Henkel reaction

from Wikipedia, the free encyclopedia

The Henkel reaction is a name reaction from the field of organic chemistry , which was first published in 1952 by the German chemist Bernhard Raecke (Henkel & Cie. GmbH). With thermal rearrangement or disproportionation , alkali metal salts of aromatic carboxylic acids are converted into symmetrical dicarboxylic acids .

Overview

An alkali salt of an aromatic carboxylic acid, here potassium benzoate , is converted by thermal- catalytic disproportionation with the addition of carbon dioxide to the corresponding 1,4-dicarboxylic acid , here potassium terephthalate , and the corresponding aromatic, here benzene :

Henkel reaction overview2

Alternatively, the alkali salt of an aromatic 1,2-dicarboxylic acid (here phthalic acid ) can be converted to an alkali salt of an aromatic 1,4-dicarboxylic acid ( terephthalic acid ) by a thermal-catalytic rearrangement with the addition of carbon dioxide :

Henkel reaction overview2

Cadmium or zinc is used as a catalyst .

Reaction mechanism

The following illustration shows a proposal for the reaction mechanism of the Henkel reaction using the example of the disproportionation of potassium benzoate to potassium terephthalate and benzene:

Henkel reaction mechanism

First, potassium benzoate ( 1 ) is converted into a deprotonated benzene by splitting off carbon dioxide . This anion now deprotonates a second molecule of potassium benzoate ( 1 ) in the para position. This creates the potassium benzoate ( 2 ) and benzene deprotonated in the para position . The potassium benzoate anion ( 2 ) is then reacted with carbon dioxide to form potassium terephthalate ( 3 ).

Yield and other by-products

The yield rises and falls with the pressure at which the carbon dioxide is supplied. The higher the pressure, the higher the yield. In addition, the choice of alkali metal has an influence on the yield. If potassium, rubidium and cesium are used, a high yield can be expected. In contrast, sodium and lithium worsen the yield.

Further by-products are to be expected in the Henkel reaction. When using potassium benzoate, for example, diphenyl methane , benzophenone and triphenyl methane can be formed.

Individual evidence

  1. ↑ a b Patent DE936036 : Process for the production of terephthalic acid. Registered December 1, 1952 , published December 1, 1955 , inventor: B. Raecke. ‌
  2. ↑ Reza Dabestani, Phillip F. Britt, AC Buchanan: Pyrolysis of Aromatic Carboxylic Acid Salts: Does Decarboxylation Play a Role in Cross-Linking Reactions? In: Energy & Fuels . tape 19 , no. 2 , March 2005, p. 365-373 , doi : 10.1021 / ef0400722 .
  3. ↑ a b c d Z. Wang (Ed.): Comprehensive Organic Name Reactions and Reagents, 3 Volume Set . John Wiley & Sons, Hoboken, New Jersey, 2009, ISBN 978-0-471-70450-8 , p. 1380.
  4. ↑ Hiroaki Mori, Hiroaki Nakao: 2, 3, 6, 7-Naphthalenetetracarboxylic Dianhydride (NTCDA) as a Monomer for Polyimide . In: JFE technical report . tape 8 , no. 6 , 2005, p. 54–61 ( PDF [accessed December 23, 2016]).
  5. ↑ Edward McNelis: Reactions of Aromatic carboxylate. II. The Henkel Reaction . In: The Journal of Organic Chemistry . tape 30 , no. 4 , April 1965, p. 1209-1213 , doi : 10.1021 / jo01015a060 .