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Topic: Acidity of phenol group  (Read 7407 times)

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Offline ritwik06

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Acidity of phenol group
« on: August 26, 2009, 01:15:26 PM »
Which of these is more acidic?
1) 3,5-dimethyl-4-nitro-phenol
2) 2,6-dimethyl-4-nitro-phenol

I think 3,5-dimethyl-4-nitro-phenol is more acidic because the resultant anion will be more stable as compared to the one formed from 2,4-dimethyl-4-nitro-phenol- because when the two methyl groups are near to the - OH group they would drive it out of the plane of the benzene ring and hinder resonance.

Please let me know if I am right. I shall be glad if someone provides me with some data on Ka values for these phenols.

Offline Arctic-Nation

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Re: Acidity of phenol group
« Reply #1 on: August 26, 2009, 01:48:14 PM »
The problem of steric hindrance could also be applied to the 3,5-dimethyl nitrophenol. The ortho-substituents might hinder conjugation of the nitro group with the ring, resulting in weaker acidity.
Now, I don't know which effect will be most pronounced, but I think that because of size constraints, 2,6 will be more acidic.

Offline ritwik06

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Re: Acidity of phenol group
« Reply #2 on: August 26, 2009, 01:58:55 PM »
Thanks a lot for the reply. I really appreciate it!

But I would request other forum members to post their views on this and please do tell me if you are sure about it. Ka values data would be most helpful.
Thanks again.

Offline sjb

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Re: Acidity of phenol group
« Reply #3 on: August 26, 2009, 03:02:03 PM »
Don't know for sure, but try drawing resonance forms of the anions formed. I think the -ve charge is better stabilised in the 3,5-isomer, so that is more acidic. I'm not that sure that steric hindrance plays any part here, it may do if you were looking at the basic strength of the phenol.

Offline lmlh

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Re: Acidity of phenol group
« Reply #4 on: August 27, 2009, 06:41:30 AM »
i'm sure that you are right according to resonance stucture, and steric hindrance does not play any part in similar condition.

Offline ritwik06

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Re: Acidity of phenol group
« Reply #5 on: August 29, 2009, 07:07:57 AM »

Attention: SENIOR FORUM MEMBERS
more citations needed..
Thanks to all those who helped!!

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