Ethynylation

The (Reppe) ethynylation is an important technical process from the field of organic chemistry . It was developed in the 1930s by Walter Reppe and his employees in the main laboratories of the Badische Anilin- und Soda-Fabrik (today BASF SE ) in Ludwigshafen am Rhein . Together with vinylation , cyclization and hydrocarboxylation , it forms the well-known Reppe chemistry . The process achieved great industrial importance in the context of the high-pressure synthesis of acetylene and in the course of modern process engineering .

Ethynylation is generally understood to mean the introduction of an ethynyl group (H – C≡C – R) into organic chemical compounds with retention of the C≡C triple bond. It is a special case of an alkynylation .

historical development

A simple and obvious method is the reaction of acetylene (ethyne) with the respective organic compound. The problem with this concept was initially the concern of forming explosive and thus highly dangerous mixtures when working with acetylene under increased pressure. For safety reasons, the German Acetylene Ordinance stipulated until the 1930s that acetylene was only allowed to be handled at a maximum of 1.5 bar. However, by conducting experiments on the ignition behavior and decomposition of acetylene, the research team headed by Walter Reppe found that acetylene can be safely handled as a compressed gas up to a maximum pressure of 25 bar. After this knowledge, it was possible for Reppe to advance his actual research ideas with acetylene in order to provide new types of preliminary products for the up-and-coming plastics industry by developing a technical process . As a chemical pioneer, Walter Reppe paved the way for the large-scale production of vitamins, fragrances, pesticides and plastics with the development of ethynylation and is therefore still considered the founder of modern acetylene chemistry.

Reaction equation

In Reppe ethynylation, aldehydes and ketones are reacted with acetylene with basic catalysis to form alkynols and alkynediols . In the case of the ethynylation of aliphatic ketones, alkali metal or alkaline earth metal hydroxides such as potassium hydroxide or amides such as sodium amide in liquid ammonia are usually used. Aldehydes, on the other hand, can preferably be ethynylated with basic copper (I) acetylide catalysts , since an aldol addition or aldol condensation reaction easily occurs as a competing side reaction when using basic hydroxides (such as potassium hydroxide) .

Overview reaction of the ethynylation

In the first reaction stage, acetylene is reacted with a carbonyl compound in the presence of the respective catalyst to give the α-alkynol . This can now react one more time with the aldehyde or ketone and finally forms a γ-alkynediol.

If the carbonyl compound is an aldehyde , R ^{2 is} a hydrogen atom and R ¹ can either be a hydrogen atom ( formaldehyde ) or an organyl radical ( alkyl , aryl , alkenyl radical, etc.) be. However, if a ketone is used as the carbonyl component, both radicals R ¹ and R ² independently represent an organyl group.

Catalysts and their manufacture

As already mentioned, different catalysts are suitable for the ethynylation of aldehydes and ketones, which have different effectiveness.

If an aldehyde is ethynylated as the carbonyl compound, then in most cases copper (I) acetylide is used as the catalyst. This is produced in the laboratory by slurrying a monovalent copper salt (preferably copper (I) chloride CuCl) with water in a stirred flask filled with material containing silicate (e.g. fuller's earth ). Gaseous acetylene is then passed in at temperatures of 80–90 ° C until all of the copper salt has reacted. The copper (I) acetylide formed is almost insoluble in water and is practically quantitative. The hydrochloric acid formed is neutralized with suitable buffer solutions or smaller amounts of bases, since the copper (I) acetylide is unstable in a strongly acidic medium. The filter cake is then washed with distilled water until it is free of chloride ions and used directly for the ethynylation without further purification.

Industrial catalysts usually consist of copper (I) oxide , which is applied to a silicate-containing carrier (SiO ₂ ) and still contain about 3–6% bismuth (III) oxide (Bi ₂ O ₃ ) as a promoter and to improve selectivity as it suppresses the formation of so-called cuprenes ( polyacetylenes ). During the reaction, the copper salt is converted into copper (I) acetylide by the acetylene.

The copper (I) acetylide catalysts have little effect on the ethynylation of a ketone. Since the competing side reaction of the base-catalyzed aldol reaction does not occur as quickly as with aldehydes, it is possible here to use alkali or alkaline earth metal hydroxides (e.g. sodium or potassium hydroxide ), carbonates , tertiary amines and alkali metal alcoholates or amides as catalysts. Ethynylation works particularly well with, especially cycloaliphatic, ketones in the presence of sodium acetylide (formed in situ from acetylene and sodium amide) in liquid ammonia at lower temperatures.

Application examples

Ethynylation is used industrially in the production of 1,4-butanediol, among other things . Starting from acetylene and formaldehyde, 2-butyne-1,4-diol (alkynediol) is formed via the intermediate stage of propargyl alcohol (alkynol) , which is converted to 1,4-butanediol in a multi-stage catalytic hydrogenation .

In addition, ethynylation is used in the technical synthesis of the vitamin E precursor isophytol . In a first reaction step, acetone is ethynylated with acetylene to form 2-methyl-3-butyn-2-ol , which is then converted into isophytol via several intermediate stages (and further ethynylations).

Individual evidence

1. ↑ Entry on ethynylation. In: Römpp Online . Georg Thieme Verlag, accessed on December 30, 2019.
2. ↑ Michael Lang: Tradition of Ideas: Reppe Chemistry. In: BASF SE. April 2008, accessed December 30, 2019 . 
3. ^ ^{A b} Hans R. Christen, Fritz Vögtle: Fundamentals of organic chemistry . 1st edition. Salle and Sauerländer, 1989, ISBN 978-3-7935-5399-1 , p. 92 .
4. ↑ Walter Reppe: ethynylation III. In: Justus Liebig's Annals of Chemistry . Wiley ‐ VCH Verlag GmbH & Co. KGaA., November 14, 1955, Vol. 596, Issue 1, doi : 10.1002 / jlac.19555960106 .
5. ↑ Heinz Gräfje, Wolfgang Körnig, Hans ‐ Martin Weitz, Wolfgang Reiss, Guido Steffan, Herbert Diehl, Horst Bosche, Kurt Schneider, Heinz Kieczka, Rolf Pinkos: Butanediols, Butenediol, and Butynediol. In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley ‐ VCH Verlag GmbH & Co. KGaA., July 23, 2019, p. 2, doi : 10.1002 / 14356007.a04_455.pub2 .
6. ↑ Thomas Netscher, Werner Bonrath: Synthesis of isophytol and total synthesis of (all-rac) -a-tocopherol. In: Current newsreel. 2008, accessed January 18, 2020 .