The Varrentrapp reaction, also named Varrentrapp degradation, is a name reaction in the organic chemistry. It is named after Franz Varrentrapp, who discovered this reaction in 1840. It is an organic reaction involving the chemical decomposition of an α,β-unsaturated acid into two other acid fragments by action of molten alkali.
Varrentrapp reaction Wikipedia
Because of the complex structure of this reaction it is shortened in the following scheme.
The reaction conditions are harsh: medium molten potassium hydroxide at temperatures in the range of 250 to 300 °C. Likewise cinnamic acid is converted to benzoic acid.
Below, the reaction mechanism is shown with (E)-4-hexenoic acid.
First, the hydroxyl reacts with the caustic potash 1. After two condensations a compound with two mesomers is build 3a and 3b. After an other condensation a negative charged carbon-atom is generated 4. After a rearrangement the intermediate compound (5) is built. This reacts with a hydroxydion of the caustic potash 6. In the next step the molecule dissociates into two pieces 7 and 8. (8) is protonated and builds the acetic acid derivative (9). The second compound (7) reacts with a hydroxidion and a shortened carbonic acid is generated 10.
The reaction has been of some importance in structure elucidation of certain fatty acids but has no practical synthetic use. The original 1840 Varrentrapp reaction concerned the conversion of oleic acid to palmitic acid, acetic acid and hydrogen gas: