We found it necessary to first prepare a homogeneous, finely divided suspension of the aniline hydrochloride by vigorous stirring, before adding the sodium nitrite solution. However, diazotization of 3 in aqueous HCl presented difficulties, as was earlier reported by Frejka and Vymetal. To this end, we first prepared a formaldoxime solution as prescribed by Beech. In the second step, 4-bromo-2-nitroaniline ( 3) is converted to 4-bromo-2-nitrobenzaldehyde ( 6).
When the amount of solvent was adjusted suitably, practically pure product, in yields of up to 80%, precipitated out from the reaction mixture upon cooling. in which H 2O 2 is added at 50 ✬ to 3 in the presence of a slight excess of HBr in ethanol. We, therefore, adopted the optimized procedure of Islam et al.
In our hands, however, this led to a mixture of 3 and a dibrominated side product, which we were unable to separate. In the first step of this synthesis, we prepared 4-bromo-2-nitroaniline ( 3), following the procedure of Inoue and coworkers in which an excess of HBr and H 2O 2 with respect to 2 was used for the bromination. Due to the toxicity and possible carcinogenicity of these compounds, we initially chose the Beech synthesis instead, starting from o-nitroaniline ( 2). The traditional syntheses of 6 involve the use of p-toluidine, o-nitrotoluene, or their derivatives as starting materials or intermediates. This paper describes our efforts in developing a new, more attractive synthesis of Tyrian purple.įirst synthetic scheme for the preparation of dibromoindigo ( 1). In particular, we are interested in a synthesis which is amenable to production of relatively large amounts of the compound in an undergraduate student laboratory. A chemical synthesis of reasonable length, cost and safety is, therefore, still a very desirable research goal. Synthetic Tyrian purple is commercially available today, but at a price nearly as high as the natural pigment. However, all known syntheses are either lengthy, inefficient, or involve expensive or hazardous starting materials or reagents. Įver since the dye was identified, chemists have been interested in developing practical syntheses of the compound. Paul Friedländer, who in 1909 first identified the structure of the dye as 6,6′-dibromoindigo, required 12,000 Murex brandaris snails to produce 1.4 g of pure pigment. Due to the minute amounts of dye found in the snails, the dye has always been very costly. Structure of 6,6’-dibromoindigo ( 1, Tyrian purple).įrom ancient times the dye has been produced from secretions of various species of snails found off the Atlantic and Mediterranean coasts.
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