[swati]

   Hello everyone

Explain why ethyl isobutyrate does not undergo Claisen Condensation reaction in presence of sodium ethoxide ?

[Borek]

It is up to you to explain. Then someone can tell you if you are right.

[Dan]

The ester must possess an ?-hydrogen atom to facilitate the first step of the reaction - deprotonation.

Isn't ethyl isobutyrate Me₂HCCOOEt? in which case it can enolise in the first step.

But I would suggest that the bulky enolate formed will not add to the hindered carbonyl of the ester on steric grounds.

[movies]

It could add, but there might be a problem after that.  Think about what drives the Claisen condensation to completion.

[swati]

It could add, but there might be a problem after that. Think about what drives the Claisen condensation to completion.

After the addition of enolate to carbonyl of ester ^-OEt is removed & the reaction is drawn to completion by deprotonation of the active methylene in the product by the ethoxide. But there's no acidic hydrogen in the compound after the removal of ^-OEt.

But I have a doubt .
I think formation of enolate ion won't take place due to +I effect of 2 CH₃ groups attached to ?-carbon atom.
CH₃ groups will increase the electron density at ?-carbon atom making the release of H⁺ difficult.

[movies]

I agree with you that the two methyl groups do make the alpha carbon somewhat less acidic, but the difference is pretty small.  For example, imagine a ketone with a phenyl group on one side and a methyl group on the other.  The pKa of that molecule is 24.7 (DMSO solvent and reference, 25 degrees C).  If you change the methyl group to an ethyl group, the pKa is 24.4 (pretty much the same), and if you change to an isopropyl group, the pKa is 26.3.  So it does go up, but not a whole lot.  This should hinder whether or not you can form the enolate; you don't need to make the enolate quantitatively to do the Claisen condensation.

[Dan]

But I would suggest that the bulky enolate formed will not add to the hindered carbonyl of the ester on steric grounds.

...yeah, that was a quite a poor argument. I realised as soon as I read movies' post what it was - I remember it being strongly emphasised in a lecture last year.

[movies]

Don't feel bad, I always forget that too.  I only remember because they have just learned the Claisen condensation in the class that I am TAing right now!

[swati]

 
Thank You Everyone.

[Donaldson Tan]

Don't feel bad, I always forget that too.  I only remember because they have just learned the Claisen condensation in the class that I am TAing right now!

I just learned it in lecture too. I am taking organic chemistry lectures with first year chemists..