[discodermolide]

There is another N atom in imidazole, but I am not sure on which atom of imidazole will the H⁺ be attached to make the conjugate acid and why.

Surely both the N atoms in the imidazole are equivalent!

[Rutherford]

So both can bind a H₊ ion from water and make OH^- ions, but in pyrrole there is only 1N atom, therefore, pyrrole is less basic, right?

[discodermolide]

So both can bind a H₊ ion from water and make OH^- ions, but in pyrrole there is only 1N atom, therefore, pyrrole is less basic, right?

Yes.

[orgopete]

So both can bind a H₊ ion from water and make OH^- ions, but in pyrrole there is only 1N atom, therefore, pyrrole is less basic, right?

No, pyridine is more basic than pyrimidine, 5.25 vs 1.1 for pyridinium, pyrimidinium ions.

Hint, Huckel aromaticity rules.

[Rutherford]

It aren't pyridine and pyrimidine, in every molecule there is a N atom bounded only with single bonds to other atoms.

Does trifluoromethylimidazole go in the middle because of the big electronegativity of F atoms, they attract the lone pairs from N atoms so they are less available for the H⁺ ion?

[orgopete]

Pyrimidine has two nitrogen atoms and is much less basic than pyridine with one.  If that argument were applied to pyrrole, it should be more basic.

Why isn't pyrrole more basic? The answer does not depend on the number of nitrogens, but the availability of the electrons. If the electrons are readily available, the nitrogen will be more basic, or why is pyridine more basic than pyrrole?

Again, think aromaticity. If you look at models, look at the orientation of the electrons in pyrrole, imidazole, and pyridine. How is imidazole like pyridine?

[Rutherford]

Okay, then I use Huckel's rule, pyrrole N atom needs to donate its electron pair to the ring so it will follow the rule, in imidazole to make the same effect, only the electron pair from one N atom is enough to follow the rule, so there is a pair that can be donated to a H⁺ ion, so imidazole is more basic. The compound in the middle has 3 F atoms that push the N electron pair from it, so it is less available, while the pair in the other N atom follows Huckel's rule.
That is clear I think, only one more thing: I don't understand why is pyrimidine less basic than pyridine. The N atom in both molecules needn't to donate an electron pair.

[orgopete]

… why is pyrimidine less basic than pyridine. The N atom in both molecules needn't to donate an electron pair.

I argue this is an inductive effect. Nitrogen has a larger nuclear charge and pulls electrons away. A similar effect can be found with hydroxyl amine (NH2OH) or hydrazine (NH2NH2), but now there is an sp2-carbon intervening. 

[Rutherford]

"Nitrogen has a larger nuclear charge and pulls electrons away."
I don't understand, away from what?

[orgopete]

"Nitrogen has a larger nuclear charge and pulls electrons away."
I don't understand, away from what?

If you begin with pyridine and replace a CH with a N, that nitrogen can pull electrons from the basic nitrogen making it less basic.

[Rutherford]

How can an atom pull electrons from another atom that has the same electronegativity?

[orgopete]

Which is more acidic of X-OH?

HOH
HOOH
ClOH

The pK_as are 16, 12, and 7.5. This seems reasonable to me.

If I compare CH2=CHOH, HN=CHOH, and O=CHOH, I would expect the O=CHOH to me the most acidic.

If I compare CH2=CHNMe2, HN=CHNMe2, and O=CHNMe2, I would expect O=CHNMe2 to be the least basic.

Even if you compare HOCH2CH2NH2 or CH2=CHCH2NH2 with ethyl or propylamine, they are less basic.

**Edit**

I am continuing to research this topic (amine basicity). Here is an interesting exception, guanidine is far more basic does not follow the trend. See http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/amine1.htm#amin2

[sjb]

How can an atom pull electrons from another atom that has the same electronegativity?

If you are talking, for instance, about the 2 nitrogens in 4-trifluoromethylimidazole, then these are not identical nitrogens. Electronegativity scales are a bit misleading in my opinion as they suggest an absolute value. Perhaps you know for instance that the CH₃ protons in methanol, methyl chloride, methyl benzoate and acetone have different δ values (we have touched on this a little in your NMR assignment post), but simplistically they can call be described as Me-(electron withdrawing group), right?

[Rutherford]

I was talking about the two N atoms in pyrimidine. Orgopete, the thing that confuses me is that the N atoms in pyrimidine are equivalent (they are in a circle) and have the same effect on the C atom and on themselves, so I don't understand how an inductive effect can appear here.

[orgopete]

I don't mean to resurrect this post on ammonium ion acidities, especially has it contains several irrelevant threads. However, I wanted to write my opinion about the pKa issues, solvation, gas phase, and structural effects. As this is long, I have included it in a blog.