Hydrocarbons are compounds of carbon and hydrogen only and are the main sources of energy (LPG, CNG, petrol, diesel). They are classified by the type of carbon-carbon bonds into saturated (alkanes, cycloalkanes), unsaturated (alkenes, alkynes) and aromatic hydrocarbons.
Alkanes (CnH2n+2) are saturated open-chain hydrocarbons with only single bonds. They show chain isomerism (C4H10 has 2 isomers, C5H12 has 3) and conformational isomerism due to free rotation about C-C sigma bonds. Carbons are classified primary, secondary, tertiary or quaternary by the number of attached carbons. Preparation: hydrogenation of alkenes/alkynes (Pt/Pd/Ni); reduction of alkyl halides (Zn + dil HCl); Wurtz reaction (alkyl halide + Na in dry ether, even-carbon alkanes); decarboxylation of sodium carboxylate with soda lime (one carbon less); Kolbe electrolysis. Reactions: free radical substitution (halogenation, reactivity F2 > Cl2 > Br2 > I2), combustion, controlled oxidation, isomerisation, aromatization and pyrolysis. Conformations of ethane: staggered (least torsional strain, most stable) and eclipsed (maximum strain); energy difference about 12.5 kJ/mol, shown by sawhorse and Newman projections.
Alkenes (CnH2n) contain a C=C double bond (one sigma + one pi). They show structural and geometrical (cis-trans) isomerism due to restricted rotation about the double bond; cis is more polar than trans. Preparation: partial reduction of alkynes (Lindlar gives cis, Na/liq NH3 gives trans); dehydrohalogenation of alkyl halides (alcoholic KOH, beta-elimination); dehalogenation of vicinal dihalides (Zn); acidic dehydration of alcohols. Reactions: electrophilic addition of H2, halogens, hydrogen halides and water. Markovnikov's rule: the negative part adds to the carbon with fewer hydrogens (via the more stable carbocation). Peroxide / Kharasch effect: HBr (only) adds anti-Markovnikov by a free radical mechanism. Oxidation with Baeyer's reagent (cold dilute KMnO4) gives vicinal glycols (test for unsaturation); ozonolysis (O3 then Zn/H2O) cleaves the double bond to carbonyls, locating its position; polymerisation gives polythene, polypropene etc.
Alkynes (CnH2n-2) contain a C-C triple bond (one sigma + two pi); ethyne is linear (sp hybridised). Terminal alkynes are acidic because the C-H is on an sp carbon (50 percent s-character), giving metal acetylides with Na or NaNH2. Preparation: calcium carbide + water; dehydrohalogenation of vicinal dihalides. They give electrophilic addition (two molecules of H2, X2, HX following Markovnikov to gem-dihalides), water addition (HgSO4/H2SO4 to carbonyls), and polymerisation (linear to polyacetylene; cyclic, three molecules to benzene at 873 K on red-hot iron).
Aromatic hydrocarbons (arenes) contain a benzene ring (or other highly unsaturated ring). Benzene (C6H6) is a planar, sp2-hybridised resonance hybrid with all six C-C bonds equal (139 pm) and six delocalised pi electrons, giving unusual stability. Aromaticity (Huckel rule) requires planarity, complete pi delocalisation and (4n+2) pi electrons. Arenes characteristically undergo electrophilic substitution: nitration (NO2+ electrophile), halogenation (Lewis acid like FeCl3/AlCl3), sulphonation (oleum), Friedel-Crafts alkylation and acylation. The SE mechanism has three steps: generation of electrophile, formation of the arenium ion (sigma complex), and loss of a proton to restore aromaticity. Directive influence: -OH, -NH2, -CH3 and halogens are ortho/para directing (-OH, -NH2, alkyl are activating; halogens are deactivating); -NO2, -COOH, -CN, -SO3H are meta directing and deactivating. Benzene and polynuclear hydrocarbons with more than two fused rings are toxic and carcinogenic, formed by incomplete combustion of tobacco, coal and petroleum.