Board-style MCQs and past-paper reactions in the BIEK / Sindh Board pattern. Tap an option to check yourself instantly. Solved questions are at the bottom.
Multiple-choice questions
Solved reactions & self-assessment (past papers)
Write the mechanism of chlorination of methane
Initiation: Cl₂ →(hν) 2Cl•
Propagation: Cl• + CH₄ → HCl + •CH₃ ; •CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: Cl• + Cl• → Cl₂ ; •CH₃ + Cl• → CH₃Cl ; •CH₃ + •CH₃ → C₂H₆
It is a free-radical substitution chain reaction.
State Markovnikov's rule and apply it to propene + HBr
Rule: the H of HX adds to the doubly-bonded carbon that already has more hydrogens.
Propene CH₃–CH=CH₂ + HBr: H → terminal CH₂, Br → middle C (via the more stable 2° carbocation) →
CH₃CHBrCH₃ (2-bromopropane).
Why does benzene undergo substitution rather than addition?
Benzene has six delocalised π electrons forming a stable aromatic ring (resonance / delocalisation energy ≈ 150 kJ/mol). Addition would break this delocalisation and destroy the stability, so benzene prefers
electrophilic substitution, which keeps the aromatic ring intact.