CBSE Class 10 Chemistry Carbon and its Compounds Notes Set 04

Organic Chemistry

Organic chemistry is the chemistry of carbon compounds. Wohler synthesised urea from ammonium cyanate in 1828, disproving the idea that organic compounds need a vital force. Carbon is tetravalent, shows catenation (long chains and rings) and forms single, double and triple bonds, which is why millions of carbon compounds exist.

Homologous Series and Isomerism

A homologous series is a family of compounds with the same functional group and a general formula, whose members differ by CH₂: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂. Isomers have the same molecular formula but different structures, for example n-butane and isobutane (C₄H₁₀).

Methane (Alkanes)

Marsh gas, tetrahedral, saturated. Made by heating sodium acetate with soda lime: CH₃COONa + NaOH → CH₄ + Na₂CO₃. Alkanes burn in air (2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O) and undergo substitution with chlorine: CH₄ + Cl₂ → CH₃Cl + HCl. They are unreactive to acids and alkaline KMnO₄.

Ethene (Alkenes)

Unsaturated with a C=C bond. Made by dehydration of ethanol with conc. H₂SO₄ at 170°C. They undergo addition (with H₂, Br₂, HCl), decolourise bromine water and alkaline KMnO₄ (a test for unsaturation), and polymerise to polythene.

Ethyne (Alkynes)

Contains a C≡C bond. Made from calcium carbide: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂. Gives a red precipitate with ammoniacal cuprous chloride and a white one with ammoniacal silver nitrate (acidic hydrogen). With dilute H₂SO₄ and HgSO₄ it forms acetaldehyde, and when passed through a red-hot tube it polymerises to benzene (3C₂H₂ → C₆H₆).

Benzene

C₆H₆ is a ring of six carbon atoms with alternating double bonds (a delocalised hybrid). Although unsaturated, it mainly undergoes substitution: nitration gives nitrobenzene and chlorination with iron as catalyst gives chlorobenzene. In sunlight chlorine adds to give benzene hexachloride (gammexane), an insecticide.