lecture recording on 20 February 2025 at 22.06.12 PM

Organic Chemistry Overview

  • Definition: Organic chemistry is the branch of chemistry that studies the structure, properties, composition, reactions, and preparation of carbon-containing compounds.

  • Importance: It is significant in various fields such as medicine, biology, and environmental science due to the diversity of carbon compounds.

Benzene: A Fundamental Organic Compound

  • Structure:

    • Benzene (C6H6) is a cyclic hydrocarbon with a ring structure composed of six carbon atoms.

    • Each carbon atom is bonded to one hydrogen atom, resulting in the molecular formula C6H6.

  • Skeletal Formula:

    • The skeletal formula of benzene shows its carbon skeleton.

    • It is represented as a hexagon with alternating double bonds or simply as a hexagon with a circle inside, depicting resonance structures.

Bonding in Benzene

  • Resonance:

    • Benzene exhibits resonance, meaning the actual structure is an average of two or more valid Lewis structures.

    • The electrons involved in bonds are delocalized over the carbon atoms, causing equal bond lengths that are intermediate between single and double bonds.

  • Bond Length:

    • All C-C bond lengths in benzene are approximately 1.39 Å, which is shorter than a typical C-C single bond and longer than a C=C double bond.

Properties of Benzene

  • Aromaticity:

    • Benzene is classified as an aromatic compound due to its stability and unique bonding, fulfilling Huckel's rule (having 4n + 2 π electrons, where n is an integer).

    • This stability is often referred to as "aromatic stabilization," making benzene less reactive than typical alkenes.

  • Physical Properties:

    • State: Benzene is a colorless liquid at room temperature.

    • Smell: It has a sweet odor and is flammable.

    • Solubility: Although benzene is a non-polar molecule, it is slightly soluble in water but miscible with many organic solvents.

Chemical Reactions of Benzene

  • Electrophilic Aromatic Substitution (EAS):

    • Benzene primarily undergoes EAS reactions rather than addition reactions due to its aromatic nature.

    • Common EAS reactions include:

      • Nitration: Introduction of a nitro group (NO2) via reaction with nitric acid (HNO3) in the presence of sulfuric acid (H2SO4).

      • Sulfonation: Addition of a sulfonyl group (SO3H) using sulfur trioxide (SO3).

      • Halogenation: Substitution of hydrogen with halogens (Cl, Br) using a Lewis acid catalyst (e.g., FeBr3).

Industrial Significance of Benzene

  • Uses:

    • Benzene is a precursor for many chemicals, including:

      • Styrene: used to produce polystyrene plastics.

      • Phenol: used in the manufacture of products such as plastics and adhesives.

      • Cyclohexane: used in the production of nylon and other synthetic fibers.

  • Health Concerns:

    • Benzene is toxic and a known carcinogen, leading to regulations on its handling and use in industry.

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