Hydrocarbons — Class 11 Chemistry

Hydrocarbons, compounds of carbon and hydrogen only, are the principal sources of energy and the feedstock for polymers, dyes and drugs. They are classified as saturated alkanes (general formula , with single bonds), unsaturated alkenes () and alkynes (, with multiple bonds), and aromatic hydrocarbons. Alkanes are prepared by hydrogenation of unsaturated hydrocarbons, reduction of alkyl halides, the Wurtz reaction, decarboxylation and Kolbe electrolysis; they are fairly unreactive but undergo free-radical halogenation, combustion, controlled oxidation, isomerisation and pyrolysis, and their staggered conformation is more stable than the eclipsed because of lower torsional strain. Alkenes contain a sigma plus pi double bond, show chain, position and geometrical (cis-trans) isomerism, and undergo electrophilic addition of hydrogen, halogens, hydrogen halides (Markovnikov rule, with the anti-Markovnikov peroxide effect for HBr), water and sulphuric acid, as well as oxidation, ozonolysis and polymerisation. Alkynes have a triple bond between sp-hybridised carbons, terminal alkynes showing acidic hydrogens. Aromatic chemistry centres on benzene, whose unusual stability is explained by resonance delocalisation of its six pi electrons and the Hückel rule; benzene undergoes electrophilic substitution — nitration, halogenation, sulphonation and Friedel-Crafts reactions — with substituents directing further substitution to ortho/para or meta positions. Polynuclear aromatic hydrocarbons are noted as carcinogenic and toxic.