Strongest Base

[cc609]

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Between negative on Alkyne with a negative on a NH2, why is NH2 - a stronger base? When I follow the basicity trend which increases basicity left and up, Carbon is left of Nitrogen so I assumed it would be the alkyne

 

[FeralisExtremum]

This is an exception to the CARDIO rules, and one worth knowing. The sp orbital conformation of the acetylide ion stabilizes it enough to be a weaker base than -NH2. Normally orbitals get bottom priority in CARDIO rules, but in this case it does not. Important to know because if -NH2 wasn't a stronger base, we wouldn't be able to use it to create the acetylide ion from alkynes.

 

[DAT Destroyer]

Between negative on Alkyne with a negative on a NH2, why is NH2 - a stronger base? When I follow the basicity trend which increases basicity left and up, Carbon is left of Nitrogen so I assumed it would be the alkyne

Think in terms of Pka values.....The NH4+ is about 9......alkynes about 25,,,,,,, The weaker acid....namely the alkyne will be the stronger base. As a matter a fact.....NH2-...the amide ion.....is one of the very strongest bases.......few bases can beat it.

Also.....for a bit more theory.....the carbon on the alkyne has the electrons in an sp orbital......these are held tightly.....thus a bit " reluctant " to share them with a proton.

Hope this helps

Dr. Romano