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.