Perkin reaction

The Perkin reaction is a reaction name and named after the English chemist Sir William Henry Perkin ( 1838-1907 ). The first such compound was synthesized from him cinnamic acid. Generally, the Perkin reaction is a condensation of aromatic aldehydes with acetic anhydride in the presence of sodium acetate to form α, β - unsaturated carboxylic acids. Aromatic derivatives of these compounds are also known as cinnamic acids. The Perkin reaction takes place according to the principle of the aldol condensation, and is also closely related to the Knoevenagel reaction.

Mechanism

The mechanism of the Perkin reaction proceeds under basic conditions, initially the anion of the acid anhydride forms. This is due to the CH- acidity of the acid anhydride in α -position. In the next step the anion nucleophilically added to the carbonyl carbon of the aldehyde.

The simplest case, which was also conducted by Perkin 's reaction with methyl groups in α - position (R = H) and a phenyl group as the aryl group. From benzaldehyde and acetic anhydride while cinnamic acid was synthesized ( and acetic acid). If only one α - hydrogen atom is present, the reaction with the β -hydroxycarboxylic acid remains. The result is preferably ( but not only) the D diastereomer. Another variant of the Perkin reaction, the Erlenmeyer Plöchl synthesis for the synthesis of amino acids and α -keto acids.

Practice

It is for this reaction is a so-called one-pot reaction, that is, aldehyde, acid anhydride and base are mixed together and heated to 170-200 ° C for several hours. As base is usually used as the sodium salt of the corresponding carboxylic acid anhydride. However, it has been found that cesium salts sometimes drastically improve the reaction yield and shorten the required reaction time. Were crown ether used, the complex cation, the reaction remained completely. This suggested the finding that the increased basicity is not crucial for a better enolization and hence are weaker due to contact ion pairs for the heavier alkali cations accelerate the reaction, but rather that these ions catalyze the reactants vorkomplexieren or better.