[movies]

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).

[AWK]

http://www.chem.ucalgary.ca/courses/351/Carey/Ch24/ch24-1.html#Acidity

[ahmed alzawalaty]

fluorine atom doesnot increase acidity of phenol obviously due to the fact that in the 4 position ut is very far from oh group

[movies]

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!

[Mitch]

nah, its gotta be hyperconjugation.

[cbee]

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

[movies]

Excellent!!

Your prize, a scooby snack.