[Anthasci]

I have a question regarding the substituent constants in comparison of some more common functional groups. So, I know that if σ>0, it indicates an EWG and σ<0 indicates an ERG. I know the σ-values of Me are:

σ_m=0.11
σ_p=-0.29

and since the para-value is smaller, this means that the acidity of the benzoic acid is greater when Me is bonded to the meta spot (it then acts as an EWG, as opposed to ERG behaviour on para).

On the contrary, CN values are:

σ_m=0.62
σ_p=0.67

Now I understand that CN is a relatively good EWG, and I understand why benzoic acid is more acidic with CN bonded on it. What i don't understand, is how the CN is able to make benzoic acid more acidic when on the para spot, since it's further away than meta and thus the induction effect is lesser. Could it be the steric effects? Similarly, how is the methyl group, which is both +I and +M is able to stabilize the anion - and in turn, increase the acidity - BETTER when bonded to a meta spot? Analogically to the CN group, shouldn't this destabilize the anion, because the methyl group would further saturate it with electrons? I can't understand why it's not the opposite (meaning the acid would be more acidic with Me on the para spot).

[Dan]

I know the σ-values of Me are:

σ_m=0.11
σ_p=-0.29

These are the Hammett constants for methoxy (OMe), not methyl (Me). Try to rationalise the trend for OMe.

What i don't understand, is how the CN is able to make benzoic acid more acidic when on the para spot, since it's further away than meta and thus the induction effect is lesser

Draw resonance structures for benzonitrile. Where are the most δ⁺ centres? Would you expect a -CO₂^- substituent to be more stable at the meta or para position?

[Anthasci]

Yeah, my bad, I later changed it to Me, but forgot to change the numbers.

Anyway, would the structures be something like this?

And, of course, the delocalization inside the ring - this leads me to believe that δ⁺ centres are on the ring itself?

I don't understand the second question - you mean in relation to CN or COOH as in benzoic acid?

[Dan]

Anyway, would the structures be something like this?

Yes

And, of course, the delocalization inside the ring - this leads me to believe that δ⁺ centres are on the ring itself?

Yes

I don't understand the second question - you mean in relation to CN or COOH as in benzoic acid?

I mean:

[Anthasci]

I'd say the first one, but that's only knowing that the M effect is lessened on the meta spot and thus the electron withdrawing effects are greater on the p-spot, thus stabilizing the anion.

I don't quite know how to explain this. I looked it up on wikipedia (Hammett equation) and it seems the meta effect works across. In this case, the CN group on the meta spot would be withdrawing the electrons via aligned p-orbitals from the C atom directly across it and not from the carboxylate ion.

[Dan]

You said

Now I understand that CN is a relatively good EWG, and I understand why benzoic acid is more acidic with CN bonded on it. What i don't understand, is how the CN is able to make benzoic acid more acidic when on the para spot, since it's further away than meta and thus the induction effect is lesser.

Then

the M effect is lessened on the meta spot and thus the electron withdrawing effects are greater on the p-spot, thus stabilizing the anion.

Which is fine. I'm not sure I completely understand the source of the confusion.

It is important to realise that the carboxylate anion is not stabilised directly by resonance as the -ve charge cannot be delocalised across the π system in either case.

By considering the -M effect of CN, we can see that the ortho and para positions of the ring are positively polarised (δ⁺). These δ⁺ positions withdraw electron density from substituents attached to them. In the case of the carboxylate substituent, the δ⁺ C is inductively withdrawing (stabilising the anion). I would argue that a carboxylate at the para position experiences a greater -I effect than if it were at the meta position. The origin of the stronger -I effect at the para position is the -M effect of the CN group.

[Anthasci]

Ok, let me recap and see if I get this now;

I just drew the resonance structures for benzonitrile and indeed, the orto and para spots were the ones able to have a carbocation (in relation to the CN group). Therefore, when a carboxylate anion is located on the para spot and the carbocation as well, it stabilizes the anion, but the real question lies here - carbocation is an EWG? That would clarify things quite a bit for me.

[Dan]

carbocation is an EWG?

You tell me, what do you think?

[Anthasci]

Everything seems to add up to the fact that it is, indeed, an EWG