JEE Mains Organic Chemistry 2027 — Hydrocarbons: Complete Notes and MCQ Practice
Hydrocarbons is a foundational Organic Chemistry chapter for JEE Mains, forming the basis for all further organic chemistry reactions. Contributing 3-5 questions per year, this chapter covers alkanes, alkenes, alkynes, their preparation, properties and reactions. This guide provides complete notes and 10 practice MCQs for JEE Mains 2027.
Classification of Hydrocarbons
| Class | General Formula | Hybridisation | Example |
|---|---|---|---|
| Alkanes (saturated) | CnH2n+2 | sp3 | Methane CH4, Ethane C2H6 |
| Alkenes (one double bond) | CnH2n | sp2 | Ethene C2H4, Propene C3H6 |
| Alkynes (one triple bond) | CnH2n-2 | sp | Ethyne C2H2, Propyne C3H4 |
| Dienes | CnH2n-2 | sp2 + sp3 | 1,3-Butadiene |
| Benzene/Arenes | CnH2n-6 | sp2 | Benzene C6H6 |
Alkanes — Preparation and Reactions
Preparation: Kolbe electrolysis (RCOONa + electrolysis gives RR + CO2), Wurtz reaction (2RX + 2Na gives R-R + 2NaX), Corey-House synthesis, hydrogenation of alkenes. Reactions: Free radical halogenation (reactivity: F greater than Cl greater than Br greater than I; selectivity: tertiary greater than secondary greater than primary). Combustion: complete gives CO2 + H2O. Cracking and pyrolysis for industrial applications.
Alkenes — Preparation and Reactions
Preparation: Dehydration of alcohols (using H2SO4, follows Saytzeff rule), dehydrohalogenation of alkyl halides (using alc. KOH, Saytzeff rule), reduction of alkynes. Addition reactions follow Markovnikov rule: HX adds with H going to carbon with more H. Anti-Markovnikov (Peroxide effect/Kharash effect): HBr adds opposite to Markovnikov in presence of peroxides. Ozonolysis: cleaves double bond (ozone + Zn/H2O gives aldehydes/ketones).
Alkynes — Key Reactions
- Preparation: Dehydrohalogenation of vicinal/geminal dihalides using NaNH2
- Acidic character: terminal alkynes (R-C triple bond-CH) react with NaNH2, AgNO3, CuCl to form metal acetylides
- Addition of HX: follows Markovnikov rule twice (vinyl halide then geminal dihalide)
- Hydration (using HgSO4/H2SO4): ethyne gives acetaldehyde (Markovnikov); higher alkynes give ketones
- Lindlar catalyst: partial hydrogenation gives cis-alkene; Na/liquid NH3 gives trans-alkene
Arenes — Benzene and Aromatic Compounds
| Reaction | Reagent | Product | Type |
|---|---|---|---|
| Nitration | HNO3/H2SO4 | Nitrobenzene | EAS |
| Sulphonation | Fuming H2SO4 | Benzene sulphonic acid | EAS |
| Halogenation | X2/Lewis acid | Halobenzene | EAS |
| Friedel-Crafts Alkylation | RX/AlCl3 | Alkylbenzene | EAS |
| Friedel-Crafts Acylation | RCOCl/AlCl3 | Acylbenzene (ketone) | EAS |
Frequently Asked Questions — JEE Hydrocarbons 2027
What is Markovnikov rule?
Markovnikov rule: during addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon with more hydrogen atoms, and the halide adds to the carbon with fewer hydrogen atoms (more substituted carbon). The rule is explained by the stability of the more substituted carbocation intermediate.
What is the difference between Wurtz reaction and Corey-House synthesis?
Wurtz reaction: 2RX + 2Na gives R-R + 2NaX — joins two identical alkyl groups, good for symmetric alkanes. Corey-House synthesis: uses organocuprate (R2CuLi) with alkyl halide to form R-R’ — allows synthesis of unsymmetrical alkanes. Corey-House is more versatile for JEE problems.
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