Organic Chemistry

Module 4. Amine and phenols

Lesson 12
ACIDIC CHARACTER OF PHENOLS AND EFFECT OF NUCLEAR SUBSTITUTION ON IT. IMPORTANT REACTIONS OF PHENOLS

12.1 Introduction

• Phenols are benzene ring hydroxy compounds represented by the general formula Ar-OH
• Where Ar- is phenyl (C6H5-) group, substituted phenyl group or aryl group derived from polynuclear aromatic hydrocarbon
• May monohydric, dihydric, trihydric, etc- depending on number of –OH groups attached to the aromatic nucleus
• The simplest member of this family – hydroxybenzene – phenol
• Other phenols are generally named as derivatives of phenol

Fig. 12.1

• Phenols containing more than one –OH groups in benzene ring are better known their by special names

 

Fig. 12.2

• Phenols differ from alcohols in having –OH group attached directly to the aromatic nucleus benzene ring
• Being hydroxyl compounds, they resemble alcohols in certain respects – e.g. can be converted into ethers and esters
• They differ substantially in most of their properties – therefore they deserve to be classified as different family

12.2 Physical Properties

• Pure phenols are generally colourless solids or liquids, but they turn reddish due to atmospheric oxidation
• Phenol itself is somewhat soluble in water (9 g/ 100g) – because of H bonding with water
• Most other phenols are insoluble in water
• Its boiling point is very high due to intermolecular H bonding

12.3 Chemical reaction

12.3.1 Salt formation (acidic character)

• Phenols are fairly acidic compounds which form salts –
• Phenoxide – on reaction with alkali metal hydroxide

Fig. 12.3

• The salt decomposes by mineral acids, carboxylic acids or even by carbonic acid- gives back the free phenol
  

Fig. 12.4

R-COONa + H2CO3 - No reaction
Therefore phenols are less acidic than carboxylic acids or even H2CO3

• Acid strength
  

• Phenols are much more acidic than alcohols but less than carboxylic acids even carbonic acid

• Electron-withdrawing substituents -NO2 , -Cl, etc. enhance the acidity of phenols, but electron releasing substitutes -CH3 - decreases it.

12.3.2 Esterfication

• Phenols react with acids, in presence of polyphosphoric acid or toluene sulphonic acid to give phenyl esters.

• The yields are poorer than those obtained in case of alcohols

Fig. 12.5

• Phenyl esters are better prepared by the action of acid chlorides or acid anhydrides

Fig. 12.6

12.3.3 Etherification

• Phenols can be converted into ethers by treatment with alkyl halides or alkylsulphates (Fig. 12.7)

12.3.4 Reaction with ferric chloride

• Phenols (unlike alcohols) gives characteristic colours with neutral ferric chloride – green, blue, violet, red etc
• Due to formation of complexes, but their precise nature is unknown

12.3.5 Libermann Nitroso reaction

• When phenol is treated with NaNO₂ and concentrated H₂SO₄, it develops a deep green or blue colour which turns red on careful dilution with water. Addition of NaOH solution brings back the green or blue color. (Fig. 12.8)

12.3.6 Phthalein reaction

• When phenols are heated with phthalic anhydride in presence of concentrated H₂SO₄ or anhydrous ZnCl₂ - phthaleins are formed – give characteristic colour depending on reaction medium pH. (Fig. 12.9)

12.3.7 Reaction with formaldehyde

• Results into formation of polymer – phenol-formaldehyde resins - also known as Bakelite
• It is among the oldest of the synthetic polymers and still extremely important
• When phenol is treated with formaldehyde in the presence of acid or alkali – polymer is formed. (Fig. 12.10)

• Stages involved in the formation of the polymer

• Phenol reacts with formaldehyde to form o- or p- hydroxymethyl-phenol
• Hydroxymethyl phenol than reacts with another molecule of phenol with loss of water to form compound in which two rings are joined by a –CH2- link
• This process than continues to yield a product of high molecular weight- a polymer (Thermosetting polymer)
• Since three positions in each phenol molecule are susceptible to attack, the final product contains many cross-links
• Hence has a rigid linear or cross-linked structure.