Class 11 Chemistry Chapter 9 Hydrocarbons – Revision Notes

Hydrocarbons are classified here by saturation, from alkanes (C_nH_2n+2) through alkenes and alkynes to the ring structure of arenes, each with its own characteristic reactions. Alkenes and alkynes favor electrophilic addition following Markovnikov’s rule, while arenes undergo electrophilic substitution directed by activating or deactivating substituents.

Last Updated: September 23, 2026

Classification

• Alkanes C_nH_2n+2 (saturated); Alkenes C_nH_2n; Alkynes C_nH_2n−2; Arenes (benzene ring).

Key Reactions

• Alkanes: substitution (free radical), Wurtz reaction.
• Alkenes/Alkynes: electrophilic addition; Markovnikov’s rule.
• Arenes: electrophilic substitution (nitration, halogenation, sulphonation, Friedel-Crafts).

Directive Effects

• Ortho/para-directing: -OH, -NH₂, -CH₃ (activating).
• Meta-directing: -NO₂, -COOH (deactivating).

One-Line Summary

Hydrocarbons range from saturated alkanes to unsaturated alkenes/alkynes and aromatic arenes, each with characteristic reaction patterns (substitution, addition, or aromatic substitution) governed by their bonding and structure.

Frequently Asked Questions

What is the key structural difference between alkanes, alkenes, and alkynes?
Alkanes contain only single bonds between carbon atoms, alkenes contain at least one carbon-carbon double bond, and alkynes contain at least one carbon-carbon triple bond, which directly affects their reactivity.

Why are alkenes and alkynes generally more reactive than alkanes?
The double and triple bonds in alkenes and alkynes contain pi electrons that are more loosely held and exposed than the electrons in single sigma bonds, making them more susceptible to addition reactions with various reagents.

Written by Satish

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