[Kithairin]

Hi everyone, I would like to ask you something appeared in this morning's test:
Put these anilinium ions in order of acidity and explain the reasons:

Anilinium Ion; 3-methoxy Anilinium Ion; 4-methoxy Anilinium Ion; 3-Nitro Anilinium Ion; 4-Nitro Anilinium Ion

I thought that the most acid compound is the 3-Nitro anilinium Ion, but I'm not certain of it, can you give me an hand?

[azmanam]

I think 3-nitro is incorrect.

Which substituents are electron withdrawing?  which are electron donating?  how does that effect acidity?  why does it matter where the substitutent is positioned?  which substituent in which position will cause the anilinium to be most acidic?

[Kithairin]

Yes, the 3-nitro is incorrect, i wrote it wrong and I couldn't find the edit function

Methoxy should increase the acidity because it is an electron donating substituent, but it has an effect only in orto and para position because the charge dolocalyzes only in orto and para, right?

[azmanam]

charge can theoretically delocalize regardless of the position of the substituent.  In certain positions, however, the delocalization of certain substituents can alter the electronics of other substituents (such as acidic protons).

Does adding electron density or removing electron density make a proton more acidic?  which substituents add electron density, and which remove it?  which substituent in which position will cause the anilinium to be most acidic?

[Kithairin]

If the charge theoretically delocalizes regardless of the position, then an electon withdrawing group should stabilize the cation, making it less acidic; the opposite thing happens with an electron donating group

[azmanam]

an electron withdrawing group should stabilize the cation, making it less acidic;

no...

Which is more acidic: acetic acid, or trichloroacetic acid?

And while charge will delocalize regardless of position, one position will cause the acidic proton to be even more acidic than the same substituent in a different position.  So both substituent and position will be important for this answer.

[Vidya]

if you make it more electron rich then it will become more difficult to remove positive charge .Do you agree ?
So think on this line
then decide who will increase acidity -methoxy or nitro

now check for position -nitro is meta directing  and methoxy is ortho /para directing  .
Now decide which position is best to make it electron deficient or more acidic on the basis of nitro or methoxy group.