Acidity of Phenols - Pharmanotes

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Acidity of Phenols

Contents

• Resonance structures of phenol and phenoxide

• Effect of substituents on acidity of
phenol

• Qualitative tests for phenols

• Structure and uses of phenol,
cresols, resorcinol, naphthols

Learning Objectives

At the
end of this lecture, the student will be able to

Qualitative Tests

• Action with sodium hydrodide

• Liebermann’s reaction

• Phthalein test

• Azo dye formation

• Scotten Baumann reaction

• Phenol dissolves in sodium hydroxide

Experiment	Observation	Inference
To one drop of dilute solution of phenol , a few drops of neutral ferric chloride solition are added	Violet colour	Presence of phenolic group
Liebermann’s reaction: To two drops of melted phenol, added little solid sodium nitrite. Cooled and added 4 drops of concentrated sulphuric acid. Diluted causiously with water	A deep red solution is obtained which turned green or blue with excess of alkali	Characteristic reactions of phenol
Phthalein test: About 2 drops of melted phenol is mixed with a small quantity of phthalic anhydride in a dry test tube. 2 drops of concentrated sulphuric acid are added. The mixture is heated at about 150ο C for two minutes. Cooled and excess of 10 % sodium hydroxide solution is added	Pink solution	Due to formation of phenolphthalein
Azo-dye formation: Dissolved two drops of aniline in 1 ml of dilute hydrochloric acid and well cooled in ice. A few drops of saturated sodium nitrite solution are added. The diazonium solution thus obtained is added to a well cooled solution of phenol in aqueous sodium hydroxide solution	A red colour	Formation of azo dye
Scotten-Baumann reaction: Dissolved about 0.25g of phenol in about 5 ml of 10 % sodium hydroxide solution contained in a boiling tube. About 1 ml of benzoyl chloride is added. The boiling tube is corked and shaken vigorously for about 15 minutes	A white precipitate is formed	Formation of phenyl benzoate

Structure and Uses of Phenol

• Phenols are widely used in
household products and as intermediates for industrial synthesis

• used (in low concentrations) as a disinfectant
in household cleaners and in mouthwash

Structure and Uses Cresols

• Different isomers are present

• Antibacterial and insecticidal
properties

Structure and Uses of
Resorcinol

• To treat acne, seborrheic
dermatitis, eczema, psoriasis, and other skin disorders

• To treat corns and warts

• Resorcinol works by helping to remove hard,
scaly, or roughened skin

Structure and Uses of
Naphthols

• 2-Naphthol is a
widely used as intermediate for the production of dyes and other compounds

Summary

• Acidity of phenol is explained by
difference in the resonance stabilization of phenol and phenoxide ion

• Electron withdrawing groups
increases the acidity of phenol

• Electron donating groups decreases’
the acidity of phenol

• Liebermann’s reaction and Phthalein
test are characteristic reactions of phenol

• Phenol is used (in low concentrations) as a
disinfectant in household cleaners and in mouthwash

• Cresol has antibacterial and
insecticidal properties

• Resorcinol is used to
treat acne, seborrheic dermatitis, eczema, psoriasis, and
other skin disorders

• 2-Naphthol is a
widely used as intermediate for the production of dyes and other compounds

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