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Topic: Basicity of Nitrogen Heterocycles  (Read 2512 times)

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Offline GabrielOPerez

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Basicity of Nitrogen Heterocycles
« on: September 10, 2013, 12:18:58 PM »
Hey guys, I got this problem wrong on a problem set and I am seeking the correct answer. It's the picture attached. The question is: which is the most basic?

The bottom nitrogen in each case is the most basic atom. So when you protonate the first structure, you'll get resonance among the two nitrogens & carbon. When you protonate the second one, you'll get the same, but you'll also have a resonance structure in which both the nitrogens have a positive charge, and the oxygen has a negative charge. The final structure possess that Sulfur atom, and I thought when you protonated the nitrogen, the sulfur would pull on the nitrogen's electrons leading to instability. But there's resonance with the sulfur, so I'm not sure.

Answer was: Last structure is most basic, middle is least basic.

And you can't use pKa values.

Offline spirochete

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Re: Basicity of Nitrogen Heterocycles
« Reply #1 on: September 11, 2013, 09:46:12 PM »
This requires some hand waving but:

In all cases we're considering the stability of the conjugate base. Since the conjugate base is positively charged, we will think about things that concentrate or delocalize the positive charge. All 3 are identical in that they have that imidazole type unit with one nitrogen donating to the other by resonance.

In the middle, all you have is that electronegative oxygen pulling electron density away by both induction and resonance. That's destabalizing to a structure with positve charge.

On the left you have an amine substituent a few carbons away that is electron donating by resonance, but if you work out the lewis structures it is not actually conjugated with the basic nitrogen we care about. So overall it's hard to say.

on the right you have that additional sulfur next door that can push some electron density onto the basic nitrogen by resonance. This is in contrast to the amine substituted one where the lone pair was not conjugated with the basic nitrogen.  Sulfur is not that electronegative, so it's electron donation should win out.
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