Jul 31, 2016
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I feel like I've gone over this a million times but still can't wrap my head around it conceptually. I just got this problem on a practice test: Which of the following is the weakest base? C shows pyrrole and D shows pyridine, but I don't understand why pyrrole is the weaker base? It has something to do with lone pair availability/aromaticity I'm pretty sure but I just don't get this whole concept at all- any input would be greatly appreciated
 

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mOlar1!

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Jun 24, 2016
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okay so you are on the right track. the lone pair is responsible for the base strength. which mean you wanna look for a one pair that is not part of the cyclic conjugated system. if your N has double bonds in the the conjugated system, then it's lone pair doesn't count which is why the option you chose is wrong. on the other hand, in the first option, N doesn't have a lone pair in the conj cyclic system, so you'll count it's lone pair. Hence, granting it basic. hope that helped.
 
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pulp_fiction
Jul 31, 2016
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So if the lone pair is not part of a conjugated system it's more basic? And if it is part of the conjugated system it's less basic?
 

rolltide15

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May 10, 2015
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Okay I could totally be wrong because I get these confused to but the way I looked at it was the weaker base is the one that is the stronger acid. Acids are stabilized by resonance so pyrrole has more resonance structures due to the lone pair not being in the conjugated system. since pyridine has the lone pairs participating in the cyclic conjugated system there isn't anywhere for the electrons to move if you try to draw the resonance structures you only get the same one but the double bond is on the other side of the nitrogen. The pyrrole has a lot more resonance so it is able to stabilize the charge better making it a better acid and a weak base.
 
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mOlar1!

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lol okay let me clarify this in an easier way lone pair itself isn't doing anything. look at the hybridization. more losely electrons are held, more basic it is. first choice N is sp3 so electrons are more loosely held than in second option sp2.