Michael Addition Mechanism

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Michael Reaction Mechanism 

When an α,β -unsaturated carbonyl carbon is treated with a base, the base deprotonates the α,β -unsaturated carbonyl carbon. The deprotonation of α,β -unsaturated carbonyl carbon leads to the formation of an intermediate. The intermediate formed in this reaction is carbanion. The carbanion intermediate consists of a negative charge which can be stabilized by groups that are electron-withdrawing in nature. This reaction was first worked out by an American organic chemist named Arthur Michael and after his name, this reaction is named. This reaction can also be called a nucleophilic addition reaction because the electrons of the base are being donated to a carbon center making it a nucleophile. This reaction is useful because it generates carbon-carbon bonds which is a strong covalent bond. 


Michael Addition Reaction With Mechanism

The nucleophile or base which donates their electron to the proton is called Michael donor. Acyl and cyano groups act as very good nucleophiles because of their non-bonding electrons which are high in energy and are therefore ready to donate. The hydrogen attached to the substrate, methane is acidic and because of that the base or nucleophile readily abstracts it and the carbanion is formed. The substrate where the carbanion is formed is called the Michael donor whereas the other substrate which is attacked by the donor is said to be the Michael acceptor.

 

The reaction is thermodynamically controlled i.e. the product formed will be thermodynamically stable. The Michael donors are mostly active methylene which has electron-withdrawing groups attached to the carbon whose proton is abstracted. The abstracted proton is highly acidic because of the electron-withdrawing group attached to its adjacent carbon as they are capable of stabilizing the carbanion. The Michael acceptors are usually olefins which are electron deficient. 

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Example of Compounds Showing Michael's Addition Reaction

  • When diethyl malonate acts as Michael donor and diethyl fumarate acts as a Michael acceptor. 

  • Acrylonitrile acts as Michael acceptor and acetylacetone act as Michael acceptor. 

  • Malononitrile acting as a Michael donor and ethyl vinyl ether acting as a Michael acceptor. 

Michael Addition Reaction Mechanism


Step 1: 

In the first step, the α -hydrogen is deprotonated by the base which leads to the formation of carbanion. The negative charge on the carbon is stabilized by the carbonyl carbon by resonance as the negative charge on oxygen is more stable than being on the carbon. 


Step 2: 

In the second step, the Michael acceptor being deficient in electrons acts as an acceptor which then accepts electrons from the carbanion which is rich in electrons. The reaction between them forms the carbon-carbon bond. Even though the negative charge is more stable on the oxygen during the resonance structure the carbon-carbon bond is more stable as compared to the carbon-oxygen bond. This is a 1,4- addition reaction. 


Step 3: 

In the third step, the carbonyl compound is protonated by accepting an electron from the solvent which gives us the final product. The reaction mechanism is shown below which shows which bonds were broken during the course and which were formed. 

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Mukaiyama-Michael Addition Reaction 

When an organosilicon group (an organic compound attached to silicon) is bonded to the oxygen of an enolate then this functional group is called silyl enol ether and when this functional group acts as a nucleophile in a Michael reaction then that reaction is called Mukaiyama-Michael reaction. The reaction requires titanium chloride to act as a catalyst in the process of forming a new carbon-carbon bond. 

FAQ (Frequently Asked Questions)

Q1. What Kind of Reaction is a Michael Reaction?

Michael's reaction is a nucleophilic 1,4-addition reaction. The word nucleophilic indicates the presence of a nucleophile which means nucleus loving and is rich in electrons and are donor by nature. The 1,4 indicates the position of the bond that has been added or where the new carbon-carbon bond is formed. The Michael reaction includes an α,β -unsaturated carbonyl carbon which is when attacked by a nucleophile to abstract its proton, a carbanion form which then attacks the other substrate which is an electron acceptor i.e. electron deficient. The carbon-carbon bond is formed between the Michael acceptor and Michael donor. 

Q2. Why are Active Methylene Species Often Termed as Michael Donor Species?

Michael donor species are often active methylene species because they are highly acidic and can easily lose a proton which gives a negative charge to the carbon which releases a proton. The negative charge on carbon is usually unstable because carbon is an electropositive element but due to the presence of an electron-withdrawing group attached to the carbanion, the negative charge is stabilized. It gets delocalized to the electron-withdrawing groups which are carbonyl carbons and the negative charge then resides on the oxygen which is electronegative and can bear a negative charge easily as compared to the carbon. This delocalization leads to the formation of an enolate. 

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