[faust]

Hello

 that's the question !

Take a benzoate ester and a 2pyridine carboxylic acid ester. Exeperimentaly the radical anion of the benzoate ester is more stable than the one of the 2-pyridine carboxylic acid ester.

My prof ask me why... and I have no idea... for me the pyridine is more withdrawing group than phenyle... so it should be the opposite...

Do you have any idea?

Thank you!

[barcrphd]

can you give the structure? where is the radical anion generated?

[Shaun]

This must be the radical anion to which you are referring.

The group that is being stabilized is not an anion, it is a radical.  The resonance form that places the negative charge on O and the radical on C is by far the major contributor.

As for why phenyl stabilizes better than pyridyl - my guess is that pyridyl does not stabilize benzylic radicals as well as phenyl.  The main reason is that one of the resonance forms places the radical on the nitrogen atom, and radicals on N or O atoms are of substantially higher energy (and thus disfavored) relative to radicals on carbon.