Hydrocarbons: Alkanes, Alkenes, Alkynes and Aromatic Compounds

Understand preparation, addition and substitution mechanisms, and aromaticity across alkanes, alkenes, alkynes and benzene for CBSE Boards and NEET/JEE.

Hydrocarbons, compounds built from only carbon and hydrogen, form the foundation of Organic Chemistry in Class 11, classified as saturated (alkanes), unsaturated (alkenes and alkynes), and aromatic (benzene and its derivatives). Every later organic chapter, from haloalkanes to alcohols to carbonyl compounds, builds directly on the reaction patterns introduced here, making this one of the highest-leverage chapters for CBSE Boards, NEET and JEE.

Alkanes (general formula CnH2n+2) are prepared by the Wurtz reaction (2 R-X + 2Na in dry ether gives R-R) or by decarboxylation of sodium salts of carboxylic acids with soda lime, and undergo free-radical halogenation under UV light through initiation, propagation and termination steps. Alkenes (CnH2n) are prepared by dehydration of alcohols with concentrated H2SO4 or dehydrohalogenation of alkyl halides with alcoholic KOH (favouring the more substituted alkene per Saytzeff's rule), and undergo electrophilic addition governed by Markovnikov's rule, except HBr addition in the presence of peroxides, which follows anti-Markovnikov addition through the Kharasch peroxide effect.

Alkynes (CnH2n-2) are prepared industrially from calcium carbide and water to give acetylene, or by double dehydrohalogenation of vicinal dihalides using alcoholic KOH. Terminal alkynes show a distinctive acidic character because their sp-hybridised carbon holds bonding electrons closer to the nucleus, letting the terminal hydrogen be removed by strong bases or form precipitates with ammoniacal silver nitrate or cuprous chloride - a key test that distinguishes terminal from internal alkynes and from alkenes.

Benzene's exceptional stability comes from its cyclic, planar, fully conjugated ring of six delocalised pi electrons, satisfying Huckel's rule (4n+2 pi electrons). This delocalisation makes benzene strongly favour electrophilic substitution, such as nitration, sulphonation, halogenation, and Friedel-Crafts alkylation or acylation, all proceeding through a resonance-stabilised arenium ion intermediate, over addition reactions that would destroy its aromaticity. Substituents already on the ring direct further substitution: electron-donating groups like -OH, -NH2 and alkyl groups activate the ring toward ortho and para positions, while electron-withdrawing groups like -NO2, -COOH and -SO3H deactivate the ring and direct meta. Dr. Aarzoo Saini has students memorise this director-effect table as a fixed reference chart at We-Gyaan Classes Roorkee, since it appears in nearly every board and entrance paper.