Aniline

[MedPR]

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Are phenyl groups EWG or EDG? I'm asking to help determine the relative basicity of aniline (aminobenzene)

I'm guessing it is an EWG because it can delocalize electrons. So Methylamine should be more basic than aniline since EWG reduce basicity by making it harder for the amine to accept protons.

 

[SaintJude]

Benzene is an EDG. Aniline has a pkb of 9.3

Methylamine is a pKa of 10.6 (Via wiki, I don't memorize these value before I sleep)

So aniline is more basic, right?

 

[MedPR]

Benzene is an EDG. Aniline has a pkb of 9.3

Methylamine is a pKa of 10.6 (Via wiki, I don't memorize these value before I sleep)

So aniline is more basic, right?

Aniline has a pkb of 9.3? TBR says methylamine has a pkb of 3.35. So methylamine is a way better base. I'm pretty sure EDG's will increase basicity (decrease pkb), but basicity is also reduced by steric hindrance, so I'm not sure of the effects of benzene.

 

[milski]

SJ: note that you have pKa for and pKb for the other, you cannot make the comparison directly.

MPR: EWG increase acidity, so it makes sense that EDG will increase how basic the compound is. Benzene is not good at absorbing additional electrons since all the pi orbitals are already full. It would seem that donating will happen easier.

PS: just thinking here, I'm not expert in ochem.


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[MedPR]

SJ: note that you have pKa for and pKb for the other, you cannot make the comparison directly.

MPR: EWG increase acidity, so it makes sense that EDG will increase how basic the compound is. Benzene is not good at absorbing additional electrons since all the pi orbitals are already full. It would seem that donating will happen easier.

PS: just thinking here, I'm not expert in ochem.


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Phenyl alcohol is significantly more acidic than alkyl alcohols, right? Why? Is the reasoning applicable here?

 

[chiddler]

Phenyl alcohol is significantly more acidic than alkyl alcohols, right? Why? Is the reasoning applicable here?

The difference is the cationic state of oxygen vs nitrogen. Nitrogen can happily exist as a cation and oxygen cannot. So oxygen acts much much less as an EDG than does nitrogen.

I think a better comparison is ammonia vs aniline. Ammonia is pKb 4ish, aniline 9ish. Therefore, in this case, the nitrogen is losing electron density to the aromatic ring.

I'm not sure why it can absorb electrons more easily than it can donate electrons.

 

[SaintJude]

SJ: note that you have pKa for and pKb for the other, you cannot make the comparison directly.

Oops you're so right. 😳 how embarrassing...

pKb for methylamine is 3.36
pkB for aniline is ~9.3

but good questions raised in this thread-i myself would like answers to these...