Preparation of phenyl-azo B naphthol dye (Diazotisation).

 

Aim: To prepare phenyl–azo–β-naphthol (an azo dye).

Theory:

Aniline is an aromatic primary amine. It forms diazonium salt when treated with nitrous acid at 0-5°C. Nitrous acid is generated in situ by the reaction of sodium nitrite with hydrochloric acid. The process is called diazotisation. The diazonium salt is coupled with an alkaline solution of β-naphthol to form an orange-red azo dye.

Material Required:

Beaker (250mL), Conical flask (100mL), Glass rod, Thermometer (210°C), Filter paper,  Funnel, Melting point assembly.

Chemicals:

Aniline (2mL), Conc. HCl (6.5 mL), Sodium nitrite (1.6gm)  β-naphthol (3.2 gm), Sodium hydroxide (2.0 gm), Glacial acetic acid (12.0 mL),

Procedure:

 (i) Take 6.5 mL of concentrated hydrochloric acid in a 100 mL beaker. Dilute it with 6.5 mL of water and dissolve 2 mL of aniline in it.

(ii) Cool the above mixture by placing the beaker in an ice bath maintained at 0-5°C temperature.

(iii) Diazotise the above mixture by adding a solution of 1.6 g of sodium nitrite in 8 mL water.

(iv) Dissolve 3.2 g β-naphthol in 18 mL of 10% sodium hydroxide solution. Add about 25 g of crushed ice to it.

(v) Stir the β-naphthol solution well and add chilled diazonium chloride solution very slowly to it with constant stirring.

(vi) An orange red dye of phenyl-azo-β-naphthol is formed.

(vii) Allow the mixture to stand in the bath for 30 minutes with occasional shaking.

(viii) Filter the crystals obtained and wash them well with cold water.

(ix) Recrystallise about one-fourth of the crude product from glacial acetic acid.

(x) Filter the recrystallised sample, wash with a little alcohol to remove acetic acid. Dry the recrystallised sample between the folds of a filter paper.

 (xi) Record the yield and the melting point of the compound.

 Result:

(a) Yield of phenyl–azo–β–naphthol is ______g and its

 (b) Melting point of phenyl-azo-β-naphthol is _______ °C.