2-Naphthol, coupling with diazonium salts

Both 2- and 3-nitrothiophenes are reduced by tin and hydrochloric acid to the corresponding aminothiophenes. In contrast to anilines, the free bases are very unstable their salts and acyl derivatives, however, are stable. 2-Aminothiophene can be diazotized and the resulting diazonium salt coupled with / - naphthol. The chemical instability of aminothiophenes compared with aniline is illustrated by the ring opening of 2-amino-3-ethoxycar-bonylthiophenes (157) with ethanolic sodium ethoxide to give cyanothiolenones (158) (75JPR861). 

The free bases are much less stable than aniline, particularly 2-aminopyrroles and -furans which are very easily oxidized or hydrolyzed. 2-Aminofurans substituted with electron-withdrawing groups (e.g., N()2) are known and 3-amino-2-methylfuran is a relatively stable amine which can be acylated and diazotized. 2-Aminothiophene can be diazotized and the resulting diazonium salt coupled with -naphthol. 2,3-Diaminothiophene has been prepared and isolated as the hydrobromide the free base is not stable. 

The kinetics of coupling a diazonium salt (D) with naphthol (N) to form a dye (A) is complicated to a minor extent by spontaneous decomposition of D to form inert products P. Bata of A, but not of other participants, are in the table (Hanna et al, JACS 96 7222, 1974). Temperature was 0 C, N0 = 0.01 mols/liter, D0 - 0.0001. Find the specific rates. 

The alternative approach is to pad the fabric with the alkaline naphthol and dry, foUowed by printing directly onto this prepared fabric diazonium salts or stabilized diazonium salts. Coupling is instant and the only further treatment needed is to remove aU the uncoupled naphthol and surface azo pigment in a subsequent washing treatment. Because the choice of colors is limited from one naphthol component, other shades are obtained by using other classes of dye alongside the azoic colors, eg, reactives. This approach is widely used in the production of African prints. 

Amino-l,2,4-thiadiazoles are diazotized at — 10°C in phosphoric acid, and couple with naphthol to give moderate yields of diazo dyes.182,388 In their limited stability, the 3-diazonium salts resemble those of 3-aminoiso-thiazole, but differ from the more stable 1,2,4-thiadiazole- and isothiazole-5-diazonium salts a consideration of the 7t-electron densities leads to the opposite conclusion.182... 

A suspension of o-chlorometanilic acid in aq. HCl treated at 0-2° with a 20%-Na-nitrite soln. and the resulting diazonium salt coupled at 2-4° with 2-naphthol in NaOH-NagCOg-soln. Na-4-chloro-3-(2-hydroxy-1-naphthylazo) benzenesul-... 

Dimethyl-4-pyrimidone and l,4,6-trimethyl-2-pyrimidone show similar behaviour at the 6-methyl group.1-Aminophenazine fails to form a diazonium salt that will couple with -naphthol (though it will couple with phloroglucinol) because the phenazine-1-diazonium cation reacts rapidly with water and is oxidized to a 1-diazo-phenazinone that has weak coupling ability. 2-Amino-phenazine behaves similarly. ... 

Diazo coupling involves the N exocyclic atom of the diazonium salt, which acts as an electrophilic center. The diazonium salts of thiazoles couple with a-naphthol (605). 2-nitroresorcinol (606), pyrocatechol (607-609), 2.6-dihydroxy 4-methyl-5-cyanopyridine (610). and other heteroaromatic compounds (404. 611) (Scheme 188). The rates of coupling between 2-diazothicizolium salts and 2-naphthol-3.6-disulfonic acid were measured spectrophotometrically and found to be slower than that of 2-diazopyridinium salts but faster than that of benzene diazonium salts (561 i. The bis-diazonium salt of bis(2-amino-4-methylthiazole) couples with /3-naphthol to give 333 (Scheme 189) (612). The products obtained from the diazo coupling are usuallv highly colored (234. 338. 339. 613-616). 

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

With a phenol, naphthol or keto-enol coupling component the mechanism is given by Scheme 4.4, in which blocking of the p-position forces coupling at the o-position. In certain cases involving the use of feebly reactive diazonium salts, loss of the proton from the transition state (4-1) in Scheme 4.4 may be slow but may often be facilitated by the addition of a tertiary base, such as pyridine, to the coupling mixture [7,8]. 

These pigments are obtained by coupling substituted aryl diazonium salts with ary-lides of 2-hydroxy-3-naphthoic acid (2-hydroxy-3-naphthoic acid anilide = Naph-tol AS). They provide a broad range of colors from yellowish and medium red to bordeaux, carmine, brown, and violet their solvent fastness and migration resistance are only marginal. Naphthol AS pigments are used mainly in printing inks and paints. 

Nitrosylsulfuric acid, prepared by dissolving sodium nitrite in concentrated sulfuric acid, is employed for amines of low basicity, whose diazonium salts will hydrolyze in dilute acid. In order to synthesize Pigment Orange 5, for instance, 2,4-dinitroaniline is dissolved in concentrated sulfuric acid and diazotized preferably with nitrosylsulfuric acid. Coupling is carried out with a (3-naphthol suspension, produced by acidifying a sodium naphtholate solution. 

A sample of the reaction mixture should not become turbid when sodium acetate solution is added. But if now a few drops of a solution of an aniline salt are poured in, there is precipitated yellow diazoaminobenzene, which can be re-dissolved with concentrated hydrochloric acid after a few small pieces of ice have been added. Further, when a few particles of j8-naphthol or of R-acid are dissolved in a small excess of 2 A-sodium hydroxide solution and a sample of the diazo-solution is added, an intense red colour is produced. The development of this colour, which results from coupling , is an infallible test for a diazonium salt, and hence also for the corresponding primary aromatic amine. 

Whilst the conversion of the simple phenyl-syw-diazotate into its isomeride requires strong alkali, the syn-iotms of other diazotates are so labile that, almost instantaneously after formation from the diazonium salts, they change into the anti-iorm and hence can never be obtained in solution. An important example of this change is described under (/), namely, p-nitrophenyldiazotate, which, on coupling with j8-naphthol, yields the much-used dye para-red . 

The completeness of coupling may be checked by the R-salt test as follows A few drops of the clear supernatant liquid is added to a small quantity of a solution of R-acid (jS-naphthol-3,6-disulfonic acid) in an excess of 2 TV sodium hydroxide. An intense red color is produced if unreacted diazonium salts are present. Obviously this test is satisfactory only if R-acid couples more rapidly with the diazonium salt than the coupling agent involved in the reactions and if the change to a red color is not obscured by other colors present in the reaction system.]... 

Coupling of 2-naphthol-l-sulfonic acid (199) with diazonium salts leads in several well-defined steps (Scheme 8) to l-hydroxy-3,4-dihydrophthalazine-... 

Diazonium Salts from 3-Amino-1,2,4-thiadiazoles The 3-amino group in 1,2,4-thiadiazoles (e.g. in the 5-phenyl homolog) is also capable of being diazotized, preferably in concentrated phosphoric acid. The resulting diazonium salt may be coupled in the usual way, but with sufficiently reactive partners only (e.g. phenol and 0-naphthol).126... 

---