Varrentrapp reaction

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The Varrentrapp reaction , also called Varrentrapp degradation , is a name reaction in organic chemistry . It was named after its discoverer, Franz Varrentrapp . This reaction was first reported in 1840. This is the breakdown of unsaturated carboxylic acids with the help of potassium hydroxide to carboxylic acids with two fewer carbon atoms, acetic acid and hydrogen .

Overview reaction

The Varrentrapp reaction is the breakdown of unsaturated carboxylic acids. The reaction is very complex, in order to ensure clarity, it is outlined below in a shortened and simplified manner.

Overview reaction of the Varrentrapp reaction

With only one double bond, yields of up to 90% can be achieved. Otherwise, the yield depends on the length of the carbon chain and the location of the double bonds. If the double bond is in the α, β-position to the carbonyl group , then the retro-Claisen condensation or the retro- aldol condensation is more likely to take place. The Varrentrapp reaction only takes place at very high temperatures (around 300 ° C).

mechanism

The mechanism is shown here using ( E ) -4-hexenoic acid as an example :

Mechanism of the Varrentrapp reaction

First, the hydroxyl group of the carbonyl group reacts with the potassium hydroxide solution 1 , and the potassium atom binds instead of the hydrogen atom 2 . In addition, water is split off, this is called condensation . In the next step, water is split off again and a compound with two mersomeric limit formulas 3a and 3b is formed . After water has been split off again, a compound with a negative charge is formed on carbon atom 4 . Intermediate product 5 is then formed through rearrangements . This then reacts again with the hydroxide ion of the potassium hydroxide solution and forms compound 6 in an intermediate step . In the next step, the molecule 6 splits into two parts, ie into butanal 7 and the carbanion 8 . The anion 8 is then only protonated and thus forms potassium acetate ( 9 ). Butanal ( 7 ) then reacts with a hydroxide ion. Then molecular hydrogen (H 2 ) is split off, with the “shortened” butyric acid anion 10 being formed. Acidic work-up then provides the corresponding carboxylic acid, butyric acid in the specific example .

application

With the help of this reaction certain carboxylic acids can be synthesized from unsaturated fatty acids . However, it is more worthwhile for industrial syntheses as it is very complex and time-consuming.

Individual evidence

  1. ^ F. Varrentrapp: About oleic acid . In: Liebigs Ann. Chem. Band 35 , 1840, pp. 196-215 , doi : 10.1002 / jlac.18400350205 .
  2. ↑ a b c d e Zerong Wang: Comprehensive Organic Name Reactions and Reagents . John Wiley & Sons, New Jersey 2009, ISBN 978-0-471-70450-8 , pp. 2864-2868 .
  3. ↑ a b M. Windholz: The Merck Index . Merck & Co, Rahway 1976, ISBN 0-911910-26-3 , pp. ONR-90 .
  4. ^ HF Harwood, Reactions of the Hydrocarbon Chain of Fatty Acids . In: Chem. Rev. Band 62 , 1962, pp. 99-154 , doi : 10.1021 / cr60216a002 .