Acidity of Phenols

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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