Synthesis of 2-phenylindole from Phenyl Hydrazine

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BACKGROUND

2-phenylindole

Aim:

To prepare 2-phenylindole from phenyl hydrazine.

Principle:

Ayrl hydrazones are formed from a condensation reaction of an arylhydrazine and an aldehyde or ketone. Here phenylhydrazine condenses with acetophenone to produce acetophenone phenylhydrazone. Finally, by Fischer indole synthesis the arylhydrazone (acetophenone phenylhydrazone) converts into the indole (2-phenylindole) in the presence of an acid catalyst.¹

Reaction:

Mechanism:

Step 1:

Step 2:

Use:

2-Phenylindole is a reactant for preparation of organic light emitting diodes (OLEDs), anti-inflammatory agents, antibacterial, antifungal agents and fluorescent probes. It is a reactant in difluorohydroxylation reactions and Mannich-type reactions.

REQUIREMENTS

Chemicals:

Acetophenone
Phenylhydrazine
Glacial acetic acid
Dilute hydrochloric acid
Ethanol
Polyphosphoric acid
Calcium chloride etc.

Apparatus:

Round bottomed flask 500 ml
Reflux condenser
Thermometer
Beaker
Buchner funnel
Measuring cylinder
Filter paper etc.

PROCEDURE

Prepare acetophenone phenylhydrazone by boiling a mixture of 20 g (0.167 mol) of acetophenone and 18 g (0.167 mol) of phenylhydrazine carefully with ethanol 60 ml and a few drops of glacial acetic acid. Filter the cold reaction mixture, wash the solid with dilute hydrochloric acid followed by about 12 ml of cold rectified spirit.

Recrystallise a small portion from ethanol and thus obtain a sample of pure acetophenone phenylhydrazone as a white solid, m.p. 106 °C. Place 28 g of the crude phenylhydrazone in a 250 ml beaker containing 180 g of polyphosphoric acid.

Heat on a boiling water bath, stir with a thermometer and maintain at 100-120 °C for 10 min (the reaction is exothermic). Add 450 ml of cold water and stir well to complete solution of the polyphosphoric acid.

Filter at the pump and wash well with water. Boil the crude solid under the reflux along with 300 ml of rectified spirit, add a little decolourising charcoal and filter through a preheated Buchner funnel.

Wash the residue with 40 ml of hot rectified spirit. Cool the combined filtrates to room temperature, filter off the 2-phenylindole and wash it three times with 10 ml portions of cold alcohol. Dry in a vacuum desiccator over anhydrous calcium chloride. The yield of pure 2-phenylindole, m.p. 188-189 °C, is 20g (79%).

Calculation:

Here limiting reagent is phenylhydrazine; hence yield should be calculated from its amount taken.

C₆H₈N₂ = Molecular formula of phenylhydrazine

C₁₄H₁₁N =Molecular formula of 2-phenylindole

Molecular weight of phenylhydrazine = 108 g/mole

Molecular weight of 2-phenylindole = 193 g/mole

Theoretical yield:

108 g phenylhydrazine forms 193 g 2-phenylindole

Therefore, 18 g phenylhydrazine will form …….? (X) g 2-phenylindole

= 32.16 g

Theoretical yield = 32.16 g

Practical yield = ————- g

% Yield = (Practical Yield)/(Theoretical Yield) × 100

CONCLUSION

2-phenylindole was synthesized and the percentage yield was found to be………..%

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. 1161.

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