[tilly94]

In a week I've got my organic chemistry final coming up (again), I've already failed it two times before. One of the main questions is always about acidity in relation to mesomeric (resonance) and inductive effects and I always score the lowest on this question. Can someone please explain me how I should handle this question? The question starts with a reference molecule and the goal is to explain, by ticking boxes, if the molecule is less or more acidic than the one above and why. MD stands for mesomeric donor and MA for mesomeric acceptor, ID for inductive donor and IA for inductive acceptor. 'Andere' means 'other' and apparently this is for when there is steric hindrance (? sorry I don't know the English word for it) or any other effect that can influence the acidity. The underlined 'H' is the one who is leaving. I would appreciate it so much if someone could just explain with the help of this one exercise why each of the molecules is more or less acidic than the one above. Oh and 'benzoëzuur' means 'benzoic acid', and 'nieuwe referentie' means 'new reference molecule'  

[Irlanur]

to you know how and why mesomeric/inductive donors/acceptors (also called pi and sigma) influence the acidity?
do you know how to recognize if its MD/MA/ID/IA ?

[Babcock_Hall]

Forum rules require you to show an attempt before we can help you.

[tilly94]

The things I know and learned so far: The molecule is more acidic if the conjugate base is more stabilized. Inductive effect is usually less powerfull than the mesomeric effect except for halogens. But it's the recognizing of mesomeric effect and inductive effect and how they work that is the hardest for me. I know the answers to the exercise because of the answersheet, but in some cases I don't understand why the answer is the right one. For instance: the first molecule is less acidic than the reference. The difference between the molecules is a -CH3 bond, which is supposed to be a inductive donor and therefore it unstabilizes the conjugate base? Is this right?  And why is CF3OH less acidic than CF3-COOH? And why is the benzoïc acid a mesomeric acceptor, and therefore more acidic?
And for the bottom section: I know that molecules in the m-place stabilize the conjugate base more than in the p-place. But why is p-NO2 benzoïc acid more acidic than m-Cl-benzoïc acid? And why is p-NO2 a mesomeric acceptor, while the rest from the bottom section are all mesomeric donors? I guess you can say I just don't understand the mesomeric effect

[tilly94]

This one is even harder: what is happening with molecule numer 4? why is this a mesomeric acceptor, and 'other'? and why does this make it less acidic than 3?  and why is the H+ going of at question number 5 even less acidic? And what is happening at the 12th molecule? why is it a mesomeric donor?
I'm completely lost I'm sorry   

[Irlanur]

I guess you can say I just don't understand the mesomeric effect

That's the first step to improvement;)

let's recap:

Acid/Base Questions are all about Equilibrium, which is all about Thermodynamics. We look at an acid and it's conjugated base. To understand this equilibrium, you have to look at both sides of the equation. The acidity is determined by the energy difference of the acid and the conjugated base. So:

if you stabilize the protonated form, you make the molecule less acidic.
if you destabilize the protonated form, you make the molecule more acidic.
if you stabilize the deprotonated form (the conjugated base), you make the molecule more acidic.
if you destabilze the deprotonated form, you make the molecule more acidic.

usually, one of the latter two is explaining these kind of questions.

Now further. What do we mean by "mesomeric effect". To put it simple: usually after deprotonation, the conjugated base carries a negative charge, and charge separation means higher energy. If the molecule is able to distribute this charge over several atoms, we can assume that this stabilizing. how do we know if the molecule can distribute/delocalize this negative charge? We draw all the sensible lewis structure, all resonance structures. do you know how to do that? do you see the difference between RCOOH and ROH?

"Inductive effects" are usually not that strong. But some groups can "pull or push" electron density. Pulling electrons from the negative charge is stabilizing. and vice versa.

the first molecule is less acidic than the reference. The difference between the molecules is a -CH3 bond, which is supposed to be a inductive donor and therefore it unstabilizes the conjugate base? Is this right?

yes, alkylgroups are inductive donors.

(it's always like that in OC (for me), there are concepts which seem to work quite well. but after all, you have to have seen it somewhere to really know;))