Phenols

• Phenols are a class of organic compounds consisting of a hydroxyl group (-OH) attached directly to an aromatic hydrocarbon ring, usually a benzene ring.
• They are derived from and named after phenol, the simplest member of the class.
• Phenols are known for their antioxidant, antiseptic, and preservative properties and are widely used in various industries, such as pharmaceuticals, plastics, resins, and cosmetics.

Types and Classification of Phenols

• • Phenols can be classified into different categories based on the number of hydroxyl groups, the type of aromatic ring, and the presence of other substituents.
  • The common types and classifications include:

    1. Monophenols

       • These phenols have one hydroxyl group attached to the aromatic ring.
       • Examples include phenol itself and cresols.

    2. Polyphenols

       • These phenols have two or more hydroxyl groups attached to the aromatic ring.
       • They can be further divided into different categories:
         1. Diphenols: They have two hydroxyl groups. Examples include catechol, resorcinol, and hydroquinone.
         2. Triphenols: They have three hydroxyl groups. An example is phloroglucinol.
         3. Tetraphenols: They have four hydroxyl groups. An example is pyrogallol.
         4. Pentaphenols: They have five hydroxyl groups. An example is gallic acid.

    3. Substituted Phenols

       • Phenols can also be classified based on the presence of other substituents on the aromatic ring.
       • Examples include:
         1. Alkylphenols: Phenols with alkyl groups.
         2. Halophenols: Phenols with halogen atoms.
         3. Nitrophenols: Phenols with nitro groups.
         4. Aminophenols: Phenols with amino groups.

Applications of Phenols:

• • Phenols are versatile compounds with wide-ranging applications across various industries:
    1. Pharmaceuticals: Phenols serve as precursors in drug synthesis, including analgesics like aspirin, antipyretics, and antiseptics such as hexachlorophene.
    2. Plastics and Resins: Phenol-formaldehyde resins are key components in adhesives and plastic production, notably in Bakelite.
    3. Cosmetics: Phenolic compounds are used as preservatives and antioxidants in cosmetics and personal care products.
    4. Food Industry: Phenolic antioxidants like butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are used in food packaging and processing to prevent spoilage and extend shelf life.
    5. Chemical Industry: Phenols are intermediates in the synthesis of various chemicals, including dyes, detergents, and herbicides.

Examples of Phenols:

1. 1. Phenol: The simplest phenol, used as a disinfectant and in the production of phenolic resins.
   2. Cresols: Monophenols with a methyl group, used in disinfectants and solvents.
   3. Catechol: A diphenol used as a precursor in pharmaceuticals and dyes.
   4. Resorcinol: A diphenol used in resins, adhesives, and pharmaceuticals.
   5. Hydroquinone: A diphenol used as a reducing agent, photographic developer, and chemical precursor.
   6. Gallic Acid: A pentaphenol with antioxidant properties.

General Methods of Preparation of Phenol

1. 1. From Cumene (Isopropylbenzene) – Cumene Hydroperoxide Process

      • Cumene is oxidized to cumene hydroperoxide, which is cleaved using an acid catalyst (e.g., sulfuric acid) to yield phenol and acetone.
      • Reaction:
        • C6H5CH(CH3)2 + O2 → C6H5CH(OOH)CH3 → C6H5OH + (CH3)2CO

   2. From Chlorobenzene – Dow Process

      • Chlorobenzene reacts with aqueous sodium hydroxide at high temperature and pressure to form sodium phenoxide, which is acidified to yield phenol.
      • Reaction:
        • C6H5Cl + NaOH → C6H5ONa + NaCl → C6H5OH + H2O + NaCl

   3. From Aniline – Raschig Process

      • Aniline is diazotized with nitrous acid to form diazonium salt, which is hydrolyzed to produce phenol and nitrogen gas.
      • Reaction:
        • C6H5NH2 + HNO2 → C6H5N2+Cl- → C6H5OH + N2

   4. From Benzene Sulfonic Acid

      • Benzene is sulfonated using concentrated sulfuric acid to form benzene sulfonic acid, which is treated with hot aqueous sodium hydroxide to form sodium phenoxide, followed by acidification to yield phenol.
      • Reaction:
        • C6H6 + H2SO4 → C6H5SO3H → C6H5ONa + H2O + Na2SO4 → C6H5OH + H2O + Na2SO4

Important Reactions of Phenol

1. Electrophilic Aromatic Substitution

• • Phenol undergoes electrophilic aromatic substitution reactions more readily than benzene due to the electron-donating nature of the hydroxyl group, which activates the aromatic ring toward electrophiles.

    1. Nitration:

       • Phenol reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid to form ortho- and para-nitrophenols.
       • Reaction:
         • C6H5OH + HNO3 → C6H4(OH)(NO2) + H2O

    2. Halogenation:

       • Phenol reacts with halogens in the presence of a suitable catalyst to form ortho- and para-halophenols.
       • Reaction:
         • C6H5OH + X2 → C6H4(OH)(X) + HX (X=Cl, Br, or I)

    3. Friedel-Crafts Alkylation and Acylation:

       • Phenol undergoes Friedel-Crafts reactions with alkyl halides and acyl halides in the presence of a Lewis acid catalyst (e.g., AlCl3).
       • Reaction:
         • C6H5OH + RCl → C6H4(OH)(R) + HCl(Alkylation

2. Reactions of the Hydroxyl Group

• • Phenol’s hydroxyl group is reactive and can undergo several important chemical transformations.

    1. Esterification:

       • Phenol reacts with carboxylic acids or their derivatives (e.g., acid chlorides) to form esters, usually in the presence of a catalyst such as concentrated sulfuric acid or pyridine.
       • Reaction: C6H5OH + RCOOH → C6H5OCOR + H2O

    2. Ether Formation:

       • Phenol reacts with alkyl halides in the presence of a base, such as potassium hydroxide, to form phenyl ethers.
       • Reaction: C6H5OH + R-X → C6H5OR + HX

3. Oxidation Reactions

• • Phenol can be oxidized to various products depending on the conditions and oxidizing agents used.

    1. Oxidation to Quinones:

       • Phenol is oxidized to form quinones, such as benzoquinone, in the presence of oxidizing agents like chromic acid or potassium dichromate.
       • Reaction: C6H5OH + [O] → C6H4O2

    2. Oxidative Cleavage to Dicarboxylic Acids:

       • Phenol undergoes oxidative cleavage to form dicarboxylic acids, such as muconic acid, in the presence of strong oxidizing agents like potassium permanganate.
       • Reaction: C6H5OH + 2[O] → HOOC-C6H2−COOH

4. Reduction Reactions

• • Phenol can be reduced to simpler aromatic compounds.

    1. Reduction to Benzene:

       • Phenol is reduced to benzene by treating it with zinc dust at high temperatures.
       • Reaction: C6H5OH + Zn → C6H6 + ZnO

Thank you for reading from Firsthope's notes, don't forget to check YouTube videos!