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The Michael addition mechanism begins with the base deprotonation of the α,β -unsaturated carbonyl compound. This results in the formation of a carbanion, which is stable due to its electron-withdrawing groups. The nucleophilic addition of a nucleophile (or a carbanion) to an, α,β -unsaturated carbonyl compound is known as the Michael Addition, and it belongs to a class of reactions that are very useful in the mild formation of carbon-carbon bonds.
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Key Takeaways: Michael Addition, Michael Addition Mechanism, Carbonyl carbon, Michael acceptor, Michael donar
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What is Michael Addition?
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When a base is applied to an, α,β-unsaturated carbonyl carbon, the base deprotonates the, α,β-unsaturated carbonyl carbon. The formation of an intermediate is caused by the deprotonation of, α,β-unsaturated carbonyl carbon. Carbanion is the intermediate formed in this reaction. The carbanion intermediate contains a negative charge that can be stabilised by electron-withdrawing groups. This reaction was discovered by an American organic chemist named Arthur Michael, and it is named after him.
This reaction is also known as a nucleophilic addition reaction because the base's electrons are donated to a carbon centre, making it a nucleophile. This reaction is beneficial because it produces carbon-carbon bonds, which are strong covalent bonds.
Michael Addition Examples
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The reactions between are a few examples of Michael addition reactions.
- Methyl crotonate with diethyl malonate
- Diethyl malonate with diethyl fumarate
- Diethyl malonate with mesityl oxide
- Methyl vinyl ketone with nitropropane
- 2-Nitropropane with methyl acrylate
Read More: Physical and Chemical Classification of Matter
Michael Addition Mechanism
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The nucleophile or base that donates an electron to the proton is referred to as a Michael donor. Because of their non-bonding electrons, which are high in energy and thus ready to donate, acyl and cyano groups are excellent nucleophiles. Because the hydrogen attached to the substrate, methane, is acidic, the base or nucleophile easily abstracts it and the carbanion is formed. The substrate where the carbanion is formed is referred to as the Michael donor, while the other substrate attacked by the donor is referred to as the Michael acceptor.
The reaction is thermodynamically controlled, which means that the product produced is thermodynamically stable. The majority of Michael donors are active methylene, which has electron-drawing groups attached to the carbon whose proton is abstracted. Because the electron-withdrawing group attached to its adjacent carbon is capable of stabilising the carbanion, the abstracted proton is highly acidic. Michael acceptors are typically electron-deficient olefins.
Michael Addition Mechanism (Step by Step)
Step 1: The base deprotonates the -hydrogen in the first step, resulting in the formation of carbanion. Because a negative charge on oxygen is more stable than a negative charge on carbon, the carbonyl carbon stabilises the negative charge on carbon via resonance.
Step 2: In the second step, the Michael acceptor, which is electron-deficient, acts as an acceptor, accepting electrons from the carbanion, which is electron-rich. The carbon-carbon bond is formed as a result of their reaction. Even though the negative charge on oxygen is more stable during the resonance structure, the carbon-carbon bond is more stable than the carbon-oxygen bond. This is a reaction of 1,4-addition.
Step 3: The carbonyl compound is protonated in the third step by accepting an electron from the solvent, yielding the final product. The reaction mechanism is depicted below, which shows which bonds were broken and which were formed during the process.
Read More: Laws of Chemical Combinations
Mukaiyama-Michael Addition Reaction
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When an organosilicon group (an organic compound attached to silicon) bonds to the oxygen of an enolate, the functional group is known as silyl enol ether, and when this functional group acts as a nucleophile in a Michael reaction, the reaction is known as Mukaiyama-Michael. Titanium chloride is needed as a catalyst in the process of forming a new carbon-carbon bond.
Points to Remember
Following are some important points:
- The Michael Addition is the nucleophilic addition of a nucleophile (or a carbanion) to an, α,β-unsaturated carbonyl compound.
- The intermediate formed in this reaction is carbanion.
- The carbanion intermediate, which has a negative charge that can be stabilised by electron-withdrawing groups.
