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IS NH2 more basic than CH2?

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csulapredental

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I know basicity goes <---- this way so the least electronegative the stronger base but one of chads quizzes there was an exception where the NH2 was more basic than c triple bond SO I WAs wondering if its same for all N compared to C
 

Don Kim

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So with NH2(-) and C2H (-), the former is more basic despite the charge being on the more electronegative atom (N > C) because the negative charge on the latter terminal alkyne is stablized by the sp orbital (triple bond), making it a weaker base. The NH2 would hold the basic electrons tighter due to Nitrogen being more electronegative, but it turns out the tight bonding (sp orbital) holds the basic electrons on C2H even tighter. So the net result is NH2 basic electrons being held more loose than those of C2H. However, it does not apply to all Ns and Cs. A CH2 (-) would be more basic than NH2(-). The previous two are exceptions for a reason.

Hope that helped!
 
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DAT Destroyer

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I know basicity goes <---- this way so the least electronegative the stronger base but one of chads quizzes there was an exception where the NH2 was more basic than c triple bond SO I WAs wondering if its same for all N compared to C
Exceptions Abound in the World of Organic Chemistry !!!! Welcome to the real world !!!! As a general rule, the more stabilized the anion, the more acidic a compound is.......For example alcohols are much less acidic than thiols because the Sulfur is larger and can stabilize the anion to a greater extent. The Pka of an amine is around 35..that of an alkyne is 25 !!!!! The anion " rule " doesnt work here.....because another factor is operating...ORBITALS !!!! Ammonia is sp3,,,,,and the alkyne is sp.......EVIDENTLY.......the hybridization in the sp allows a more stabilizing effect than looking at the atom bearing the negative charge. My suggestion......Keep life simple......Know a few pKa values,,,,,,, carboxy acids are 5........phenols and thiols about 10........alcohols about 16.......aldehydes and ketones about 18........alkynes 25.. amines 35........alkenes 44...and alkanes 52.

Hope this helps.

Dr. Romano
 
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