Indophenine reaction

The indophenine reaction is one of the oldest color reactions in organic chemistry . It is used to detect thiophene and some substituted thiophenes ( indophenine test , indophenine test ).

history

During his pioneering research into the natural dye indigo , Adolf von Baeyer found the name reaction that was also named after him . In 1879 he reported that a solution of isatin (obtained, among other things, by oxidative cleavage of indigo) in concentrated sulfuric acid turned deep blue when shaken with benzene . The benzene used later turned out to be contaminated with thiophene ( Victor Meyer ).

reaction

The reaction of isatin with pure thiophene ultimately yielded a blue, poorly soluble and poorly volatile dye powder, which Baeyer called indophenin . The poor solubility made the elucidation of the structure difficult. In the end, it was found that two molecules of isatin had combined with two molecules of thiophene with splitting off of two water molecules. Therefore, the reaction was classified as “ condensation ”. As a constitutional formula of the product with the molecular formula C ₂₆ H ₁₄ N ₂ O ₂ S ₂ was 1a proposed.

Formation of indophenine ( 1a ) from isatin and thiophene in sulfuric acid over several reaction steps

Stereoisomers

In addition to structure 1a but five more are E / Z - stereoisomers ( 1b - f ) into consideration. Which of these is in the solid state is unknown; a crystal structure analysis is missing so far. In solution (hexadeuteriodimethyl sulfoxide, DMSO -d ₆ ), several E / Z isomers were detected by NMR spectroscopy , but no structural formulas were assigned.

E / Z stereoisomers ( 1a - 1f ) of indophenine

Reaction mechanism

As for the mechanism of the reaction, there is little doubt that the indophenine reaction is initiated by an electrophilic attack by protonated isatin on a thiophene molecule:

Hypothetical reaction mechanism for the formation of indophenine (stereoisomer 1f ) from protonated isatin and thiophene

Like most of the "classic" color reactions in organic chemistry, the indophenine test has also lost its importance in analysis since the methods of instrumental analysis were established.

Individual evidence

1. ^ A. Baeyer, Reports of the German Chemical Society , 12, 1311 (1879).
2. ^ W. Steinkopf and W. Hanske: Studies in the thiophen series. IL. The constitution of the Indophenine , Justus Liebigs Annalen der Chemie , 341, pp. 238-260 (1939); doi : 10.1002 / jlac.19395410112 .
3. ↑ Ulf Pindur: New studies on the constitution of isatin-thiophene-indophenine and isatin-pyrrole-indophenine , Archiv der Pharmazie (Weinheim), 314, pp. 337-342 (1981); doi : 10.1002 / ardp.19813140409 .
4. ^ JA Joule and GF Smith: Heterocyclic Chemistry , 2nd edition, Van Nostrand Reinhold Company, London 1978, ISBN 0-442-30212-6 , p. 224.