- When a base is applied to an, α,β-unsaturated carbonyl carbon, the base deprotonates the carbonyl carbon.
- The deprotonation of, α,β-unsaturated carbonyl carbon results in the formation of an intermediate.
- When an organosilicon group bonds to the oxygen of an enolate, the functional group is known as silyl enol ether, and the reaction is known as Mukaiyama-Michael when this functional group acts as a nucleophile in a Michael reaction.
Read More: Law of Multiple Proportions
Sample Questions
Ques: A Michael Reaction is what kind of reaction? (3 Marks)
Ans: Michael's reaction is a 1,4-addition nucleophilic reaction. The presence of a nucleophile, which means nucleus loving and is rich in electrons and is a donor by nature, is indicated by the term nucleophilic. The 1,4 denotes the position of the newly added bond or the formation of a new carbon-carbon bond. The Michael reaction involves an, α,β -unsaturated carbonyl carbon that, when attacked by a nucleophile to abstract its proton, transforms into a carbanion form that then attacks the other substrate, which is an electron acceptor, i.e. electron deficient. Between the Michael acceptor and Michael donor, a carbon-carbon bond is formed.
Ques: Why are Active Methylene Species Known as Michael Donor Species? (3 Marks)
Ans: Michael donor species are frequently active methylene species because they are highly acidic and can easily lose a proton, resulting in a negative charge on the carbon and the release of a proton. The negative charge on carbon is normally unstable because carbon is an electropositive element, but the negative charge is stabilised due to the presence of an electron-withdrawing group attached to the carbanion. It delocalizes to the electron-drawing groups, which are carbonyl carbons, and the negative charge then resides on the oxygen, which is electronegative and can bear a negative charge more easily than carbon. The formation of an enolate is the result of this delocalization.
Ques: What is the difference between Michael donors and Michael acceptors? Mention some examples of Michael Addition. (3 Marks)
Ans: The nucleophile is commonly referred to as the Michael donor in Michael addition reactions. Michael donors typically have electron-drawing substituent groups. The substituent groups on the activated unsaturated compound are known as Michael acceptors. Ketone and nitro groups are two common types of Michael acceptors.
Michael addition is used in the reaction of diethyl malonate with diethyl fumarate. The synthesis of the anticoagulant warfarin from the chemical reaction of benzylideneacetone and 4-hydroxycoumarin is another important example of this named reaction.
Ques: What is Mukaiyama-Michael addition? (3 Marks)
Ans: The Mukaiyama-Michael addition is a modification of the Michael reaction that uses a silyl enol ether as the nucleophile. Silyl enol ethers are a type of organic compound that shares a functional group consisting of an enolate bonded to an organosilicon group via an oxygen atom. Titanium tetrachloride is the most common catalyst used in Mukaiyama-Michael addition reactions.
Ques: Explain Michael's asymmetric reactions. (3 Marks)
Ans: Enantioselective synthesis of the products is involved in asymmetric Michael additions. Chiral phase transfer catalysis can be used to catalyse such reactions. The production of asymmetric quaternary salts of the ammonium ion from Cinchona alkaloids is an example of an asymmetric Michael reaction.
Ques: What contribution does Michael's addition mechanism make to organic chemistry? (3 Marks)
Ans: The conjugate addition of a carbanion, enolate, enamine, or another carbon nucleophile to the α, β-carbon of an enone, enal, or other, α, β-unsaturated compound (Michael reaction). This results in the formation of a new carbon-carbon bond at the α, β-carbon. The Michael donor is the nucleophile of the Michael reaction.
Ques: What are the requirements for a Michael addition? Is the Michael addition reversible? (3 Marks)
Ans: The Michael reaction can be carried out using a wide range of, α, β-unsaturated carbonyl electrophilic acceptors and enolate donors. This reaction can involve Michael acceptors such as α, β-unsaturated ketones, aldehydes, esters, amides, and nitro compounds. Since the Michael reaction is reversible, the thermodynamically more stable product with the strong carbon-oxygen double bond is preferred.
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