[dhruv8890]

Hi,

In the ACS organic chemistry study guide, there's a question:

Arrange these compounds in the order from strongest to weakest base:

I) N2O--Ph--:NH2
II) CH3--Ph--:NH2
III) Cyclehexane--:NHCH3

where : shows an electron pair

In general, to understand which base is the stronger/strongest, I protonate each of the compounds, which adds a H to the nitrogen atom for each compound. Now, whichever compound is the most stable should be the strongest base, correct?

The book explains that the positive charge on the nitrogen cannot be delocalized in any of the conjugate acid forms. And that the base forms of I and II are resonance stabilized b/c of the aromatic ring, thus they are less basic.

Is this reasoning correct? I would think that since the aromatic ring can distribute electron density, it would stabilize the protonated forms more than the aliphatic amine would, giving them more of a "tendency" to accept a positive charge (or lose an electron pair).

Would someone kindly like to explain this to me? Am I missing something fundamental? I'd really like to understand it rather than just know it..

Thank you,
Dhruv

[stewie griffin]

The reasoning is indeed correct. Remember, it is always better to delocalize charge over as many atoms as possible. I can understand your reasoning for wanting to think that since aromatic rings are so good at distributing charge, then why can't they distribute the charge such that it helps out a poor positively charged nitrogen. That's not a crazy idea. However, draw out what would happen (for help, I did.. see below).
You see, aromatic rings are certainly willing to help out however possible. They will accept electron density if needed (equation 1), and they can even give electron density if needed (equation 2). However, what happens if you protonate the nitrogen?? How can there be any resonance now? Unlike a positively charged carbocation, which will indeed be stabilized by the aromatic ring, the positively charged nitrogen can't accept anything more. It's already got four bonds since it's protonated.
In other words, once you protonate that nitrogen, you no longer have any resonance stabilization.
Hope that helps somewhat.