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In organic chemistry, an azo coupling is an reaction between a diazonium compound (R−N≡N +) and another aromatic compound that produces an azo compound (R−N=N−R’).In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile, and the activated carbon (usually from an arene, which is called coupling agent), serves as a nucleophile.
Alizarine Yellow R is a yellow colored azo dye made by the diazo coupling reaction. It is usually commercially available as a sodium salt. In its pure form, it is a rust-colored solid. [2] It is mainly used as a pH indicator.
Notice that the coupling occurs adjacent to the hydroxy group. 3-Hydroxy-2-naphthoic acid is a precursor to many anilides, such as Naphthol AS, which are reactive toward diazonium salts to give deeply colored azo compounds. Azo coupling of 3-hydroxy-2-naphthoic acid gives many dyes as well. Heating 3-hydroxy-2-naphthoic acid in ammonia give 3 ...
To make the dyes, acetoacetanilides are coupled to diazonium salts, "azo coupling". [6] Acetoacetylation with diketene followed by diazo coupling. The ketohydrazone tautomer is shown. In the presence of sulfuric acid, acetoacetanilide dehydrates to give 4-methyl-2-quinolone. [7]
Azo pigments are similar in chemical structure to azo dyes, but they lack solubilizing groups. [8] Many so-called azo pigments are not strictly classifiable as azo compounds since they exist as keto hydrazide tautomers, which lack the -N=N- linkage. C.I. Pigment Yellow 12, an azo pigment (also classified as a diarylide pigment).
The Griess test involves two subsequent reactions. When sulfanilamide is added, the nitrite ion reacts with it in the Griess diazotization reaction to form a diazonium salt, which then reacts with N-(1-naphthyl)ethylenediamine in an azo coupling reaction, forming a pink-red azo dye.