[dun13203171]

Hi guys and gals,
                        I am reading about aromatic amines and I am trying to rationalise why they are generally less masic that thier aliphatic counterparts.

So the lone pair on the nitrogen in an P orbital, the compound is reasonance stabilised, ie. the lone pair can interact with the aromaticity of the ring where several different canonical forms can be drawn. This means as the lone pair in more invloved in the ring, it is less available for protonation? or is the compound less basic because when protonated, the compound looses stability as it now cannot be resonance stabilised?

I also read that there is a greater electron-withdrawing inductive effect of the sp2-hybridized carbon of an aromatic amine compared with that of the sp3-hybridized carbon of an aliphatic amine. Is that typical? ie an sp2 hybridised carbon is electron withdrawing in comparison to an sp3 hybridised carbon?

If someone could peep it in that would be great

[orgopete]

I favor the electron withdrawing argument of sp- and sp2-hybridized atoms. I find several examples in which this explains the answer quite well, e.g. cyanide ion. Although I prefer the chemistry or physics of this type of rationale, I'm also aware that many books and other chemists prefer the resonance model. I suggest for the purpose of a course that you adopt the explanation of your text or professor.

In your post regarding phenol acidity, I see you've adopted the resonance argument. An electron withdrawal effect of an sp2-carbon could also be used for this same effect. If you were to search my posts on this topic, you can find other examples. You will also find that I've received the greatest number of flags by those disagreeing.

[Dan]

This means as the lone pair in more invloved in the ring, it is less available for protonation? or is the compound less basic because when protonated, the compound looses stability as it now cannot be resonance stabilised?

I consider these the same argument (and both valid), just approached from different directions.

I also read that there is a greater electron-withdrawing inductive effect of the sp2-hybridized carbon of an aromatic amine compared with that of the sp3-hybridized carbon of an aliphatic amine. Is that typical? ie an sp2 hybridised carbon is electron withdrawing in comparison to an sp3 hybridised carbon?

Also valid. Both resonance and inductive effects will operate and contribute to the observed trends. Which effect is dominant is debatable and probably depends on the molecule. Induction and hyperconjugation are often not given enough weight compared to resonance (orgopete has made many interesting and valuable contributions on this subject over the years), and I think both induction and resonance effects have their place in answering the question.

[dun13203171]

Can I confirm that the electron greater elctron withdrawing effect of an SP2 carbon over an sp3 carbon is the greater percentage of S character (33.3%) in an sp2 orbital, thus the electrons are held much tighter to the carbon making a lone pair less available for protonation?

I just want to check the way I am interpreting these concepts is correct.

In terms of resonance, the fact that the charge can be delocalised along the bezene ring on loss of a proton, leads to a much more stable conjugate base in comparison to the equivalent alcohol, thus phenols are more acidic as formation of their conjugate base is more energetically feasible in comparison to its equivalent alcohol.

Correct?

Feel free to make corrections on any terminology, as I am trying to get a good grasp of these concepts as I have only just started this degree.

cheers