Alkali hydrides in org. synthesis

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In summary, the conversation discusses the reaction between C2H5COCH3 and KH, where the participating hydrogen is beta to the carbonyl group due to the electronegativity of oxygen. The reaction would not take place with an alkane such as C4H10, as the pKa for hydride is significantly lower than that of an alkane. The conversation also touches on the importance of resonance stabilization and the use of this approach in understanding reactivity in organic chemistry.
  • #1
espen180
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I am interested in reactions of the type

C2H5COCH3 + KH -> C2H5COCH2K + H2

In this paticular reaction, I understand that the participating hydrogen is beta to the carbonyl group since the oxygen's electronegativity makes the beta-hydrogen atoms acidic, and KH acts like a base. However, would the reaction take place had the reagent been an alkane, for example C4H10? If so, at what carbon atom will the reaction most likely take place?
 
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  • #2
Nope. pKa for hydride is something like 28-29 whereas that of an alkane is around 50.

Sodium hydride is sold as a solid coated with a hydrocarbon oil for safety purposes.
 
  • #3
espen: watch your terminology. We only describe carbons as alpha, beta, gamma, etc. with respect to a carbonyl. As such, those are alpha hydrogens, NOT beta hydrogens.

Also, to expand on your understanding a bit more: the reason those alpha hydrogen are more acidic than the corresponding alkane is not only due to inductive electron withdrawal by the carbonyl group. If the alpha carbon were deprotonated, there would be resonance stabilization of this anion (you can draw this out for yourself to see). This is important in the haloform reaction.

One of the best ways that I've found to approach questions of reactivity in organic chemistry is to imagine a bond breaking, then asking, "What features of this molecule would stabilize this resulting structure?" This gets to the heart of some chemical principles.
 
  • #4
Thanks for the help!
 
  • #5


I am intrigued by your interest in alkali hydrides in organic synthesis and specifically in the reaction between C2H5COCH3 and KH. This type of reaction, known as a deprotonation reaction, is commonly used in organic synthesis to create new carbon-carbon bonds. In this case, the hydrogen atom on the beta position of the carbonyl group is acidic due to the electronegativity of the oxygen atom, and the alkali hydride acts as a base to remove this proton and form a new carbon-carbon bond.

To answer your question, yes, the reaction would still occur if the reagent was an alkane such as C4H10. However, the reaction rate may be slower compared to using a more acidic compound like C2H5COCH3. The reaction would likely take place at the carbon atom that is most acidic, which is typically the one with the most substituted beta-hydrogen atoms. In the case of C4H10, this would be the beta-hydrogen atom on the second carbon, as it is surrounded by three other carbon atoms, making it more acidic compared to the first carbon.

Overall, the use of alkali hydrides in organic synthesis offers a valuable tool for creating new carbon-carbon bonds and can be applied to a variety of compounds, including alkanes. Further studies and experiments can provide more insight into the reactivity and selectivity of these reactions, leading to the development of new and efficient synthetic methods.
 

1. What are alkali hydrides?

Alkali hydrides are compounds that contain one or more alkali metals (such as lithium, sodium, or potassium) bonded to one or more hydrogen atoms. They are highly reactive and can be used as reducing agents in organic synthesis.

2. How are alkali hydrides used in organic synthesis?

Alkali hydrides can be used as powerful reducing agents to convert functional groups in organic molecules. They can also be used as catalysts in certain reactions, such as the Birch reduction.

3. What are the advantages of using alkali hydrides in organic synthesis?

Alkali hydrides are relatively inexpensive and readily available, making them a cost-effective option for organic synthesis. They are also highly reactive and can often achieve desired reactions in a shorter amount of time compared to other reducing agents.

4. Are there any safety considerations when working with alkali hydrides?

Yes, alkali hydrides are highly reactive and can be dangerous if not handled properly. They should be handled with caution and appropriate safety measures, such as wearing protective gear and working in a well-ventilated area, should be taken to avoid accidents.

5. Can alkali hydrides be used in large scale organic synthesis?

While alkali hydrides can be used in large scale synthesis, it is important to carefully consider their reactivity and potential safety hazards. It is also important to properly dispose of any excess or waste materials to avoid environmental contamination.

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