Methyl-orange, constitution

Suitably substituted azo compounds constitute an important class of dyes—the azo dyes. Some derivatives such as /j-dimethylaminoazobenzene- p -sulfonic acid Na-salt (trivial name methyl orange) are used as pH-indicators. 

Those of complex constitution and high molecular weight, e.g., sodium glycocholate, Congo red, methyl orange, sodium oleate, which show adsorption 20—100 times larger than that calculated from the formula. 

Indicators are used in small quantities so diat [H ] is determined by the other constitutents of the solution. The ratio [HIn]/[In ), which determines the color, depends on the pH of the solution, Widi methyl orange, for example, the solution is red if [Hln] > [In ], yellow if [In ] > [HIn], and varying shades of orange when [HIn] and [In ] are comparable. Table 15.1 lists some common indicators, their colors, and the pH ranges in which the colors show perceptible gradations. 

Furanones and pyranones constitute a family of compounds that derive from a common a-dicarbonyl intermediate, namely, CHj-CO-CO-(CHOHJn-CHjR (where R-H or OH). Acetylformoin, which has a fruity-caramel aroma, has been isolated from browned instant orange juice (43) and from a sugar model system containing a catalyst (38). It can be formed by dehydration and cyclization of methyl dihydroxy-ethyl reductone (enediol of the methyl dicarbonyl intermediate), but it decomposes easily in air to pungent compounds (44). 

Compound (85) constitutes one of the minor components of the extract studied by Chavez et al. [81]. It was as an orange lacquer, showing a molecular formula C28H36O5. Its H-NMR spectrum was similar to that of compound (83), except the lack of aldehyde signal, the presence of six methyl signals, one of them attached to an aromatic ring. With the aid of 13C-NMR data, the structure of (85) was established as 22p-hydroxy-3-methoxy-6-oxotingenol or 2, 22p-dihydroxy-3-methoxy-24-29-dinor-1,3,5(10),7- friedelatetraene-6,21-dione, Fig. (45). 

The acid reaction occurs, as defined in the test, when methyl red changes from yellow to orange or red at about 5.1 pH or when bromothymol blue changes from blue to yellow at about 7.1 pH. This means that most tertiary and quaternary amines are found in the ionic acid form, and this allows the thereby cationic alkaloid to precipitate with anionic iodobismuthate. The salt is believed to be constituted not of monomeric entities, of for example one nitrogen base and one iodobismuthate, but of oligomeric cations and anions. 

During extraction, 9 -ds-bixin is also partly decomposed to orange and pale yellow products. The main product is 14-methyl-hydrogen-4,8-dimethyltetradeca-2,4,6,8,10,12-hexaene-l,14-dioic acid (9-189). This compound exists in various stereoisomers such as the all-trans isomer, 15-cis, 13-cis and 9-ds isomer. Related compounds with 18 and 13 carbon atoms per molecule, m-xylene, toluene and other products also arise. Coloured degradation products of 9 -ds-bixin may constitute up to 40% of pigments in commercial products. 

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