Ch 9 covers hydrocarbons — alkanes (saturated), alkenes and alkynes (unsaturated), and aromatic hydrocarbons — their preparation, physical properties, chemical reactions, and mechanisms.
CₙH₂ₙ₊₂, sp³, tetrahedral. Preparation: Wurtz reaction (2RX + 2Na → R-R), decarboxylation (RCOONa + NaOH → RH). Reactions: halogenation (free radical substitution: initiation → propagation → termination), combustion (CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O), pyrolysis (cracking at high T).
Alkenes: CₙH₂ₙ, sp², planar. Addition reactions: HX (Markovnikov's rule — H to C with more H), H₂O (acid-catalysed), H₂ (catalytic hydrogenation). Anti-Markovnikov: HBr with peroxide. Alkynes: CₙH₂ₙ₋₂, sp, linear. Terminal alkynes are weakly acidic (react with Na, ammoniacal AgNO₃). Addition of 2 mol HX follows Markovnikov.
Benzene: C₆H₆, planar, delocalised π-electrons (4n+2 rule, n=1). Electrophilic aromatic substitution (EAS): halogenation (Cl₂/FeCl₃), nitration (HNO₃/H₂SO₄), sulphonation, Friedel-Crafts alkylation (RCl/AlCl₃) and acylation (RCOCl/AlCl₃). Directing effects: −OH, −NH₂ are o,p-directors (activating); −NO₂, −COOH are m-directors (deactivating).
Download: https://ncert.nic.in/textbook/pdf/kech202.pdf | Part II: https://ncert.nic.in/textbook/pdf/kech2ps.zip
Benzene has a stable aromatic ring with delocalised π-electrons (resonance energy ~150 kJ/mol). Addition reactions would break this aromatic system and lose this stabilisation energy. Substitution reactions replace a hydrogen without disturbing the aromatic ring, preserving the extra stability. So benzene favours electrophilic substitution over addition.
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