[jithin]

how to explain acidity of phenols? can it be explained by inductive effect?

[MissPhosgene]

Charge delocalization.

[cupid.callin]

phenol is acidic because phenoxide ion is resonance stablized!!!

But next time before posting please try to google it ... post only if you cant find it on internet or in your books!!!

[orgopete]

how to explain acidity of phenols? can it be explained by inductive effect?

That is my preferred explanation. I am wary of resonance stabilization to explain acidity. Why should product stability enable a reaction? If the reaction didn't take place, how would product stability help?

A kinetic enolate can generally be prepared from the most acidic hydrogens and if one wished to, it could be equilibrated to the more stable enolate. Methylcyclohexanone is a common example. How does enolate stability or resonance stabilization enable the thermodynamic enolate formation, especially as it is not the one that forms the fastest? I would use resonance to explain the thermodynamic properties, but I would use inductive effects to explain kinetic properties, i.e., which hydrogens are more acidic.

[MissPhosgene]

I think you can use resonance in the case of phenols in particular as compared to another type of alcohol, which it seems the question was about. There are contributing structures in which there is a cylohexadienone with a negative charge on the para or one of the ortho carbons. This increases the polarization of the bond between the oxygen and hydrogen to which it is bound rendering the hydrogen atom more acidic. So, resonance doesn't mean product stabilization, it refers to delocalization of oxygen lone pairs over the aromatic system which renders the proton more acidic.

[g-bones]

how to explain acidity of phenols? can it be explained by inductive effect?

That is my preferred explanation. I am wary of resonance stabilization to explain acidity. Why should product stability enable a reaction? If the reaction didn't take place, how would product stability help?

A kinetic enolate can generally be prepared from the most acidic hydrogens and if one wished to, it could be equilibrated to the more stable enolate. Methylcyclohexanone is a common example. How does enolate stability or resonance stabilization enable the thermodynamic enolate formation, especially as it is not the one that forms the fastest? I would use resonance to explain the thermodynamic properties, but I would use inductive effects to explain kinetic properties, i.e., which hydrogens are more acidic.

Resonance is the king for explaining the acidity of phenols.  As we know, when we approach the transition state of a reaction (in this case B: + PhOH -> PhO- + BH+) the activated complex resembles both the starting material and the product (the endo- or exothermicity of the reaction describes how much it describes one or the other... this is the Hammond Postulate) and therefore the transition state is stabilized by resonance just as the product is.  Therefore, the energy barrier is lower in this deprotonation leading to conversion to the resonance stabilized product.  THAT is why product stability helps to encourage reactivity.

[MissPhosgene]

Thanks for the answer g-bones! I wasn't sure how to explain why I thought the TS would be lower in energy and why the product would influence. . It's really simple.

[orgopete]

Resonance is the king for explaining the acidity of phenols.  As we know, when we approach the transition state of a reaction (in this case B: + PhOH -> PhO- + BH+) the activated complex resembles both the starting material and the product (the endo- or exothermicity of the reaction describes how much it describes one or the other... this is the Hammond Postulate) and therefore the transition state is stabilized by resonance just as the product is.  Therefore, the energy barrier is lower in this deprotonation leading to conversion to the resonance stabilized product.  THAT is why product stability helps to encourage reactivity.

Although I agree that resonance is king for explaining the acidity of phenols, that is simply not my preferred explanation. This is my argument. It doesn't really matter about TS or anything else like that. The question is why or what factors make the OH bond like an HCl bond or why are the electrons of the oxygen pulled away from the proton. I think of TS arguments as an ex post facto argument and should not be used to explain what enables a reaction to take place in the first place.

This is my thinking, if you look at the C-H bond lengths of ethane, ethylene, and acetylene, they become shorter and the acidity increases. The electrons are being pulled away form the proton to shorten the bond length and increases the acidity. If I have a methyl ethylene unit (propene), the acidity is similarly increased because the electrons are also pulled in, and now are pulled away form the methyl, that inductive effect is translated into an increased C-H bond length and increased proton-electron pair distance, again increasing acidity. If I use a methyl ketone (=O replacing =CH2), I can make the same argument. If I replace the group on the carbonyl with another atom (Cl, H, CH3, Ph), it will be reflected in the CH3 acidity by being passed through the C=O carbon. If I replace the CH3-group of a methyl ketone with an NH2-group or an OH, I can continue to use the same arguments. In the case of an OH, I have the combined effect of having a reasonably electron withdrawing oxygen atom attached to an electron withdrawing C=O group to further increase the H-electron pair distance and increase acidity.

