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BACKGROUND
Principle:
Sulphanilic acid with sodium carbonate converted into sodium salt of p-aminobenzene sulphonate, It undergoes diazotization reaction in presence of nitrous acid to form diazonium chloride salt. The diazonium chloride then ionizes in aquous solution giving sodium ion, chloride ion and the internal salt (–O3SC6H4N2+). N, N-dimethylaniline hydrochloride added to the internal salt, coupling occurs (electrophilic aromatic substitution) to yield methyl orange.1
Aim: To prepare methyl orange from sulphanilic acid.
Reaction:
Mechanism:
Step 1:
Step 2:
Step 3:
Use:
Methyl orange is a pH indicator.
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REQUIREMENTS
Chemicals: Sodium sulphanilate dehydrate
Anhydrous sodium carbonate
Sodium nitrite
Acetophenone
Phenylhydrazine
Conc. hydrochloric acid
Dimethylaniline
Glacial acetic acid
Sodium hydroxide
Sodium chloride
Apparatus: Conical flask
Thermomete
Beaker
Buchner funnel
Measuring cylinder
Potassium iodide-starch paper
Filter paper
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PROCEDURE
Take 10.5 g (0.05 mol) of sodium sulphanilate dihydrate, 2.65 g (0.025 mol) of anhydrous sodium carbonate and 100 ml of water in a 250 ml conical flask and warm until a clear solution is obtained. Cool the mixture solution under the tap to about 15 °C and mix a solution of 3.7 g (0.059 mol) of sodium nitrite in water (10 ml). Transfer the resulting solution slowly with constant stirring into a 600 ml beaker holding 10.5 ml of concentrated hydrochloric acid and 60 g of crushed ice. Test the presence of free nitrous acid with potassium iodide-starch paper after 15 min. Fine crystals of the diazobenzene sulphonate will be soon separated. Do not filter these off as they will dissolve during the next stage of the preparation. Dissolve 6.05 g (6.3 ml, 0.05 mol) of dimethylaniline in 3.0 ml of glacial acetic acid, and add it with vigorous stirring to the suspension of diazotised sulphanilic acid. Allow the mixture to stand for 10 min; the red or acid form of methyl orange will gradually separate. Then add slowly and with stirring 35 ml of 20% sodium hydroxide solution. The mixture will assume a uniform orange colour due to the separation of the sodium salt of methyl orange in fine particles. Direct filtration of the latter is slow, hence, while stirring the mixture with a thermometer, heat it almost to the boiling point. Most of the methyl orange will dissolve. Add about 10 g of sodium chloride (to assist the subsequent separation of the methyl orange) and warm at 80-90 °C until the salt has dissolved. Allow the mixture to cool undisturbed for 15 min and then cool in ice-water; this gives a fairly easily filterable product. Filter off the methyl orange at the pump, but apply only gentle suction so as to avoid clogging the pores of the filter paper; rinse the beaker with a little saturated salt solution and drain well. Recrystallise from hot water (about 150 ml are required); filter the hot solution, if necessary, through a hot water funnel or through a preheated Buchner funnel. Reddish-orange crystals of methyl orange separate as the solution cools. Filter these at the pump, drain well, wash with a little ethanol, and finally with a small volume of ether. The yield is 13 g (80%). Methyl orange, being a salt, has no well-defined m.p.
Calculation:
Here limiting reagent is sodium sulphanilate dihydrate; hence yield should be calculated from its amount taken.
Molecular formula of sodium sulphanilate dihydrate = C6H10NNaO5S
Molecular formula of methyl orange = C14H14N3NaO3S
Molecular weight of sodium sulphanilate dihydrate = 231 g/mole
Molecular weight of methyl orange = 327 g/mole
Theoretical yield:
231 g sodium sulphanilate dihydrate forms 327 g methyl orange
Therefore, 10.5 g sodium sulphanilate dihydrate will form …….? (X) g methyl orange
X =( 327 × 10.5)/231 = 14.86 g
Theoretical yield = 14.86 g
Practical yield = ————- g
% Yield = (Practical Yield)/(Theoretical Yield) × 100
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CONCLUSION
Methyl orange was synthesized and the percentage yield was found to be………..%
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REFERENCES
- Vogel’s Textbook of Practical Organic Chemistry by Brian S. Furniss, Antony J. Hannaford, Peter W. G. Smith & Austin R. Tatchell; Fifth Edition; Page No.- 951
- Practical in organic chemistry, by Hitesh G. Raval, Sunil L. Baldania and Dimal A. Shah, Nirav Prakashan, Page No. 320.
