Phenol acidity.. contradiction??

[minutemen11]

why does the OH group which is supposed to be highly electron donating on a benzene ring make it acidic?? furthermore.. why does wikipedia call phenol aromatic? I get it satisfies hukels rule.. but does that mean that any substituted benzene is aromatic?

also am i correcting in saying that Electron donor LOWERs acidity (more basic) and E withdrawers INCREASE acidity?? This would make -NO2 on a benzene a crazy good acid right?

i've always seen something as acidic or very basic as being quite reactive... but E withdrawing groups deactivate the ring.. and still make it acidic? huh? i get its more acidic due to resonance but its still hard for me to understand conceptually...

---

Members don't see this ad.

 

[inaccensa]

http://www.chemguide.co.uk/organicprops/phenol/acidity.html

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch24phenols.html

 

[inaccensa]

why does the OH group which is supposed to be highly electron donating on a benzene ring make it acidic?? furthermore.. why does wikipedia call phenol aromatic? I get it satisfies hukels rule.. but does that mean that any substituted benzene is aromatic?

also am i correcting in saying that Electron donor LOWERs acidity (more basic) and E withdrawers INCREASE acidity?? This would make -NO2 on a benzene a crazy good acid right?

i've always seen something as acidic or very basic as being quite reactive... but E withdrawing groups deactivate the ring.. and still make it acidic? huh? i get its more acidic due to resonance but its still hard for me to understand conceptually...

When u look at Huckels rule, 4n + 2pi, u are essentially looking at carbon skeleton for benzene. treat any substituents as if you are looking at hydrogens. Aromaticity doesn't depend on the substituents. Remember than benzene undergoes substitution and not addition, which means that it is still compeletly conjugated. All the carbons are sp2 hybridized (with an empty unhybridized p orbital) and thus the structure is flat and it is cyclic.

 

[minutemen11]

so phenol is the only exception where an E-donating group actually raises acidity?

i already knew that stuff about the resonance.. im more concered with why the acidity would RISE when an -OH group attaches to an unsubsituted benzene. aka why would the acidity of the benzene rise...

but those site did help me clear other doubts....

 

[inaccensa]

so phenol is the only exception where an E-donating group actually raises acidity?

i already knew that stuff about the resonance.. im more concered with why the acidity would RISE when an -OH group attaches to an unsubsituted benzene. aka why would the acidity of the benzene rise...

but those site did help me clear other doubts....

The answer like you said lies in the resonance stabilization effect on the phenoxide ion. The H of the OH is more acidic than the H of benzene. Think of as being attached to an electronegative atom. It can easily lose the H-bond, the resulting phenoxide is stabilized by resonance, the negative charge on oxygen is spread over 6 different carbons, increasing the stability and decreasing the energy. Benzene undergoes substitution reaction, if you simply remove a hydrogen from the benzene structure, the ring become destabilized. I think of it as in phenol, the ring is intact and the oxygen can bear the negative charge, but in benzene basicallty are disrupting its aromaticity. I do believe phenol is a special case. I'd like someone to validate that.

 

[inaccensa]

You can also technically call phenol an acid, but NOT benzene.

 

[Hemichordate]

Does anyone know if benzene is electron withdrawing or electron donating?

 

[inaccensa]

Does anyone know if benzene is electron withdrawing or electron donating?

donating