Aromatic Compounds, Phenols, and Cycloalkanes: Properties and Uses

Posted on Dec 15, 2024 in Chemistry

Hückel’s Rule

• Definition: Hückel’s Rule determines whether a cyclic compound is aromatic. A compound is aromatic if:

  1. It is cyclic and planar.
  2. It is fully conjugated (alternating single and double bonds).
  3. It contains (4n+2)π electrons, where n is a non-negative integer (0, 1, 2…).

• Example:

  • Benzene (n = 1, 6π electrons): Aromatic.
  • Cyclobutadiene (n = 1, 4π electrons): Anti-aromatic (unstable).

• Importance: Aromatic compounds are more stable due to delocalized π electrons.

Structure and Uses of DDT, Saccharin, BHC, and Chloramine

1. DDT (Dichlorodiphenyltrichloroethane):

   • Structure: C₁₄H₉Cl₅.
   • Uses:
     • Agricultural insecticide.
     • Controls diseases like malaria and typhus.
   • Concerns: Persistent organic pollutant; banned in many countries.

2. Saccharin:

   • Structure: C₇H₅NO₃S.
   • Uses:
     • Non-caloric artificial sweetener (300–400 times sweeter than sugar).
     • Used in diabetic diets.

3. BHC (Benzene Hexachloride):

   • Structure: C₆H₆Cl₆.
   • Uses:
     • Agricultural pesticide (also called Lindane).
     • Treats scabies and lice.

4. Chloramine:

   • Structure: NH₂Cl.
   • Uses:
     • Water disinfectant.
     • Used in medical antiseptics.

UNIT II: Phenols

Definitions and Structures of Phenol, Cresol, Resorcinol, and Naphthol

1. Phenol:

   • Structure: C₆H₅OH.
   • Definition: An aromatic compound with a hydroxyl group (-OH) attached to a benzene ring.
   • Uses: Antiseptic, precursor to polymers like Bakelite.

2. Cresol:

   • Structure: CH₃-substituted phenol.
   • Types: Ortho-, meta-, and para-cresol.
   • Uses: Disinfectants, preservatives.

3. Resorcinol:

   • Structure: Benzene with two -OH groups at 1,3-positions.
   • Uses: Sunscreen, dyes, and medicines.

4. Naphthol:

   • Structure: Naphthalene with an -OH group.
   • Types:
     • 1-Naphthol (α-position).
     • 2-Naphthol (β-position).
   • Uses: Dyes, antiseptics, and pharmaceuticals.

Reactions of Aromatic Acids and Aromatic Amines

1. Aromatic Acids:

   • Example: Benzoic Acid (C₆H₅COOH).
   • Reactions: Esterification, decarboxylation, and reduction.
   • Uses: Preservative, in medicines.

2. Aromatic Amines:

   • Example: Aniline (C₆H₅NH₂).
   • Reactions:
     • Diazotization (C₆H₅NH₂ → C₆H₅N₂⁺Cl^–): Produces diazonium salts.
     • Coupling reactions (e.g., synthesis of azo dyes).
   • Uses: Azo dyes, rubber chemicals, and drugs.

Nitration, Sulphonation, Friedel-Crafts Acylation, and Alkylation

1. Nitration:

   • Definition: Introduction of a nitro group (-NO₂) into benzene.
   • Reaction: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (Catalyst: Conc. H₂SO₄).
   • Uses: Nitrobenzene is used in explosives, dyes, and aniline synthesis.

2. Sulphonation:

   • Definition: Introduction of a sulfonic acid group (-SO₃H) into benzene.
   • Reaction: C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O.
   • Uses: Sulfonic acids are used in detergents and dyes.

3. Friedel-Crafts Acylation:

   • Definition: Substitution of a hydrogen atom in benzene with an acyl group (-COR).
   • Reaction: C₆H₆ + RCOCl → C₆H₅COR + HCl (Catalyst: AlCl₃).
   • Uses: Aromatic ketones like acetophenone are used in perfumes.

4. Friedel-Crafts Alkylation:

   • Definition: Substitution of a hydrogen atom in benzene with an alkyl group (-R).
   • Reaction: C₆H₆ + RCl → C₆H₅R + HCl (Catalyst: AlCl₃).
   • Uses: Used to prepare alkylbenzenes like toluene (used in solvents).

Properties of Fatty Acids

1. Hydrolysis: Breakdown of triglycerides into glycerol and free fatty acids.
2. Hydrogenation: Converts unsaturated oils into saturated fats (e.g., margarine production).
3. Acid Value: Measures the free fatty acids in fats/oils.
4. Iodine Value: Measures the degree of unsaturation in fats/oils.
5. Saponification Value: Determines the amount of alkali required to saponify fats.

