Free O-Chem Lectures' Video 3: The Acidity of Substituted Benzenes

In this video I'll explain how different substituents affect the acidity of phenols and benzoic acids. --Dr. Mike Christiansen from Utah State University

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[itzam marin]

Awesome video. Thank you

[Mike Christiansen]

You're very welcome!

[Rafael Ayala]

Professor Christiansen,

Do you have the answers to questions you said you would work in class for those of us who follow you online?

[Mike Christiansen]

Not yet, I'm afraid. Really sorry. I will get them posted here on YouTube sometime this summer.

[Mike Christiansen]

Awesome. You're quite welcome!

[Calvinang93]

"a benzene ring (as a whole) is about as electronegative as an oxygen." Interesting fact I never knew :D. After drawing the resonance structure, it became much clearer and I can understand it now. Thanks a lot for your reply! :D

By the way, great video! :)

[Mike Christiansen]

One additional comment (see the one I just posted below): you're absolutely right that having a resonance-donor, like OH, meta to your carboxylic acid will have less of a donating effect than if the OH is ortho or para to the acid. This is because, if you draw out all the resonance structures, you can never have a meta OH pump electrons directly into the carboxylic acid group, while ortho and para OH's can. Pushing electrons into an acid by resonance makes it less acidic.

[Mike Christiansen]

Good questions. Because oxygen is more electronegative than carbon, we can imagine it withdrawing from the benzene ring by induction. However, it also has lone pairs that can donate into the ring by resonance. As it turns out, a benzene ring (as a whole) is about as electronegative as an oxygen. Hence, oxygen's donation by resonance outweighs its withdrawing by induction. It is therefore an o,p director.

[Calvinang93]

I'm very confused by the fact that come substituent is 'electron withdrawing by induction' but 'electron donating by resonance'. For example -OH. How do we know which outweigh effect the other?

In addition, the position of the substituent o,m,p do make a difference. What about those?

From what I know, p-hydroxybenzoic acid is less acidic than benzoic acid BUT m-hydroxybenzoic acid is more acidic than benzoic acid..

Can anyone please explain?