Varrentrapp reaction

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.[1]

General Reaction Scheme

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.[2]

Reaction Mechanism

Below, the reaction mechanism is shown with (E)-4-hexenoic acid.[3]

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.

Applications

The reaction has been of some importance in structure elucidation of certain fatty acids but has no practical synthetic use.[4][5] The original 1840 Varrentrapp reaction concerned the conversion of oleic acid to palmitic acid, acetic acid and hydrogen gas:

References

  1. ↑ F. Varrentrapp, Ann., 36, 196 (1840)
  2. ↑ Windholz, M. (1976) (in German), The Merck Index, Rahway: Merck & Co, pp. ONR-90, ISBN 0-911910-26-3
  3. ↑ Wang, Zerong (2009) (in German), Comprehensive Organic Name Reactions and Reagents, New Jersey: John Wiley & Sons, pp. 2864-2868, ISBN 978-0-471-70450-8
  4. ↑ Organic reactions in strong alkalis-I : Fission of ethylenic acids (the varrentrapp reaction) Tetrahedron, Volume 8, Issues 3–4, 1960, Pages 221–238 R. G. Ackman, Patrick Linstead, B. J. Wakefield and B. C. L. Weedon doi:10.1016/0040-4020(60)80031-2
  5. ↑ A Mechanism for the Cleavage of Unsaturated Acids with Molten Alkali William A. Bonner, Robert T. Rewick J. Am. Chem. Soc.; 1962; 84(12); 2334–2337. doi:10.1021/ja00871a013
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