Ozonolysis

The ozonolysis , also Harries reaction is a reaction in the field of organic chemistry . The name is derived from the ozone (O ₃ ) used and the decisive reaction step : the destruction / dissolution ( lysis ) of a carbon-carbon double bond . This process was discovered by Carl Dietrich Harries in 1904 and published in 1905.

R is an organyl radical (e.g. alkyl radical ) or a hydrogen atom. Depending on how the alkene is worked up, the products obtained are carbonyl compounds (in particular ketones , aldehydes ), alcohols or carboxylic acids . By analyzing the products, conclusions can be drawn about the structure of the starting material. In the past , structure elucidation was carried out in this way - without modern methods such as NMR spectroscopy .

Other oxidation processes for double bonds work with osmium tetroxide , potassium permanganate or chromium compounds . In contrast to these methods, ozonolysis not only breaks the π bond , but also the σ bond .

The three-stage mechanism of ozonolysis was clarified in 1949 by Rudolf Criegee , whose name is therefore sometimes associated with ozonolysis.

Reaction conditions and mechanism

Carbon oxide (Criegee zwitterion); R = organyl radical (e.g. alkyl radical)

This reaction works particularly well at low temperatures . Commonly used solvents are methanol , ethyl acetate and dichloromethane .

Mechanistically, in the first step, the polar ozone is added to the alkene via a 1,3-dipolar cycloaddition . This forms the so-called primary ozonide (also called Molozonid). This breaks down by breaking the CC bond and one of the two OO bonds in the ozone (cycloreversion) into a carbonyl compound and a carbonyl oxide , which is extremely unstable and only occurs as an intermediate. These again form what is known as the secondary azonide through 1,3-dipolar cycloaddition .

By different processing of ozonolysis, different products can be obtained: The two carbonyl compounds are primarily formed by simple hydrolysis of the secondary zonide. However, since hydrogen peroxide is formed at the same time , this can under certain circumstances further oxidize the products .

Under certain conditions, ozonation does not lead to the formation of primary zonides, but rather epoxides , especially when bulky substituents are present on the double bond. These epoxides can rearrange to form aldehydes , which through further oxidation lead to carboxylic acids with an unchanged carbon structure.

Preparatively, one can work up either under oxidative or under reductive conditions. Maintaining the oxidation state of the two primary products is also conceivable. With sodium borohydride or lithium aluminum hydride, primary or secondary alcohols are obtained , with dimethyl sulfide , triphenylphosphine or zinc (under acidic conditions) aldehydes or ketones , with hydrogen peroxide carboxylic acids or ketones.

An alkene [R = organyl radical (e.g. alkyl radical )] 1 reacts with ozone to form a primary zonide 2 . This breaks down into a carbonyl compound 4 and a carbonyl oxide 3 . Under cycloaddition, these form a secondary azonide 5 . Under reducing conditions with e.g. B. dimethyl sulfide, two ketones are formed 6 . If one or all of the organyl radicals [R] were to be replaced by hydrogen, then additional / exclusively aldehydes would be produced. Using chromium (IV) oxide or H ₂ O ₂ and NaOH, two carboxylic acids are formed instead of the ketones. Using NaBH ₄ results in two alcohols.

literature

• Rudolf Criegee : Ozonolysis . In: Chemistry in Our Time . tape 7 , no. 3 , 1973, p. 75-81 , doi : 10.1002 / ciuz.19730070303 .
• Reinhard Brückner : reaction mechanisms . 3. Edition. Springer-Verlag, 2009, ISBN 3-8274-1579-9 , pp. 761 f .

Web links

Commons : Ozonolysis  - collection of pictures, videos and audio files

• organic-chemie.ch: ozonolysis

Individual evidence

1. ^ Brockhaus ABC chemistry. VEB FA Brockhaus, Leipzig 1965, p. 524.
2. ↑ C. Harries: About the action of ozone on organic compounds. In: Justus Liebig's Annals of Chemistry. 343, No. 2-3, 1905, pp. 311-344, doi: 10.1002 / jlac.19053430209 .
3. ^ Mordecai B. Rubin: The History of Ozone Part III. CD Harries and the Introduction of Ozone into Organic Chemistry. In: Helvetica chimica acta. 86, No. 4, 2003, pp. 930-940 ( classes.yale.edu PDF).
4. ↑ Rudolf Criegee : Mechanism of Ozonolysis. In: Angewandte Chemie 87, No. 21, 1975, pp. 765-771, doi: 10.1002 / anie.19750872104 .