[NewtoAtoms]

which substituent increases the acidity of a phenolic hydroxyl group when located para to it?
The choices are:

A. Ch3
B. Ch3C=O
C. C(triplebonded)N

I understand that electron donating substituent increase the acidity of phenols by stabilizing phenoxide ion.

Am I correct to assume that ONLY B would make the phenolic hydroxyl group more acidic because it could more completely stablize the phenoxide ion by donating electrons?

I am so lost on this question, and I am torn between A and B.

The true options for this question are:

a.  Only A
b.  Only B
c.  Only C
d.  Both A and B
e.  Both B and C

[macman104]

You have it backwards.  Electron withdrawing substituents, and those that have resonance are going to stabilize the phenolic O that forms when it loses the hydrogen.  Which one of those have dipoles or resonance going away from the ring?

[NewtoAtoms]

-  Ch3 does not have a dipole that goes away from the ring.
-  Ch3C=O does have a dipole that goes away from the ring.
-  C(triplebond)N also does have a dipole that goes away from the ring. 

Therefore would it be correct to assume that both B and C are substituents that would increase the acidity of a phenolic hydroxyl group, when located para to it.

Just so that I completely understand, electron withdrawing substituents (those that can ultimately accept electrons and therefore carry a negative charge) stablize the charge by bringing the O- charge down when it loses its hydrogen.  It does this due to its dipoles.

I am deeply sorry for my elementary question, but I just CANT get my head around this one part of the chapter.

Thank you for your time.

[macman104]

You are correct, B and C are more acidic than regular phenol.  Also, I hope the following picture helps clear the picture better. 

*Note*  When I say decrease or increase in acidity, I refer to their strength of the acidity, not the numerical value (since ya know, decrease would go down, and be more acidic)

The two compounds are more acidic, because the phenol oxygen is more willing to give up a proton (increased acidity) when it has somewhere for those extra electrons to go.  There are two ways to accomplish this, through inductance (electron pull), or resonance (electron delcalization).  Obviously, the opposite is also true.  If the extra electrons destabilize the system, then we observe a decrease in acidity.

CH₃ destabilizes because electron density is being pushed into the ring, when we are already trying to deal with an excess from the loss of the proton.

C(triplebond)N, has *some* resonance capability, but is also has a dipole in it's favor as well.

C(=O)CH₃ has a much larger resonance AND inductance capability, and should be the most acidic.

[NewtoAtoms]

Wow macman I thank you so much for your help and explanation, I totally understand it now.

Thank you so much.