Nucleophiles and carbonyl compounds .

[metalInferno]

Homework Statement

Why do bases like OH^- etc. remove \alpha-H from carbonyl compounds like ketones and aldehydes (aldol condensation) whereas amines and derivatives like NH_{3}attack the carbon centre of carbonyl groups ?

Homework Equations

none.

The Attempt at a Solution

Thought of all possible reasons but am still confused .

 

[chemisttree]

Many good nucleophiles are not strong enough bases to abstract protons from ketones and aldehydes. The only game in town is nucleophilic addition. Is an amine a strong enough base to deprotonate a proton alpha to a carbonyl?

In the case of hydroxide addition to a carbonyl, you have an oxyanion intermediate that is not delocalized... it is pretty much localized on the oxygen. The structure resembles an alkoxide. Stable? In a protic solvent it will pretty rapidly pick up a proton and become the geminal diol. Are geminal diols stable? In the case of alpha hydrogen abstraction, the anion is delocalized over both the oxygen and the alpha (or beta) anion. That is a more stable intermediate.

In a nucleophilic addition, the intermediate localizes the negative charge on the oxygen but it can pick up a proton from solvent and form an alcohol. Pretty stable.

In an aprotic solvent what happens?

 

[metalInferno]

ok ... i am getting some of it . Let us take a base like NH^{-}_{2} . This is a very good base . But as far as i know , this does not abstract the hydrogen , but attacks the carbon center to form amides .

 

[metalInferno]

One more question , how come NH^{-}_{2} forms amides with acid derivatives and not with aldehydes . This might help in understanding .