Comparing acidity of Phenol

[teefdocta]

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So I got a question. Which position of the withdrawing group make phenol a stronger acid? O or P? Chad said in his video that CF3 at O position would make a stronger acid since it's closer to the OH group and thus make the base more stabilized with the dipole induction nearby. But then Bootcamp says otherwise, the P position of withdrawing group be better stabilized the conjugate base and make it the strongest one out of 3.

My test is on Monday and these little things are just come to irritate me so bad 🙁

 

[DAT Destroyer]

So I got a question. Which position of the withdrawing group make phenol a stronger acid? O or P? Chad said in his video that CF3 at O position would make a stronger acid since it's closer to the OH group and thus make the base more stabilized with the dipole induction nearby. But then Bootcamp says otherwise, the P position of withdrawing group be better stabilized the conjugate base and make it the strongest one out of 3.

My test is on Monday and these little things are just come to irritate me so bad 🙁

Let us use a source from a PhD chemist. I will use values from the Graham Solomon text..... Ortho and para nitrophenols are about the SAME......pKa 7.17 vs 7.15....The Carey text pegs them BOTH at 7.2 .

The meta isomer is much less acidic...it has a pKa of 8.4. In nitrophenols, BOTH resonance and Inductive effects are operating. In the ortho and para isomers, we can write an additional resonance form NOT seen in the meta isomer.

This is a HUGE test question that professors including myself LOVE !!!

Please consult a text and be able to draw these structures out to prove this point. Lets us look at another electron-withdrawing group...a chlorine atom. In chlorophenols......the ortho isomer is actually the more acidic, probably due to an inductive effect. Induction is indeed competing with resonance.....but as you can see, it is not always 100% predictable. The ortho and para isomers are usually VERY close.....with meta usually the weakest acid.

Now you try....... How would the acidities of meta and para cyanophenols compare ? Here is the answer..... The values are.......para 7.95.........while the meta is 8.61. More resonance forms can be written for the para isomer......Meta loses again. Even though the meta is closer to the OH group, ...Resonance TRUMPS INDUCTION.

I hope this helps.

Dr. Jim Romano

 

[teefdocta]

Now I get it. If we take the resonance into account, then the meta would have a more stabilized resonance. And the induction effect of such Halogen like Cl and F would be best obtain at ortho?
Thank you Dr. @orgoman22! I also want to say thank you for the very good structure of both Gen and Organic Chemistry in Destroyer. Wish I would have more time to tackle the Bio section thou!

 

[510586]

Now I get it. If we take the resonance into account, then the meta would have a more stabilized resonance. And the induction effect of such Halogen like Cl and F would be best obtain at ortho?
Thank you Dr. @orgoman22! I also want to say thank you for the very good structure of both Gen and Organic Chemistry in Destroyer. Wish I would have more time to tackle the Bio section thou!

How come the meta would have a more stabilized resonance? Didn't he say that resonance trumps induction in the last paragraph?

 

[DAT Destroyer]

How come the meta would have a more stabilized resonance? Didn't he say that resonance trumps induction in the last paragraph?

510586

No....You missed my point. As far as resonance goes....the meta isomer has FEWER resonance forms ...thus is the least acidic !!! Go to any Organic text, and look at the structures !!!!

Dr. Romano

 

[DAT Destroyer]

Now I get it. If we take the resonance into account, then the meta would have a more stabilized resonance. And the induction effect of such Halogen like Cl and F would be best obtain at ortho?
Thank you Dr. @orgoman22! I also want to say thank you for the very good structure of both Gen and Organic Chemistry in Destroyer. Wish I would have more time to tackle the Bio section thou![/QUOTE
]

Good luck on Monday!

 

[510586]

No....You missed my point. As far as resonance goes....the meta isomer has FEWER resonance forms ...thus is the least acidic !!! Go to any Organic text, and look at the structures !!!!

Dr. Romano

Yes that was what I interpreted. I thought the user said that meta would have a more stabilized resonance. Wouldn't it have less resonance? You said that meta loses because resonance trumps induction

Now I get it. If we take the resonance into account, then the meta would have a more stabilized resonance. And the induction effect of such Halogen like Cl and F would be best obtain at ortho?
Thank you Dr. @orgoman22! I also want to say thank you for the very good structure of both Gen and Organic Chemistry in Destroyer. Wish I would have more time to tackle the Bio section thou!

 

[DAT Destroyer]

Yes that was what I interpreted. I thought the user said that meta would have a more stabilized resonance. Wouldn't it have less resonance? You said that meta loses because resonance trumps induction

There is NOTHING wrong with the meta isomer at all.......resonance is stabilizing......but resonance is MORE stabilizing for the ortho and para isomers. I hope that is now cleared up ! Keep up the great work.

 

[510586]

There is NOTHING wrong with the meta isomer at all.......resonance is stabilizing......but resonance is MORE stabilizing for the ortho and para isomers. I hope that is now cleared up ! Keep up the great work.

Alright awesome. So was that bootcamp example some exception we should just memorize then? (para beating out ortho when ortho usually has more induction and same amount of resonance)

 

[DAT Destroyer]

Alright awesome. So was that bootcamp example some exception we should just memorize then? (para beating out ortho when ortho usually has more induction and same amount of resonance)

Absolutely not, reread the post that I just wrote, the ortho and para are very close and difficult to predict. You will never see a question on the DAT where you will have to discriminate between values that are this close.

Hope this helps.

Dr. Romano

 

[510586]

Absolutely not, reread the post that I just wrote, the ortho and para are very close and difficult to predict. You will never see a question on the DAT where you will have to discriminate between values that are this close.

Hope this helps.

Dr. Romano

Oh ok. I think in a destroyer or odyssey question you also had ortho being the most acidic in one of the problems so if it ever comes up I think it would probably be safest to choose ortho.

 

[DAT Destroyer]

Oh ok. I think in a destroyer or odyssey question you also had ortho being the most acidic in one of the problems so if it ever comes up I think it would probably be safest to choose ortho.

Fair enough...

 

[teefdocta]

How come the meta would have a more stabilized resonance? Didn't he say that resonance trumps induction in the last paragraph?

It was my bad and you're right

 

[510586]

It was my bad and you're right

Oh alright was just checking. I'm not clear on a lot of stuff too