Destroyer OChem #207 Question

[ebadworld]

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It's a question about activating and deactivating groups....

207. Which compound is most acidic?
(a) p-chlorophenol
(b) p-nitrophenol
(c) phenol
(d) p-ethylphenol
(e) p-methylphenol

Since acidity requires more stable conjugate base, I narrowed it down to (a) and (b) but since OH is in para and ortho activating group, I thought chlorophenol would be better answer because Cl is a para, ortho deactivating group.

the answer is (b) p-nitrophenol....

I know that NO2 is a better deactivating group than Cl but since Cl is p,o directing, shouldn't it be the answer ? 😕

 

[Bereno]

The nitro group is the grand-daddy of all electron withdrawing groups. This said, it reduces the electron density between the oxygen and the hydrogen the most. This makes the p-nitrophenol the most acidic.

 

[Bereno]

Also, on a side note, I always try to stay away from the "activating" and "deactivating" terms... this is because it depends on what types of reactions are want to perform (electrophilic vs nucleophilic). This is why I tend to refer to then as either electron releasing groups or electron withdrawing groups. just my $0.02

 

[FROGGBUSTER]

Nitro withdraws thru RESONANCE (pi network) while Chlorine withdraws only thru the sigma network. The electrons are actually, truly delocalized when you have a substituent like nitro that can you can donate electrons into.

 

[mh0000]

draw the resonance structures. YOu will notice that the negative charge will be ortho and para relative to the phenoxide oxygen. Therefore, a electron withdrawing group -o or -p to the phenoxide will stabalize it. Don't worry about the o/m/p directing effects of the nitro group itself.

 

[ebadworld]

Thank you! that totally makes sense now! 🙂