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Author Topic: Phenol Acidity  (Read 26001 times)

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Phenol Acidity
« on: December 09, 2004, 06:22:04 AM »

Qualititaively rationalize the following observations.  What accounts for the differences in pKa?


- The pKa of phenol, C6H5OH, is 9.95 (H2O reference, 25 oC).  

- The pKa of 4-fluorophenol, C6H4OHF, is 9.95 (H2O reference, 25 oC).  

- The pKa of 4-chlorophenol, C6H4OHCl, is 9.38 (H2O reference, 25 oC).
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ahmed alzawalaty

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Re:Phenol Acidity
« Reply #2 on: April 26, 2005, 08:57:13 AM »

fluorine atom doesnot increase acidity of phenol obviously due to the fact that in the 4 position ut is very far from oh group
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Re:Phenol Acidity
« Reply #3 on: April 27, 2005, 07:37:13 AM »

But fluorine is a better inductive withdrawing group than chlorine and 4-chlorophenol has a lower pKa, so apparently it is close enough to have an inductive effect!
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Re:Phenol Acidity
« Reply #4 on: April 27, 2005, 08:17:19 AM »

nah, its gotta be hyperconjugation. :P
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cbee

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Re:Phenol Acidity
« Reply #5 on: June 21, 2005, 05:48:50 PM »

I have a guess.  Fluorine, although strongly electron withdrawing, also has a lone pair to  donate.  This will destabilize the resonance structures of the phenol anion (-ve charge at the ortho and para position with regard to the phenol, meta to the fluorine).  It looks like the inductive and resonance effects cancel out.

This effect is less with the rest of the halogens becuase they all have larger shells so the p orbital (or the corresponding hybrid orbital) overlap will be better for fluorine when donating into the carbon pi system.

Maybe?

Cisco
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Re:Phenol Acidity
« Reply #6 on: June 21, 2005, 06:44:42 PM »

Excellent!!

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