Methyl orange

Reduction of methyl orange to / -aminodimethylaniline. Method 1. Dissolve 2 0 g. of methyl orange in the minimum volume of hot water and to the hot solution add a solution of 8 g. of stannous chloride in 20 ml. of concentrated hydrochloric acid until decolourisation takes place gentle boiling may be necessary. Cool the resulting solution in ice a crystalline precipitate consisting of sulphanilic acid and some p-aminodimethylaniline hydrochloride separates out. In order to separate the free base, add 10 per cent, sodium hydroxide solution until the precipitate of tin hydroxide redissolves. Extract the cold solution with three or four 20 ml. portions of ether, dry the extract 

Method 2. Suspend 2-0 g. of methyl orange in 4 ml. of water, and add a small quantity of sodium hyposulphite (Na2S204). Heat the mixture and add more sodium hyposulphite until the colour is discharged. The sulphanilic add remains in the solution as sodium sulphanilate and the p-aminodimethyl-aniline may be extracted with ether as in Method 1. 

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On the addition of sodium hydroxide, the sparingly soluble sodium salt of methyl-orange, NaOsSCeHgN .NCgHgNfCHj), will crystallise out. 

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. 

The use of methyl-orange as an indicator is based on the fact that the sodium salt in aqueous solution furnishes a yellow anion, which when treated with acids (except weak acids such as H2CO3 and H SOj) apparently gives rise to a red... 

Whilst the solution is still hot, add dilute hydrochloric acid until the stirred solution is just acid to litmus, and then distil off as much ethanol as possible, using the water-bath. Now add more dilute hydrochloric acid to the residual hot solution until it is just acid to methyl-orange. The 5,5-dimethyl-cyclohexan-1,3-dione separates as an oil which solidifies on cooling. Filter the product at the pump, wash it with ice-cold water, and dry it in a desiccator. Yield of the pale cream-coloured crystals, 12 g. m.p. 136-145 (preliminary softening). 

Example. Dissolve 0 3 g. of benzoic acid in a minimum of hot water (about 70 ml.) and add 5% aqueous sodium hydro.xide until the solution is just alkaline to methyl-orange, then add i drop of dilute hydrochloric acid. Pour this solution of the sodium salt into a solution of 0 5 g, of benzylthiouronium chloride in 5 ml. of water, and cool the stirred mixture in ice-water. Filter off the benzylthiouronium salt which has separated, and recrystallise from ethanol con taining 10% of water cream-coloured cr> stals, m.p. i66 . (M.ps., pp. 543 545.)... 

Formation of methyl-orange cf. p. 214). Dissolve about 0 3 g. of sul-phanilic acid in 2 ml. of 10% aqueous NajCOj solution. Cool m ice-water and add 2 to 3 drops of 20% NaNOj solution. Now add about I ml. of cold dil. HCl, shake and leave for 2-3 minutes. Meanwhile dissolve i drop of dimethylaniline in a few drops of dil. HCl, cool thoroughly in ice-water and then add to the cold diazo solution. Shake well and make alkaline with aqueous NaOH solution note the formation of a deep orange-yellow coloration or precipitate. On the addition of HCl, a bright red coloration is produced. 

Hydrazine hydrate may be titrated with standard acid using methyl orange as indicator or, alternatively, against standard iodine solution with starch as indicator. In the latter case about 0-1 g., accurately weighed, of the hydrazine hydrate solution is diluted with about 100 ml. of water, 2-3 drops of starch indicator added, and immediately before titration 6 g. of sodium bicarbonate are introduced. Rapid titration with iodine gives a satisfactory end point. 

It is interesting to note that azo dyestuffs may be conveniently reduced either by a solution of stannous chloride in hydrochloric acid or by sodium hyposulphite. Thus phenyl-azo-p-naphthol 3delds both aniline and a-amino-p-naphthol (see formula above), and methyl orange gives p-aminodimethylaniline and sulphanilic acid ... 

Sodium hyposulphite (dithionite) NajS O, may also bo employed for the reduction see under Methyl Orange, Section IV,78. 

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. 

Determine the methylamine content of the commercial solution by titration with standard acid using methyl orange as indicator. Adjust the quantity of methyl-amine solution in accordance with the methylamine content for some commercial samples, the figure may be 33-40 per cent. 

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. 

The indicators methyl orange and phenolphthalein are indicated by the abbreviations MO and pH, respectively. 

Methyl yellow, p-dimethylaminoazobenzene, benzeneazodimethylaniline (indicator) dissolve 0.1 g in 200 mL alcohol pH range red 2.9-4.0 yellow. The color change from yellow to orange can be perceived somewhat more sharply than the change of methyl orange from orange to rose, so that methyl yellow seems to deserve preference in many cases. See also under methyl orange. 

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