Chugayev reaction
The Tschugajew reaction (also Chugaev reaction or Chugaev elimination ) is a name reaction from the field of organic chemistry . The reaction was discovered in 1899 by the Russian chemist Lev Alexandrowitsch Tschugajew (1873-1922) when he was looking for a milder alternative to dehydration using ester pyrolysis .
In the Chugayev reaction or elimination, primary , secondary or even tertiary alcohols are converted into the corresponding terminal alkenes by dehydration . Xanthates occur as intermediate products , which break down into the corresponding olefins by thermolysis . Formally, the Tschugajew reaction can be regarded as a type of ester pyrolysis , which, however, takes place at temperatures of 100–200 ° C due to the low stability of xanthates. Any side or rearrangement reactions can thus be largely avoided.
Overview reaction
Primary , secondary or tertiary alcohols are first reacted with a base, carbon disulphide and methyl iodide to form a xanthate ester, which is then thermally reloaded to the corresponding terminal alkene, with carbonyl sulphide and methanethiol being split off .
Reaction mechanism
In the first reaction step, the alcohol ( 1 ), which must have a hydrogen atom in the ß-position to the oxygen atom, is converted by a base (e.g. NaH , NaOH , KOH etc.) to an alcoholate ( 2 ). This is added in a nucleophilic attack on the electrophilic carbon in the carbon disulfide ( 3 ), whereby a xanthate salt ( 4 ) is formed, which - usually with methyl iodide - is alkylated (methylated) to a xanthate ester ( 5 ). If this is heated to temperatures of 100–200 ° C, a stereospecific syn elimination (E i reaction) occurs, which proceeds via a six-membered transition state ( 6 ). A hydrogen atom is transferred from the β-carbon atom to the sulfur of the xanthate ester, the CO bond fragments and the terminal alkene ( 7 ) being formed. The by-product - a dithiocarbonate ( 8 ) - breaks down further to methanethiol ( 9 ) and carbonyl sulfide ( 10 ).
The pyrolysis of xanthates of primary alcohols yields only one alkene. In secondary alcohols can also regioisomers occur.
Pyrolysis of other esters and analogous substances
Other esters such as B. urethanes , carbonic acid - and acetic acid esters or amine oxides ( Cope elimination ). The pyrolysis tendency decreases as follows:
Amine oxides can be at 80 to 160 ° C, xanthogenic at 120 to 200 ° C and Carbonsäureester only at 400 to 500 ° C pyrolyze .
literature
- Reinhard Brückner: reaction mechanisms - organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Springer Spectrum, Berlin-Heidelberg 2015, ISBN 978-3-662-45683-5 , p. 169 f .
- Hans P. Latscha: Basic knowledge of chemistry . Springer, Berlin 2002.
- Organikum . VEB Deutscher Verlag der Wissenschaften, Berlin 1976.
Individual evidence
- ↑ L. Tschugaeff: About a new method for the representation of unsaturated hydrocarbons. In: Reports of the German Chemical Society . 32, No. 3, 1899, pp. 3332-3335, doi : 10.1002 / cber.18990320398 ( PDF ).
- ↑ Organikum . 23rd edition, Wiley-VCH, 2009, ISBN 978-3-527-32292-3 , p. 286.
- ↑ Thomas Laue, Andreas Plagens: Name and catchword reactions of organic chemistry . 5th edition. Vieweg + Teubner Verlag, Wiesbaden 1998, ISBN 978-3-519-23526-2 , pp. 59-62 .