Preparation and properties of phenol

Introduction to phenol

Phenols are a group of organic compounds that contain a hydroxyl group (-OH) directly attached to an aromatic hydrocarbon group. Unlike alcohols, where -OH group is attached to a saturated carbon atom, in phenols, the hydroxyl group is bonded to a carbon atom that is part of the aromatic ring. The simplest phenol has the formula C_6H_5OH, known as phenol or carbolic acid.

Structure of phenol

The structure of the phenol molecule is unique because it contains a hydroxyl group (-OH) attached directly to the aromatic ring, resulting in different chemical properties. The resonance-stabilized benzene ring affects the properties and reactivity of the hydroxyl group.

      Hey 
     , 
    C - C 
    , 
    C - C 
     , 
      C 
    

Preparation of phenol

1. From benzene sulfonic acid

Phenol can be prepared from benzene sulfonic acid. The process involves the following steps:

1. Sulfonation of benzene:

   Benzene sulfonic acid is made by reacting benzene with concentrated sulfuric acid (H_2SO_4).

               C_6H_6 + H_2SO_4 → C_6H_5SO_3H + H_2O
               

2. Conversion to phenol:

   The benzene sulfonic acid is then combined with solid sodium hydroxide (NaOH) at high temperature, followed by acidification, to yield phenol.

               C_6H_5SO_3H + 2 NaOH → C_6H_5ONa + Na_2SO_3 + H_2O
               C_6H_5ONa + HCl → C_6H_5OH + NaCl
               

2. From diazonium salts

Another method of preparing phenol is from benzene diazonium chloride, an intermediate formed from aniline.

1. Preparation of benzene diazonium chloride:

   Benzene diazonium chloride is produced by reacting aniline (C_6H_5NH_2) with nitrous acid (HNO_2, produced from sodium nitrite and hydrochloric acid) at 0-5°C.

               C_6H_5NH_2 + NaNO_2 + 2 HCl → C_6H_5N_2^+Cl^- + 2 H_2O + NaCl
               

2. Conversion to phenol:

   When diazonium salt is heated with water, hydrolysis occurs, forming phenol.

               C_6H_5N_2^+Cl^- + H_2O → C_6H_5OH + N_2 + HCl
               

3. From cumene (isopropylbenzene)

This industrial process is known for producing phenol and acetone. The process involves:

1. Oxidation of cumene:

   Cumene is oxidized in the presence of air to form cumene hydroperoxide.

               C_9H_12 + O_2 → C_9H_12O_2
               

2. Acid-catalyzed decomposition:

   Cumene hydroperoxide decomposes in acidic medium to form phenol and acetone.

               C_9H_12O_2 → C_6H_5OH + (CH_3)_2CO
               

Properties of phenol

Physical properties

• Appearance: Pure phenol is a white, solid crystalline compound at room temperature.
• Melting point and boiling point: Phenol has a melting point of about 40.5°C and a boiling point of about 181.7°C.
• Solubility: Due to hydrogen bonding, phenol is moderately soluble in water and readily soluble in most organic solvents.

Chemical properties

Phenols exhibit a number of chemical reactions characteristic of their structure:

1. Acidic nature

Phenols are weak acids and can release H^+ ions in aqueous solution, forming phenoxide ions:

    C_6H_5OH ⇌ C_6H_5O^- + H^+
    

The acidic nature of phenol is greater than that of aliphatic alcohols due to stabilization by resonance of the phenoxide ion.

Illustration

    
    [ C_6H_5O^- ] ↔ { C_6H_5-O^- }
    
    

2. Electrophilic substitution reactions

Due to the presence of the hydroxyl group, phenols are highly reactive towards electrophilic substitution reactions, especially at the ortho- and para-positions.

    C_6H_5OH + Br_2 → 2,4,6-tribromophenol + HBr
    

3. Reaction with bases

Due to its acidic nature, phenol reacts with strong bases to form phenoxide salts:

    C_6H_5OH + NaOH → C_6H_5ONa + H_2O
    

Uses of phenol

Phenols are essential in a variety of industrial and pharmaceutical applications:

• Disinfectant and antiseptic: Phenol is used as an antiseptic and disinfectant due to its ability to destroy proteins.
• Production of plastics: Phenol is a precursor to important polymers such as Bakelite.
• Pharmaceuticals: Some phenolic compounds are used in the production of medications such as aspirin.

Conclusion

Understanding the preparation and properties of phenols is important due to their wide applications in the industrial and pharmaceutical fields. The ability to easily undergo electrophilic substitution reactions makes phenols versatile in synthesis processes, while their acidic nature provides additional utility in a variety of chemical reactions.

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