I argue that it is the increased proton-electron pair distance that enables the TS, not the other way around. If a methyl group were attached to a poly-ene, it wouldn't be the resonance energy that would make the proton acidic (or not). I think it would be increased in acidity for the reasons I outlined above. Because the net electron withdrawing properties of ethylenes are insufficient to create an equilibrium TS with many bases, that no reaction would take place (except with very strong bases).

If I can go from a pKa of 50 to 44 in ethane to propene, then to go from 16 to 10 in ethanol to phenol seems about right for a simple inductive effect being transmitted through an oxygen rather than a CH3 group. I concede this is not the common explanation, but it is the one that I prefer.

[MissPhosgene]

I argue that it is the increased proton-electron pair distance that enables the TS, not the other way around. If a methyl group were attached to a poly-ene, it wouldn't be the resonance energy that would make the proton acidic (or not). I think it would be increased in acidity for the reasons I outlined above. Because the net electron withdrawing properties of ethylenes are insufficient to create an equilibrium TS with many bases, that no reaction would take place (except with very strong bases).

 

Phenol is a prime example of increased proton electron-pair distance enabling the TS, and it can be explained as an effect of electronic delocalization over the ring. There is partial carbonyl character at the phenol oxygen and by NMR you can see that the proton is significantly deshielded. In addition, the chemical shifts of the five protons on the ring show chemical shifts in the order predicted by resonance.

I think the inductive effect is the minor player here. The acidity of a methyl substituted poly-ene can be explained in a similar fashion to the way the acidity of phenol can be explained using resonance arguments. Hyperconjugation can be used to explain the proton-electron pair distance or lack of electron density at the methyl group hydrogens. This happens because the pi-system is delocalized. I attached a scheme. 

Sigma-withdrawl is a major factor when dealing with saturated systems as a method to explain effects of say, the number of carbon atoms bound to the carbon with protons whose acidity is under question, but resonance is "king" for explaining the acidity of unsaturated systems or ones that can become conjugated.

[orgopete]

First, I don't mind the kind of argument you make for phenol itself. If you look back at the post I was commenting upon, it was justifying phenol's increased acidity with the resonance structure of phenoxide, "Phenol is acidic because phenoxide ion is resonance stablized!!!". The poster even suggested this could be found in a textbook or on the internet. You may agree with that argument or you may agree with me if for only that it is an ex post facto argument.

For the other examples being presented, I still prefer an inductive effect. Here is why. I generally think of double bonds and triple bonds as being more electron withdrawing than aliphatic carbons to explain their increase in acidity. The resonance structures drawn were actually a resonance lengthened inductive effect. If the terminus were a CH3-CH=, then the resonance structures could be drawn with the opposite polarization. This would not help to explain an increase in acidity, but an inductive effect would. If you compare the pKa of acetone and acetaldehyde, I argue acetaldehyde is more acidic due to the inductive electron withdrawal of the hydrogen. Even if you prefer your resonance and hyperconjugation, you will must still explain the difference with an inductive effect.

I am not opposed to resonance effects. I agree that they are king. I just don't happen to think this is an example in which they best explain the acidity of phenol and I am leery of using product stability to explain reactivity. I simply prefer to explain reactivity by the functionality of the starting material. That may include inductive or resonance effects, e.g., enamines.

[MissPhosgene]

I don't think it is an ex post facto argument. If the product were extremely unstable, would phenol be as acidic as it is?

Think about deprotonating ethane.

It doesn't seem that all parts of a reaction can necessarily be treated separately. The TS is related to both SM and products (or intermediate). You have the initial barrier going in the forward direction, but it's not a unique element of the reaction coordinate and cannot be defined without a product or TS. Why is that?

[orgopete]

Two things; first I am assuming this is the topic that I am receiving the most flags for. Secondly, I wish the forum would indicate the answer that is being flagged.

On the first, I am disappointed if I am being flagged for stating an opinion in which I thought induction might be preferred. I do understand and have used the resonance argument myself. However, I also concede that I did so because it was easier to explain.

Let me try again. If you take acetone and its enol, they form the exact same enolate. If resonance is king and acidity is determined by the stability of the product, then rationally they should have the same acidity because they form the same enolate. However, I believe the enol is more acidic than the ketone for inductive reasons, oxygen is more electron withdrawing than carbon. If an sp²-carbon is attached to the oxygen, the electron withdrawing properties of the sp²-carbon should further increase the OH acidity. This would be similar to replacing the methyl group of methanol with a chlorine to give hypochlorous acid. The chlorine draws electrons from the oxygen and increases the acidity.

If I am wrong in my analysis, explain how or why resonance should not or does not apply to the ketone-enol argument and why it should in phenol (or perhaps why an inductive rational may not be so wrong after all).

To beat a dead horse, I have written more about it here.