Rancidity of Oils

• Definition: Spoilage of oils/fats due to oxidation or hydrolysis.
• Types:
  1. Oxidative Rancidity: Formation of aldehydes and ketones.
  2. Hydrolytic Rancidity: Release of free fatty acids.
• Prevention:
  • Use antioxidants (e.g., vitamin E).
  • Store in cool, dark places.

Preparation of Naphthalene

• Naphthalene: A polycyclic aromatic hydrocarbon with the formula C₁₀H₈.

• Structure: Two fused benzene rings.

• Methods of Preparation:

  1. From Coal Tar:

     • Naphthalene is extracted from the middle oil fraction of coal tar.
     • It is purified by crystallization and sublimation.

  2. From Phthalic Anhydride:

     • Phthalic anhydride is reduced using zinc or aluminum powder.

• Uses:

  • Manufacturing mothballs.
  • Synthesis of dyes, plastics, and insecticides.

Preparation of Anthracene

• Anthracene: A polycyclic aromatic hydrocarbon with the formula C₁₄H₁₀.

• Structure: Three fused benzene rings in a linear arrangement.

• Methods of Preparation:

  1. From Coal Tar:

     • Anthracene is obtained from the green oil fraction of coal tar.
     • It is purified by recrystallization from solvents like ethanol.

  2. Cyclization of Benzyl Compounds:

     • Benzyl derivatives can undergo cyclization to form anthracene derivatives.

• Uses:

  • Production of anthraquinone (used in dyes).
  • UV detectors and organic semiconductors.

Preparation of Phenanthrene

• Phenanthrene: A polycyclic aromatic hydrocarbon with the formula C₁₄H₁₀.

• Structure: Three fused benzene rings in an angular arrangement.

• Methods of Preparation:

  1. From Coal Tar:

     • Phenanthrene is isolated from the higher boiling fractions of coal tar.

  2. Synthesis:

     • Phenanthrene can be synthesized by cyclization of benzene derivatives or through Diels-Alder reactions.

• Uses:

  • Synthesis of drugs and dyes.
  • Used in organic light-emitting diodes (OLEDs).

Preparation of Diphenylmethane

• Diphenylmethane: A hydrocarbon with the formula C₁₃H₁₂.

• Structure: Two benzene rings connected by a single methylene (CH₂) group.

• Methods of Preparation:

  1. Friedel-Crafts Alkylation:

     • Benzyl chloride reacts with benzene in the presence of AlCl₃ as a catalyst: C₆H₆ + C₆H₅CH₂Cl → (C₆H₅)₂CH₂ + HCl

  2. Reduction of Benzophenone:

     • Benzophenone is reduced using zinc and hydrochloric acid.

• Uses:

  • Intermediate in the manufacture of perfumes and dyes.
  • Used in pharmaceuticals.

UNIT V: Cycloalkanes

Cycloalkanes

• Definition: Cycloalkanes are saturated hydrocarbons that contain one or more carbon atoms arranged in a ring structure.

• General Formula: C_nH_2n.

• Examples:

  • Cyclopropane (C₃H₆).
  • Cyclobutane (C₄H₈).
  • Cyclohexane (C₆H₁₂).

Methods of Preparation of Cycloalkanes

1. From Dihalides (Wurtz Reaction):

   • When a dihalogen derivative of an alkane reacts with sodium in dry ether, cycloalkanes are formed.
   • Example: Br-(CH₂)₃-Br + 2Na → (CH₂)₃ + 2NaBr. This produces cyclopropane.

2. By Reduction of Aromatic Compounds:

   • Aromatic compounds like benzene can be reduced to cyclohexane in the presence of hydrogen and a nickel catalyst: C₆H₆ + 3H₂ → C₆H₁₂

3. By Heating Calcium or Barium Salts of Dicarboxylic Acids:

   • Calcium adipate, when heated, forms cyclohexane: CaOOC-(CH₂)₄-COOCa → C₆H₁₂ + CaCO₃

4. Diels-Alder Reaction:

   • Cycloalkanes are synthesized by the cyclization of dienes with alkenes.
   • Example: CH₂=CH-CH=CH₂ + CH₂=CH₂ → C₆H₁₀

Properties of Cycloalkanes

1. Physical Properties:

   • Non-polar, insoluble in water.
   • Boiling points increase with ring size.

2. Chemical Properties:

   • Combustion: Burn to produce CO₂ and H₂O.
   • Substitution: React with halogens under UV light to form halocycloalkanes.

Applications of Cycloalkanes

1. Industrial Applications:

   • Cyclohexane is used in the production of nylon (precursor to adipic acid and caprolactam).

2. Pharmaceuticals:

   • Cyclopropane is used as an anesthetic.

3. Fuel Industry:

   • Cycloalkanes are found in crude oil and contribute to the production of fuels like gasoline.