Organic Chemistry: Carbon Compounds, Reactions & Mechanisms

Organic chemistry is the branch of chemistry dealing with carbon-containing compounds. With over 10 million known organic compounds, it underpins pharmaceutical development, materials science, food science, petroleum refining, and the study of life itself. The central theme is carbon's extraordinary ability to form four covalent bonds and create an enormous diversity of stable structures.

Why Carbon is Unique

  • Carbon has four outer electrons and forms exactly four covalent bonds.
  • Catenation: carbon bonds extensively with other carbon atoms, forming chains, branched chains, rings, and aromatic systems.
  • Carbon forms single bonds (alkanes), double bonds (alkenes, C=C), and triple bonds (alkynes, C≡C).

Homologous Series & Functional Groups

  • Alkanes (CₙH₂ₙ₊₂): saturated hydrocarbons. Undergo combustion and halogen substitution.
  • Alkenes (CₙH₂ₙ): contain C=C double bond. Decolorise bromine water — the test for unsaturation.
  • Alcohols (−OH), carboxylic acids (−COOH), esters (−COO−), and amines (−NH₂) are key functional groups.

Alkanes: Properties & Reactions

  • Increasing chain length → increasing boiling point (stronger London dispersion forces).
  • Complete combustion: CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O.
  • Halogen substitution requires UV light: CH₄ + Cl₂ → CH₃Cl + HCl (free radical mechanism).

Alkenes: Addition Reactions

  • Addition of H₂ (hydrogenation): C₂H₄ + H₂ → C₂H₆ (Ni catalyst). Used to harden vegetable oils.
  • Addition of Br₂: C₂H₄ + Br₂ → CH₂BrCH₂Br. Bromine water decolorises.
  • Hydration: C₂H₄ + H₂O → C₂H₅OH (H₃PO₄ catalyst, 300°C). Industrial ethanol production.
  • Polymerisation: n(CH₂=CH₂) → −[CH₂−CH₂]ₙ− (poly(ethene)).

Alcohols, Carboxylic Acids & Esters

  • Ethanol can be made by fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
  • Esterification: carboxylic acid + alcohol ⇌ ester + water (conc. H₂SO₄ catalyst).
  • Saponification (reverse): esters + NaOH → carboxylate salt + alcohol (soap-making).

Polymers: Addition & Condensation

  • Addition polymerisation: alkene monomers join without losing atoms (e.g. poly(ethene), poly(propene), PVC, PTFE).
  • Condensation polymerisation: monomers join with loss of H₂O or HCl at each bond.
  • Polyesters (PET): diol + dicarboxylic acid. Polyamides (nylons): diamine + dicarboxylic acid.

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