Benzoic acid and phenol acidity substituent effect

[gangazi]

do benzoic acid and phenol share same trend of acidity?
For example, EWG on orth and para make both more acidic? And if EWG is on ortho and para, the ones that delocalize by resonance lower pKa more than the ones that does inductive effect (halogen)? And if EWG is on meta, only inductive force matters?
Thank you!

---

Members don't see this ad.

 

[Likkriue]

Yes they share the same trend. EWGs will stabilize the negative charge on the conjugate base. Remember always that the more stable the conjugate base the more acidic the species is.

I don't understand what you mean by it being meta it will only take inductive effect into account. What I learned, is that the closer the ewg is to the withdrawing group the greater the effect. Can't help you on that one.

 

[dentalstudent2021]

do benzoic acid and phenol share same trend of acidity?
For example, EWG on orth and para make both more acidic? And if EWG is on ortho and para, the ones that delocalize by resonance lower pKa more than the ones that does inductive effect (halogen)? And if EWG is on meta, only inductive force matters?
Thank you!

benzoic acid is much more acidic than a phenol.....OH on phenol is EDG and COO- on benzoic acid is EWG...and this is the trend for EWG substituent on acidity . Ortho EWG > para EWG > meta EWG (hi to lo acidity)
http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch12/subseff.gif

 

[DAT Destroyer]

do benzoic acid and phenol share same trend of acidity?
For example, EWG on orth and para make both more acidic? And if EWG is on ortho and para, the ones that delocalize by resonance lower pKa more than the ones that does inductive effect (halogen)? And if EWG is on meta, only inductive force matters?
Thank you!

Yes.....The same general trend is normally followed. Both Inductive and Resonance effects are operating. When an EWG is positioned in the meta , we still have resonance but NOT to the same extent. Consult any organic text and see how fewer resonance forms are drawn for this specie. The meta isomer is usually the less acidic.

Consider the o p , and m nitrobenzoic acids.....ortho isomer has pka of 2.2.....para is 3.4........Here RESONANCE and Induction are operating......while meta is 3.6......also resonance and induction.....but more INDUCTION. Now lets consider the o, p and m nitrophenols.....ortho is 7.2 and para is 7.2...while meta....once again loses....it has a pka of 8.4. The reason is similar . For the DAT exam.....simply make sure that you understand that Groups like NO2, CN, SO3H, halogens, benzene INCREASE the acidity.....i.e lower the pka because the anion is stabilized to a greater extent.
Hope this helps.

Dr. Romano

 

[gangazi]

Yes.....The same general trend is normally followed. Both Inductive and Resonance effects are operating. When an EWG is positioned in the meta , we still have resonance but NOT to the same extent. Consult any organic text and see how fewer resonance forms are drawn for this specie. The meta isomer is usually the less acidic.

Consider the o p , and m nitrobenzoic acids.....ortho isomer has pka of 2.2.....para is 3.4........Here RESONANCE and Induction are operating......while meta is 3.6......also resonance and induction.....but more INDUCTION. Now lets consider the o, p and m nitrophenols.....ortho is 7.2 and para is 7.2...while meta....once again loses....it has a pka of 8.4. The reason is similar . For the DAT exam.....simply make sure that you understand that Groups like NO2, CN, SO3H, halogens, benzene INCREASE the acidity.....i.e lower the pka because the anion is stabilized to a greater extent.
Hope this helps.

Dr. Romano

Thank you Dr. Romano that was very helpful!
Just a quick question-- if the substituent is on m position -- should a more electronegative group decrease acidity more than a less electronegative group that is capable of doing resonance

 

[DAT Destroyer]

Thank you Dr. Romano that was very helpful!
Just a quick question-- if the substituent is on m position -- should a more electronegative group decrease acidity more than a less electronegative group that is capable of doing resonance

As a general rule, yes. Nitro is about as acidic of a group as you are going to get, thus this should be a good guideline. Placing a group in the meta position will usually result in a less acid compound.