Dec 15, 2013
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Tetramethyl guanidine conjugate acid pKa around 12 is much stronger base than N,N-dimethylacetamide conjugate acid pKa around 0.5? I need a very good explanation of this. I would greatly appreciate it?
 

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Tetramethyl guanidine conjugate acid pKa around 12 is much stronger base than N,N-dimethylacetamide conjugate acid pKa around 0.5? I need a very good explanation of this. I would greatly appreciate it?
Look at the structure of tetramethyl guanidine.....we call this a nitrogen superbase ! If this molecule accepts a proton and acts as a base ,,,we get an enormous amount of RESONANCE stabilization. The resonance structures that you draw out will show that all 3 nitrogen atoms partake in delocalization of the positive charge. Protonation of the amide nitrogen would cause a loss of resonance stabilization...... assuming protonation did occur,,,,,the positive Nitrogen would also be flanked by a carbonyl group which would also act as a destabilizing factor. Draw this out to convince yourself.

Hope this helps.

Dr. Romano
 
Feb 9, 2012
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Here's another simple way to think about it. Methyl groups are electron donating groups, and help a molecule become a better lewis base (electron donor). The amide has a carbonyl group, and fewer methyl / electron donating groups. Carbonyl groups as you know from aromatic additions are electron withdrawing groups, which make compounds better lewis acids.
 
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