Colour tests, aldehydes

Phase-transfer catalysed colour test for aldehydes... 

The Reissert reaction exemplifies the B pathway (equation 73), and is of course a notable method for aldehyde and alkaloid synthesis. The Fujiwara reaction (81JOC3175), a colour test, provides an example of Type D behavior (Scheme 50). 

Acetaldehyde and other aldehydes arc tested for by ammoniacal silver nitrate this should not become coloured or turbid on standing for a short time. Kesslcr s reagent should give only a slight precipitate. 

This is a test for the >C = 0 group. Most aldehydes and ketones readily condense with this reagent giving yello v- or orange-coloured precipitates. 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

Colour Reactions. Rochelmeyer (1939) has provided a list of colour reactions given by solasodine and solasodiene (solanosodine), with reagents usually applied to the sterols, and Briggs et al. have found that when concentrated sulphuric acid (1 mil) is carefully added to a solution of solasonine or solasodine in hot alcohol (1 mil) a characteristic, intense, greenish-yellow fluorescence is produced, a reaction which is not given by solanine or solanidine. They have also found that intense colours are formed when solasonine or solasodine is mixed with resorcinol, or one of a variety of aldehydes, and boiled with concentrated hydrochloric acid. Colours are also produced with this test by cholesterol, digitonin, jacobine carbazole, pyrrole, or nicotine, the most intense colours being formed with p-hydroxybenzaldehyde or anisaldehyde. 

A solution of magenta decolourised by sulphur dioxide becomes violet on the addition of a drop of aldehyde (Schiff . Prepare a weak solution of magenta by dissolving a crystal in half a test-tube of water and bubbling in sulphur dioxide until the colour disappears. Now add a few drops of aldehyde. 

The simple spectrophotometric thiobarbituric acid (TBA) test has been frequently used for many years as an indicator of the peroxidation of PUFAs present in biological matrices. This test involves the reaction of aldehydes in the sample with TBA at c. 100°C under acidic conditions (Equation 1.13) to produce a pink-coloured chromogen, which absorbs light strongly at a wavelength of 532 nm (Nair and Turner, 1984). 

Experiment 11.—A few drops of acetaldehyde, dissolved in about 2 c.c. of water, are heated in a test tube with 0-5 c.c. of dilute sodium hydroxide solution. A yellow colour develops and the acetaldehyde is converted by way of aldol into crotonaldehyde, which can be recognised in the boiling solution by its pungent odour. If acetaldehyde is heated with concentrated alkali solution yellow aldehyde resin is precipitated as a result of further condensation. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

Qualitative spot tests for aldehydes, in the presence of ketones, are generally only reliable for water-soluble compounds. This problem can be overcome by the use of 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (Purpald , Aldrich Chemical Company) in the presence of Aliquat (Scheme 5.27). Under aerial oxidation, the initially formed colourless cyclic adduct changes colour through red to purple. The colourless cyclic aminal can also be formed by ketones, but only the adducts derived from the aldehydes are oxidized to the purple bicyclic aromatic system [28]. Weakly electrophilic aldehydes, e.g., 4-methoxybenzaldehyde, reacts slowly, but will give the positive coloration upon gentle heating to ca. 70°C for one or two minutes. 

The aldehydic material under test (ca. 0.1 ml) is added to Purpald (50 mg) and Aliquat (0.11 g, 0.25 mmol) in PhMe (5 ml). Aqueous NaOH (10%, 1 ml) is added and the mixture is shaken well. The initial yellow coloration of the organic phase rapidly changes to a deep rust colour to confirm the presence of the aldehyde, but remains yellow for ketones. 

With potash (Liebig). A mixture of equal volumes of the alcohol to be tested and 20% potassium hydroxide solution is gradually heated in a test-tube to boiling. Pure alcohol is scarcely coloured, whereas in presence of smaller or larger amounts of aldehydes, a yellow or reddish brown coloration is formed. 

Schiff s reaction. 10 c.c. of the alcohol (50%) and 4 c.c. of Schiff s reagent (see below r Determination) are shaken together in a test-tube and allowed to stand. The presence of aldehydes is shown by the appearance, either immediately or after some time, of a more or less intense red coloration, which should be observed after about 20 minutes. If no colour appears after this time, the liquid is free from aldehydes. 

Ehrlich s aldehyde test can be used to confirm a diagnosis of acute intermittent porphyria. Equal volumes of urine and Ehrlich s reagent are mixed a pink colour indicates raised urinary concentration of either porphobilinogen or urobilinogen. In acute intermittent porphyria, raised porphobilinogen is present and the pink precipitate formed is insoluble in chloroform. 

Formaldehyde-chromotropic acid test Formic acid, H.COOH, is reduced to formaldehyde H.CHO by magnesium and hydrochloric acid. The formaldehyde is identified by its reaction with chromotropic acid (see Section III.24, reaction 9d) in strong sulphuric acid when a violet-pink colouration appears. Other aliphatic aldehydes do not give the violet colouration. 

This sample should be examined visually for colour and the presence of tablet residues or excipients (often maize starch). The odour should be noted as tiiis may indicate the presence of alcohols, aldehydes, ketones, phenols, cyanide, ethchlorvynol, nicotine, etc. The following tests, details of which are given on p. 5, should be performed Trinder s test for salicylates, cresol-ammonia test for paracetamol, FPN reagent for phenothiazines, Fujiwara test for trichloro-compounds, and the diphenyl- nine test for oxidising agents. In addition, tiie Reinsch Test for heavy metals (p. 57) should be carried out, and a diluted, filtered extract examined by direct ultraviolet spectrophotometry for drugs with highyl values. 

Method. Add the sample to 1 ml of the reagent. Indications. A violet colour indicates the presence of an aliphatic aldehyde. The longer the carbon chain length and especially if it is branched, the weaker the response to the test. 

Aldehydes and similar reducing substances are tested for by the permanganate test. A solution of 1 g. potassium permanganate in a litre of water is made up and 1 c.c. of this is added to 100 c.c. of acetone. The colour should persist for at least thirty minutes at 15 5 in the dark. In Germany, a silver nitrate test is included to detect aldehydes these can also be detected by SchifTs reagent (fuchsine-bisulphite). Light oils also reduce ammoniacal silver nitrate. They may be tested for as follows. 

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... 

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