Nucleophiles and carbonyl compounds .

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Discussion Overview

The discussion revolves around the behavior of nucleophiles, particularly bases like hydroxide and amines, in relation to carbonyl compounds such as ketones and aldehydes. Participants explore the mechanisms of proton abstraction versus nucleophilic addition, particularly in the context of aldol condensation and the formation of amides.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant questions why bases like OH- can remove \alpha-H from carbonyl compounds while amines attack the carbon center.
  • Another participant suggests that many good nucleophiles may not be strong enough bases to abstract protons from ketones and aldehydes, leading to nucleophilic addition instead.
  • A participant discusses the stability of intermediates formed during nucleophilic addition, noting that the oxyanion intermediate is localized and can lead to the formation of a geminal diol.
  • There is a query about the behavior of amines, specifically NH2-, which is a strong base but primarily attacks the carbon center to form amides rather than abstracting hydrogen.
  • Another participant raises a question about why NH2- forms amides with acid derivatives but not with aldehydes, suggesting this could clarify the underlying mechanisms.

Areas of Agreement / Disagreement

Participants express varying views on the reactivity of different nucleophiles with carbonyl compounds, indicating that multiple competing models exist regarding the mechanisms of proton abstraction and nucleophilic addition. The discussion remains unresolved with no consensus reached.

Contextual Notes

Participants highlight the stability of different intermediates and the role of solvent in reactions, but these aspects remain underexplored and not fully resolved within the discussion.

Who May Find This Useful

Students and researchers interested in organic chemistry, particularly those studying nucleophilic reactions and carbonyl chemistry, may find this discussion relevant.

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

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