Answer Key

1. (A) C_nH_2n
2. (B) eclipsed
3. (D) 2-chloropropane
4. (A) Baeyer's reagent
5. (B) 1
6. (C) –NO₂

7. On addition of HX to an unsymmetrical alkene, H adds to the carbon bearing more H atoms (more stable carbocation) and X to the more substituted carbon. Propene + HCl → 2-chloropropane.
8. Benzene's six delocalised π electrons give it a large resonance stabilisation; substitution preserves this aromatic sextet, whereas addition would destroy it, so substitution is strongly favoured.
9. A terminal alkyne gives a white precipitate (silver acetylide) with ammoniacal AgNO₃ because of its acidic ≡C-H, whereas an alkene gives no precipitate.
10. Concentrated HNO₃ with concentrated H₂SO₄ (about 330 K); the attacking electrophile is the nitronium ion NO₂⁺.

11. Initiation: Cl₂ ⟶[hν] 2 Cl·. Propagation: Cl· + CH₄ → CH₃· + HCl, then CH₃· + Cl₂ → CH₃Cl + Cl·. Termination: two radicals combine, e.g. CH₃· + Cl· → CH₃Cl.
12. –CH₃ donates electron density (+I/hyperconjugation), activating the ring and stabilising the arenium ion at ortho/para positions, so it is an o/p director. –NO₂ withdraws electrons (−I, −M), deactivating the ring and destabilising the o/p arenium ions most, so the electrophile enters at meta.

13. Step 1 (slow): conc. HNO₃/H₂SO₄ generate NO₂⁺. The benzene π cloud attacks NO₂⁺ to form a resonance-stabilised carbocation, the arenium ion (sigma complex), in which the positive charge is delocalised over three ring carbons and aromaticity is temporarily lost. Step 2 (fast): a base (HSO₄^-) removes the proton from the sp³ carbon bearing NO₂, restoring the aromatic sextet and giving nitrobenzene. The arenium ion is the key high-energy intermediate; its relative stability at different ring positions controls the rate and the orientation (directive influence) of substitution.
14. Alkanes (C_nH_2n+2) are prepared by hydrogenation of alkenes, Wurtz reaction or decarboxylation; their typical reaction is free-radical substitution, e.g. CH₄ + Cl₂ ⟶[hν] CH₃Cl + HCl. Alkenes (C_nH_2n) are made by dehydrohalogenation (alc. KOH) or dehydration of alcohols; they undergo electrophilic addition, e.g. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. Alkynes (C_nH_2n-2) come from CaC₂+H₂O or double dehydrohalogenation; they add reagents like alkenes and, if terminal, show acidic behaviour, e.g. HC≡CH + 2[Ag(NH₃)₂]⁺ → AgC≡CAg. Alkanes are least reactive because they have only strong, non-polar σ bonds; alkenes and alkynes are more reactive because their exposed, electron-rich π bonds attract electrophiles, so they react readily by addition.