Compounds, colored from aldehydes

Aldehydes and ketones react rapidly with 2,4-dinitrophenylhydrazine to form 2,4-dinitrophenylhydrazones.These derivatives range in color from yellow to red, depending on the degree of conjugation in the carbonyl compound ... 

Next, the refined oil is bleached (reacted with activated clays, diatomaceous earths, or recoverable silicas) to adsorb phospholipids, color pigments, soaps, peroxides, aldehydes, and other polar compounds, and is recovered by filtration. Spent bleaching earths in the filter cake contain pigments, occasional residual pesticides, aldehydes, and other compounds removed from the oil, and they often present disposal problems. 

The starting materials of the aldehyde method may be sulfonated. For example. Cl Acid Blue 9 [2650-18-2] Cl Food Blue 2 (Cl 42090), is manufactured by condensing a-(A/-ethylanilino)-y -toluenesulfonic acid with o-sulfobenzaldehyde. The leuco base is oxidized with sodium dichromate to the dye, which is usually isolated as the ammonium salt. In this case, the removal of the excess amine is not necessary. However, this color caimot be used in the food sector because separation of the chromium compounds from the dye is difficult. An alternative method which gives food-grade Cl Acid Blue 9 (14) and dispenses with the use of sodium dichromate employs oxidative electrolysis of the leuco base (49). 

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. 

Equimolar quantities of the aldehyde 1 (4 mmol) and the active methylene compound 2 were stirred in CH2C12 (30 mL) in the presence of K10, ZnCl2 (4 g). The solvent was evaporated in vacuo and the obtained solid was subjected to microwave irradiation in an open Erlenmeyer flask (25 mL). The condensation product was extracted into acetonitrile. The extract was filtered and the solvent was evaporated in vacuo. The colored solid was washed with ethanol-water (1 1) and recrystallized from ethanol. 

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