Aldol Condensation: Why is OH a Good LG?

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Discussion Overview

The discussion revolves around the aldol condensation reaction, specifically addressing why hydroxide (OH) is considered a good leaving group in the final step of the mechanism. The scope includes theoretical aspects of organic chemistry and the mechanistic details of the reaction.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a mechanism involving an aldehyde and a ketone, questioning the role of hydroxide as a leaving group after heating and protonation.
  • Another participant challenges the initial mechanism, suggesting that the lone pair on the carbanion cannot effectively displace the hydroxyl group due to structural constraints.
  • There is a discussion about the nature of leaving groups, with one participant emphasizing that leaving group ability exists on a spectrum rather than as a binary classification.
  • A later reply suggests that the presence of hydroxide in the reaction mixture may facilitate its role as a leaving group, despite it being considered a poor leaving group in other contexts.

Areas of Agreement / Disagreement

Participants express differing views on the mechanism and the classification of hydroxide as a leaving group. There is no consensus on the correctness of the initial mechanism or the nature of hydroxide's leaving group ability.

Contextual Notes

Participants note that the ability of hydroxide to act as a leaving group may depend on specific conditions within the reaction environment, and that the definitions of "good" and "bad" leaving groups are not absolute.

brake4country
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Homework Statement


In a typical aldol condensation (say 2 aldehydes reacted with NaOH, water, and under heated conditions) why is hydroxide considered a good leaving group in the final step?

Homework Equations



NA

The Attempt at a Solution


I drew out a mechanism with an aldehyde and a ketone, creating the aldol product. From here, heat is added and hydroxide is protonated in solution to leave behind a carbanion on the alpha carbon. Once the double bond is created by the electrons OH is push off. How is OH a good leaving group in this case?

upload_2014-12-18_21-2-51.png
 
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The mechanism you drew is not correct in that the lone pair on the carbanion will not "kick off" the hydroxyl group as there is a methylene group between the two. Re-draw the mechanism from scratch to get a better idea of this reaction.

As far as the hydroxyl leaving group, think about what is in your flask. You even drew the reagent over your arrow in the first step. Leaving group ability is not a binary characteristic but instead is a spectrum from "bad" to "good."
 
I see what I did wrong. I connected the carbons incorrectly. Does this look better? However, how can OH leave? Isn't it a bad leaving group?
upload_2014-12-19_20-15-46.png
 
That looks better.

As far as your question goes, refer to the second part of my first post. Good and bad may be used as though they describe a binary phenomenon, but in reality things are not simply yes/no or good/bad. They all fall on a spectrum, so perhaps better terms would be better/worse instead of good bad.

Again think about what is in the flask, you have plenty of hydroxide floating around. It can't be impossible to have hydroxide as a leaving group.
 

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