The Michael addition is a nucleophilic addition to α, β unsaturated carbonyl compounds. In this addition, a carbon-carbon bond is made between a nucleophilic particle and the β-carbon atom of the α, β-unsaturated carbonyl compound. The nucleophilic particle is a carbanion formed under basic conditions from a carbon compound. Only free acidic carbon compounds can produce carbanions. These carbon compounds must have a -CH- or -CH2- group flanked by two electron-withdrawing groups. The electron withdrawing groups are, for example, two -COOR groups or one -COOR and one -CN group. The result is an adduct. Examples of Michael Addition Examples of Michael Addition Mechanism of Michael Addition
The Michael Addition mechanism is discussed by way of example. The example is the addition of a propanedioic diester to an α, β-unsaturated carbonyl compound. In the first step, the carbanion is made as follows: The diester of propanedioic acid (malonic acid) as proton donor
In the negative ion, the charge is divided into a number of atoms. This can be seen in the following boundary structures of CH (COOR) 2. (See mesomerism for border structures). Because of the division of the negative charge across multiple atoms, the carbanion is weaker than a carbanion where the negative charge is concentrated on one carbon atom. This implies that the diester of propanedioic acid is less weak as acid. As a result, in the presence of a base that is strong enough, enough of the carbanion is created to allow the Michael addition. Border structures of CH (COOR) 2.
The carbanion binds through the negative carbon to the β carbon of the carbonyl compound, creating a new negative ion that is also stabilized by mesomerism. This negative ion then binds a proton to the α-carbon atom. Reaction mechanism of Michael Addition Sources and
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