State Markovnikov's rule.

In addition of HX to an unsymmetrical alkene, the H atom adds to the carbon that already has more hydrogen atoms, and X adds to the carbon with fewer H atoms.

Why does benzene prefer substitution over addition?

Benzene has aromatic stability due to delocalized π electrons. Addition would destroy the stable aromatic ring, so electrophilic substitution is preferred as it maintains aromaticity.

What is Lindlar's catalyst used for?

Lindlar's catalyst (Pd on CaCO₃ with lead acetate) catalyses partial hydrogenation of alkynes to give cis-alkenes (not full reduction to alkanes).

Alkanes, Alkenes and Alkynes

Hydrocarbons • Class 11 Chemistry • NCERT • CBSE

Alkanes (CₙH₂ₙ₊₂): saturated, undergo substitution. Alkenes (CₙH₂ₙ): one double bond, undergo addition. Alkynes (CₙH₂ₙ₋₂): one triple bond, undergo addition. Markovnikov's rule governs HX addition to alkenes.

Key Formulas

Alkanes: CₙH₂ₙ₊₂
Alkenes: CₙH₂ₙ
Alkynes: CₙH₂ₙ₋₂
Markovnikov's Rule: H adds to C with more H
Combustion: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O

Frequently Asked Questions

State Markovnikov's rule.: In addition of HX to an unsymmetrical alkene, the H atom adds to the carbon that already has more hydrogen atoms, and X adds to the carbon with fewer H atoms.
Why does benzene prefer substitution over addition?: Benzene has aromatic stability due to delocalized π electrons. Addition would destroy the stable aromatic ring, so electrophilic substitution is preferred as it maintains aromaticity.
What is Lindlar's catalyst used for?: Lindlar's catalyst (Pd on CaCO₃ with lead acetate) catalyses partial hydrogenation of alkynes to give cis-alkenes (not full reduction to alkanes).

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