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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.
The first and still main use of diazonium salts is azo coupling, which is exploited in the production of azo dyes. [12] [13] In some cases water-fast dyed fabrics are simply immersed in an aqueous solution of the diazonium compound, followed by immersion in a solution of the coupler (the electron-rich ring that undergoes electrophilic ...
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.
The nucleophilic addition of the enolate anion 2 to the diazonium salt produces the azo compound 3. Intermediate 3 has been isolated in rare cases. However, in most cases, the hydrolysis of intermediate 3 produces a tetrahedral intermediate 4 , which quickly decomposes to release the carboxylic acid 6 .
Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near 0 °C. The oxidation of hydrazines (R−NH−NH−R′) also gives azo compounds. [5] Azo dyes are also prepared by the condensation of nitroaromatics with anilines followed by reduction of the resulting azoxy intermediate:
Pigment Yellow 74 is an azo dye and classified as an arylide yellow. It is an intensely yellow-green solid. [1] It is prepared by azo coupling of the diazonium derivative of 2-methoxy-4-nitroaniline with 2-methoxyacetoacetanilide. [2] It is a flat molecule, enforced by the hydrogen-bonding via the ketohydrazide group. [3]
N-(1-Naphthyl)ethylenediamine dihydrochloride is widely used in the quantitative analysis of nitrate and nitrite in water samples by colorimetry.It readily undergoes a diazonium coupling reaction in the presence of nitrite to give a strongly colored azo compound.
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 ...