Alkali hydrides in org. synthesis

[espen180]

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?

 

[chemisttree]

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.

 

[DDTea]

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.

 

[espen180]

Thanks for the help!

 

[LudusRex]

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.