Resonance help please!! How does resonance increase stability and thus acidity?

I'm so confused about this topic. I can't find anywhere that it's explained that clearly. :/

--Does resonance only occur with the conjugate bases with a double bond (and not the acid)? And if so, is that because of the lone pair/negative charge on the conjugate base (which can be donated to form a pi bond)?

--Also, it says that resonance increases stability/more resonance forms increases stability because the electrons are more spread out. But I don't get this because the electrons themselves aren't spread out over the molecule, they're still just as concentrated/localised, but just in a different part of the molecule aren't they? (I'm thinking of how the double bond and lone pair simply seem to be in a different position).

--My friend said it increases stability due to the fact that bond forming releases energy but this confuses me aswell because for 1) you'd also be breaking the 'previous' bond which would require energy so the energy wouldn't really be lowered, it would stay theoretically the same and 2) the resonance forms are just theoretical and the bonds aren't actually being broken and formed as the true structure is a hybrid of the resonance forms?

--I don't really get how it affects acidity. It says that the more resonance forms, the more stable the structure and so the more acidic it is. I originally thought "oh, well if it's more stable then how will it dissociate more easily? :s-smilie:

'.
My friend said that they think it's because it's only for the conjugate base and not the actual acid itself, so say for a weak acid: if there were less resonance forms i.e. less places to be able to move the electron pair to form a pi bond, then the electrons will be stuck on the atom itself i.e. be a lone pair which is going to be more reactive/less stable as it will attract the H+ back more readily. But for a strong acid, there will be more resonance forms so that there's more places to form pi bonds e.g. like in a benzene ring. And because these pi bonds can keep getting formed/'move around the ring' then it's harder for the H+ to be attracted to the structure again, i.e. the acid will be stronger because it's more on the right hand side of the equillibrium and more dissociated? Is this right?

Sorry for all of the questions! Even answering one of these questions would be incredibly useful thanks!

Reply 1

username1445490

9

Resonance occurs in the conjugate base of the carboxylic acid between the C=O and the C-O^-
In a resonance structure the electrons are more spread out as they cover a larger area and are not held on one atom as a lone pair are. This means the pair of electrons are less available to bond with H⁺ (this is what makes it 'more stable'). If it is less attractive to H⁺then by definition it is a weaker base and so the corresponding acid must be a stronger acid.
Compare to alcohols which are not acidic as they do not donate their H⁺in aq solution.
Unlike the carboxylate ion, the lone pair on an alkoxide ion (an alcohol which has lost its H⁺) is localised and held on the oxygen atom. This increases the electron density and makes it much more attractive to H⁺ this means the alkoxide ion is a very strong base and so the conjugate acid (alcohol) is a very weak acid (so much so it does not donate the H⁺ aq solution.

(edited 9 years ago)

Reply 2

m121

OP

15

Original post by Madasahatter

Resonance occurs in the conjugate base of the carboxylic acid between the C=O and the C-O^-
In a resonance structure the electrons are more spread out as they cover a larger area and are not held on one atom as a lone pair are. This means the pair of electrons are less available to bond with H⁺ (this is what makes it 'more stable'). If it is less attractive to H⁺then by definition it is a weaker base and so the corresponding acid must be a stronger acid.
Compare to alcohols which are not acidic as they do not donate their H⁺in aq solution.
Unlike the carboxylate ion, the lone pair on an alkoxide ion (an alcohol which has lost its H⁺) is localised and held on the oxygen atom. This increases the electron density and makes it much more attractive to H⁺ this means the alkoxide ion is a very strong base and so the conjugate acid (alcohol) is a very weak acid (so much so it does not donate the H⁺ aq solution.

I thought it was only in conjugate bases as well, or at least in things with a double bond/pi bond and a lone pair which is what I read in my text book... but then benzene also has resonance forms and although it has the double bond(s), it doesn't have a lone pair? Also, doesn't resonance occur in things other than conjugate bases of carboxylic acids? E.g. benzene like mentioned before and I've seen like those zig zag skeletal formulas with double bonds.
Thanks for your help btw :smile: