Ehrlich colour reaction

By using an aromatic aldehyde carrying an electron-releasing group the intermediate cation can be stabilized. This is the basis of the widely-used Ehrlich colour reaction for pyrroles, indoles and furans which have a free reactive nuclear position (Scheme 21). 

Indolizine, like many of its derivatives, is air and light sensitive. It gives a colour reaction with Ehrlich s reagent and is highly fluorescent. 

Fundamentally similar to pyrromethene formation are two of the classical colour reactions associated with pyrroles. The redness produced by pyrroles with an acid-moistened pine shaving is due to reaction with aldehydes in the wood, but it is not specific for pyrroles " . Ehrlich s reaction depends on the formation of coloured products by the reaction of a pyrrole with j -dimethylaminobenzaldehyde in acid solution. The nature of the reaction is indicated by the isolation of a salt of the cation (26) after treatment of (27) with the Ehrlich reagent, a carboxyl group being eliminated (26) gives red solutions, the disodium salt of the derived free base produces yellow solutions, and treatment of (26) with excess acid the colourless di-protonated salt . ... 

The 3-vinylpyrroles (366 X = H or C02H) also react with electrophiles. No products were isolated, but it was suggested that the blue colouration produced upon the acid-catalyzed reaction with Ehrlich s reagent was due to the formation of (370) and that it was identical with that of the product obtained from the reaction of the trisubstituted pyrrole and the cinnamaldehyde. Similarly, the red colouration obtained from the reaction of (366 X = H or C02H) with diazotized sulfanilic acid was ascribed to the formation of (371) <58LA(611)205). 

Condensations with aromatic aldehydes carrying appropriate electron-releasing substituents produce cations that are sufficiently stabilised by mesomerism to be isolated. Such cations are coloured the reaction with p-dimethylaminobenzaldehyde is the basis for the classical Ehrlich test, deep red/violet colours being produced by pyrroles (and also by furans and indoles) that have a free nuclear position. Under appropriate conditions one can combine four mole equivalents of pyrrole and four of an aldehyde to produce a... 

With Ehrlich s reagent, i.e. a solution of 4-(dimethylamino)benzaldehyde in hydrochloric acid, the reaction proceeds to give only the purple-coloured azafulvenium salt ... 

Although indoles are less reactive than pyrroles, they react with arenediazonium salts to give 3-(aryl-azo)indoles. The reaction with carbonyl compounds proceeds in an analogous way in the presence of acids. For this reason, indoles unsubstituted in the 3-position give a positive colour test with Ehrlich s reagent. Indole and acetaldehyde react via an azafulvenium salt to give 3-Vinylindole, which on further reaction with indole forms l,l-di(indol-3-yl)ethane ... 

Indoles react with aldehydes and ketones under acid catalysis - with simple carbonyl compounds, the initial products, indol-3-ylcarbinols are never isolated, for in the acidic conditions they dehydrate to 3-alkylidene-3//-indolium cations those from aromatic aldehydes have been isolated in some cases reaction of 2-methylindole with acetone under anhydrous conditions gives the simplest isolable salt of this class. Only where dehydration is not possible have hydroxyalkylindoles been isolated, for example from reaction with diethyl mesoxalate. Reaction with 4-dimethylaminobenzaldehyde (the Ehrlich reaction, see section 13.1.7) gives a mesomeric and highly-coloured cation. 

Indican can be detected in urine by its reaction with Ehrlich s reagent or by the Jaffe test in which a blue colour is produced ir the presence of hydrochloric acid and calcium hypochloriu which can be extracted into chloroform. It can be quanititatec by its reaction with Ehrlich s reagent, the chromogen beinj extracted into alkaline solution and estimated spectrophoto metrically. 

An intermediate in the formation of uric acid from purines. It is excreted in the urine in large amounts in the inborn error of metabolism, xanthinuria, a condition in which xanthine stones can be found. It can be detected in urinary stones by its reaction with Ehrlich s diazo reagent to give a red colour. 

Reports of sialic acids in plants exist in the literature (Mayer et al. 1964, Onodera et al. 1966), but the analytical methods employed were insufficient to exclude other compounds such as 2-keto-3-deoxy acids noted earlier in this chapter. Several investigations produced negative results, although a reaction in the periodic acid/thiobarbituric acid assay was obtained. The compounds in question gave no colour in the direct Ehrlich, orcinol/Fe and resorcinol assays for sialic acids (Gielen 1968, Cabezas 1968, 1973, CABEZAsand Feo 1969, Unger 1981). 

If iV-acetylglucosamine is heated with dilute alkali and then treated with -dimethylaminobenzaldehyde in hydrochloric acid (Ehrlich s reagent), a reddish violet colour is obtained. The mechanism of this Morgan-Elson reaction, which was developed for quantitative estimation of iV-acetyl-... 

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