Methyl orange, preparation

It is important in this preparation to avoid an excess of nitrous acid before coupling occurs, otherwise the excess of nitrous acid will react directly with the dimethylaniline, and the deep green p-nitrosodimethylaniline so formed will contaminate the methyl-orange. 

Compounds containing two primary amino groups attached to a benzene ring can be prepared by the reduction of dinitro compounds and of nitroanilines, usually with tin or stannous chloride and hydrochloric acid or with iron and very dilute hydrochloric acid. / ara-diamines may also be obtained by the reduction of aromatic amino-azo compounds (e.g., p-aminodimethylanihne from methyl orange, see Section IV,78). p-Phenylenediamine may also be prepared from p-nitroacetanilide reduction with iron and acid yields p-amino-acetaniUde,.which may be hydrolysed to the diamine. 

The procedures for the preparation of a number of azo dyestuffs are described in Sections IV, 76-1V,82 these include the indicators methyl orange and methyl red. Experimental details for the preparation of other typical dyestuffs and indicators are given in the following pages. 

A solution of dry ammonia gas in absolute ethyl alcohol is prepared and titrated against standard hydrochloric acid, using methyl orange as the indicator. The solution should contain at least 9 per cent of ammonia by weight (Note 5). 

Aniline and other aromatic amines are valuable industrial raw materials. They form an important starting point from which many of our dyestuffs, medicinals, and other valuable products are prepared. For example, you have used the indicator, methyl orange, in your laboratory experiments. Methyl orange is an example of an anQine-derived dye, although it is used more as an acid-base indicator than for dyeing fabrics. The structure of methyl orange is as follows ... 

Procedure B. The experimental details for the preparation of the initial solution are similar to those given under Procedure A. Titrate 25 or 50 mL of the cold solution with standard 0.1M hydrochloric acid and methyl orange, methyl orange-indigo carmine, or bromophenol blue as indicator. Titrate another 25 or 50 mL of the cold solution, diluted with an equal volume of water, slowly with the standard acid using phenolphthalein or, better, the thymol-blue cresol red mixed indicator in the latter case, the colour at the end point is rose. Calculate the result as described in the Discussion above. 

Example Methyl orange offers a gradual end-point. Hence, two flasks containing the same volume of solution having approximately the same composition as the liquid being titrated may be prepared first, slightly acidic—Red solution, second, slightly basic—Yellow solution. 

This procedure has been used successfully for many years in the preparation of ethyl laurate, caprylate, and myristate by the alcoholysis of cocoanut oil (1 kg.) in ethanol (1900 g.) with hydrogen chloride (50 g.) as a catalyst.3 The method differs slightly from the one described above. The alcoholysis is complete after fifteen or twenty hours, and the solution is then neutralized to methyl orange with barium carbonate. The mixture is added to an equal volume of a saturated sodium chloride solution, whereupon 1100-1300 g. of the mixture of crude ethyl esters separates. This mixture of esters is washed with water and fractionated as described above. The yields are approximately 50 g. of ethyl caprylate, 350 g. of ethyl laurate, and 60 g. of ethyl myristate from 1000 g. of cocoanut oil. 

Liquid/liquid partition chromatography was explored by Willstatter from 1913. The process was extensively developed by Martin and Synge (ca. 1941-1948) who partitioned amino acid derivatives between chloroform and water using precipitated silica as support for the aqueous phase. The preparations of silica were again very variable and it was difficult to prevent adsorption which interfered with the expected behavior of the aminoacids. At first methyl orange was added to the water phase to visualize the amino acids the separation of the acids then caused a red band to move down the columns. The quantitative reaction with ninhydrin was introduced by Moore and Stein in 1948 for both the detection and estimation of the amino acids. Consid-... 

Some modifications to the cyclodextrin structure have also been found to improve their complexing ability. Casu and coworkers prepared 2,3,6-tri-O-methyl and 2,6-di-O-methyl derivatives of alpha and beta cyclodextrin. They observed that tri-O-methyl-alpha cyclodextrin shows an almost ten-fold increased stability of the complex with the guest, Methyl Orange, compared with the unmodified alpha cyclodextrin. A possible reason for this increase in stability is that the methyl groups are responsible for an extension of the hydrophobic cavity of the cyclodextrin. Other workers,however, observed a much smaller enhancement of stability of complexes on methylation of the cyclodextrin, and a decrease in stability has even been reportedfor the one host-two guests complex of tropaeolin with beta cyclodextrin. Thus, the effect of methylation on the stability of a complex varies with the guest species involved, and cannot be readily predicted. 

Preparation 383.—Methyl Orange (i-Dimethylamino-i -azobenzene-Na-sulphonate). 

When the liquid is coloured it is difficult to observe the change of colour of the indicator in such case, use is made of papers prepared by immersing filter-paper in a o-i% methyl orange solution and drying in an oven. 

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