Questions
4 questions per board paper
Difficulty
Medium
Importance
Core — never skip
Overview
Hydrocarbons form the fundamental skeleton of organic chemistry, serving as the basis for understanding functional group transformations. Mastering this topic is essential for scoring in CBSE Class 11/12 boards and competitive exams, as it forms the bedrock of reaction mechanisms and IUPAC nomenclature. Focus on the reactivity patterns of saturated versus unsaturated systems and electrophilic substitution in aromatics.
Alkanes: Saturated Hydrocarbons
Alkanes are characterized by C-C and C-H sigma bonds, making them relatively unreactive under standard conditions. Their reactivity is primarily governed by free-radical substitution mechanisms.
- General formula: CnH2n+2
- Wurtz reaction for higher alkanes
- Halogenation via free-radical substitution
- Kolbe's electrolytic method
- Conformational analysis of ethane (staggered vs. eclipsed)
Alkenes and Alkynes
These unsaturated hydrocarbons contain pi-bonds, which serve as sites for electrophilic addition reactions. Understanding the regioselectivity of these additions is a high-yield area for exam questions.
- General formula Alkenes: CnH2n
- General formula Alkynes: CnH2n-2
- Markovnikov's rule for electrophilic addition
- Anti-Markovnikov's rule (Peroxide effect)
- Ozonolysis for cleavage of double bonds
- Hydroboration-oxidation sequence
Aromatic Hydrocarbons
Aromatic systems follow Hückel's rule (4n+2 pi electrons) and are characterized by extra stability due to resonance. Electrophilic aromatic substitution is the defining reaction type for these molecules.
- Hückel's Rule: (4n+2) pi-electrons
- Nitration using nitrating mixture (conc. HNO3 + H2SO4)
- Friedel-Crafts alkylation and acylation
- Directing effects of substituents (o/p-directing vs meta-directing)
- Resonance energy importance
Formula Sheet
CnH2n+2 (Alkanes)
CnH2n (Alkenes)
CnH2n-2 (Alkynes)
(4n+2) pi-electron rule
Exam Tip
Always identify the stability of the intermediate carbocation or free radical first, as this dictates the major product in 90% of hydrocarbon reaction mechanisms.
Common Mistakes
- Ignoring the stability of carbocation intermediates during electrophilic addition reactions.
- Misapplying Markovnikov’s rule by forgetting that it only applies to unsymmetrical alkenes.
- Confusing the directing effects of functional groups on the benzene ring during substitution.
More Revision Notes
Ready to test yourself?
Play topic-wise Hydrocarbons questions in Aspirant Arcade — gamified MCQ practice.
Download Free