Hydrocarbons notes

Hydrocarbons are organic compounds made only of carbon and hydrogen. NCERT classifies them mainly as aliphatic hydrocarbons, which include alkanes, alkenes and alkynes, and aromatic hydrocarbons, mainly benzene and its derivatives. Alkanes are saturated and generally undergo substitution, alkenes and alkynes are unsaturated and mainly undergo addition, while benzene is resonance-stabilized and prefers electrophilic substitution. This chapter connects structure, hybridisation, nomenclature, preparation, physical properties, reactions, mechanisms, industrial uses and environmental effects. For NEET, the most important ideas are homologous series, IUPAC naming, Markovnikov rule, peroxide effect, acidity of alkynes, benzene resonance, electrophilic substitution, directive influence, combustion and pollution.

Alkanes are saturated hydrocarbons containing only carbon-carbon and carbon-hydrogen single sigma bonds. Their general formula is CnH2n+2 for open-chain members, and every carbon is sp3 hybridised with tetrahedral geometry. They form a homologous series in which successive members differ by -CH2-. Alkanes are relatively unreactive because their sigma bonds are strong and nearly non-polar, so they are also called paraffins. NCERT expects students to know classification, IUPAC nomenclature, preparation by hydrogenation, reduction, Wurtz reaction, decarboxylation and Kolbe electrolysis, physical properties, conformations of ethane, and reactions such as halogenation, combustion and pyrolysis. NEET often asks stability of conformers, selectivity in halogenation and product prediction.

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. For open-chain monoalkenes, the formula is CnH2n. Each double-bonded carbon is sp2 hybridised, trigonal planar and has one sigma and one pi bond. The pi bond is electron-rich and weaker than a sigma bond, making alkenes more reactive than alkanes. NCERT focuses on IUPAC naming, geometrical isomerism, preparation by dehydration of alcohols and dehydrohalogenation of alkyl halides, physical properties, electrophilic addition, Markovnikov's rule, peroxide effect, oxidation and polymerisation. For NEET, the key is to predict major products using carbocation stability, understand anti-Markovnikov addition only for HBr with peroxide, and remember oxidation products.

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. Open-chain monoalkynes have the formula CnH2n−2. Each triply bonded carbon is sp hybridised and linear with a bond angle of 180°. The triple bond has one sigma and two pi bonds, so alkynes undergo addition reactions like alkenes, but often in two steps. A unique NCERT feature is the acidic nature of terminal alkynes because sp carbon has 50% s-character and holds electrons more tightly, stabilising the acetylide ion. Alkynes are prepared by double dehydrohalogenation of dihalides and from calcium carbide. NEET commonly tests acidity order, metal acetylide formation, addition reactions, oxidation and conversion of ethyne to benzene.

Aromatic hydrocarbons contain benzene-like cyclic, planar and conjugated π systems with special stability. Benzene, C6H6, has six sp2 carbons in a hexagonal ring; all C-C bonds are equal because π electrons are delocalised. It obeys Huckel's 4n+2 rule with six π electrons, making it aromatic. Because addition reactions would destroy aromaticity, benzene mainly undergoes electrophilic substitution reactions such as nitration, halogenation, sulphonation, Friedel-Crafts alkylation and Friedel-Crafts acylation. The mechanism involves generation of electrophile, attack on benzene to form sigma complex, and loss of proton to restore aromaticity. NEET also tests directive influence: activating groups usually direct ortho/para, while deactivating groups usually direct meta, with halogens as exceptions.

Hydrocarbons are not only reaction types in organic chemistry but also major fuels and industrial raw materials. Their important reactions include combustion, substitution, addition, oxidation, cracking, reforming, polymerisation and electrophilic aromatic substitution. Complete combustion produces carbon dioxide, water and heat, making hydrocarbons useful fuels; incomplete combustion produces carbon monoxide and soot, which are harmful. Petroleum and natural gas are natural mixtures of hydrocarbons separated and processed for petrol, diesel, kerosene, LPG, CNG and petrochemicals. Alkenes are key monomers for plastics, alkynes are useful in welding and synthesis, and aromatic hydrocarbons are used in dyes, drugs and polymers. NEET also asks environmental effects: greenhouse gases, photochemical smog, carcinogenic polynuclear aromatic hydrocarbons and toxicity.