larger)? That's according to the book. According to the sets she gave us, Cl trumps O as a substituent because Cl is larger, though O is more electronegative (Both posted below) . Very confused 
This is the problem she gave us. She said this...

Is less acidic than this...

Though Cl is less electronegative than O, but bigger.

[kauzs-17]

Any ideas? Open to anything at this point..

[Borek]

For example, NH₃ is weaker than HF, as Fluorine is more electronegative.

This is a very, very bad example.

You can do comparisons like that under assumption "all other things kept constant". So it would be a good idea to compare HN (if that existed) and HF, but in NH₃ and HF you change not only electronegativity of the central atom, but also molecule shape and hybridization, and that's asking for troubles, as you have at least two competing trends and you can't easily predict which one will be more important.

Molecules you posted are so different I would not risk comparing them this way.

[kauzs-17]

For example, NH₃ is weaker than HF, as Fluorine is more electronegative.

This is a very, very bad example.

You can do comparisons like that under assumption "all other things kept constant". So it would be a good idea to compare HN (if that existed) and HF, but in NH₃ and HF you change not only electronegativity of the central atom, but also molecule shape and hybridization, and that's asking for troubles, as you have at least two competing trends and you can't easily predict which one will be more important.

Molecules you posted are so different I would not risk comparing them this way.

Hey Borek, thanks for the reply.

Yeah, I realized that was different and NH definitely would have been better. This is simply the example that the book laid out.

I think they kind of dumb it down for us in the beginning, y'know? To get the hang of some of the concepts. I'm sure in a few weeks, they'll say "Well, we told you to do this, but THIS is how you actually do it."

I just spoke with the professor. She was actually wrong - the compound with the oxygen substituents was the more acidic.

Thanks again Borek.

[orgopete]

When this was initially posted, I had overlooked the structural connection that would likely affect the acidity. The oxygen is simply closer to the carboxylic acid than the chlorines, even if there are two.

[Babcock_Hall]

When H-X (where X is a halogen) dissociates, X^1- bears a full negative charge.  Why do you think that size might be a factor in the acidity in this case?  On the other hand, for molecules of the form X-CH₂COOH, X does not bear a full negative charge; instead, it draws electron density toward it.  Can you think of a name for what this effect is called?