SaitechAI – Key Sheet

Haloalkanes & Haloarenes • 100 One-Mark Questions

Answer Key
  1. 1-Chloropropane.
  2. 2-Bromopropane.
  3. Chlorobenzene.
  4. Ethyl bromide.
  5. CₙH₂ₙ₊₁X (for mono-haloalkane).
  6. Carbon attached to halogen is bonded to only one other carbon atom.
  7. Carbon attached to halogen is bonded to three other carbon atoms.
  8. Example: 2-bromobutane.
  9. Aromatic compound in which one or more hydrogen atoms of an arene are replaced by halogen (Ar–X).
  10. Example: bromobenzene / iodobenzene (any mono-haloarene).
  11. Due to electronegativity difference between carbon and halogen, giving dipole moment.
  12. C–F bond.
  13. C–I < C–Br < C–Cl < C–F (so for given, CH₃I < CH₃Br < CH₃Cl in bond strength).
  14. Presence of polar C–X bond and higher molecular mass leading to stronger intermolecular forces.
  15. CH₃Br (bromomethane) has higher boiling point than CH₃Cl.
  16. Boiling point increases with increase in halogen atomic mass (Cl < Br < I).
  17. They do not form hydrogen bonds with water and are less polar than water; they are also heavier and form a separate layer.
  18. Density increases with increase in halogen atomic mass.
  19. CH₃Cl is more reactive; CCl₄ has no C–H bonds and no site for substitution.
  20. The individual C–Cl bonds are polar but tetrahedral symmetry cancels the dipoles, giving net zero dipole moment.
  21. R–Mg–X.
  22. Wurtz reaction.
  23. 2CH₃CH₂Cl + 2Na → C₂H₅–C₂H₅ + 2NaCl.
  24. Metallic fluorides like AgF, CoF₂, SbF₃, etc. (any one).
  25. Swarts reaction.
  26. By heating ethanol with conc. HCl in presence of anhydrous ZnCl₂ (Lucas reagent).
  27. Conc. HCl + anhydrous ZnCl₂ (Lucas reagent).
  28. HI > HBr > HCl > HF (HI most reactive).
  29. HF < HCl < HBr < HI (rate of formation of haloalkanes from alcohols).
  30. Phosphorus pentachloride (PCl₅).
  31. Reaction in which a nucleophile replaces another group (leaving group) in a molecule.
  32. Unimolecular nucleophilic substitution; rate depends on concentration of substrate only.
  33. Bimolecular nucleophilic substitution; rate depends on concentration of substrate and nucleophile.
  34. Tertiary haloalkanes (3°).
  35. Primary haloalkanes (1°).
  36. 1° > 2° > 3° (reactivity: primary highest in SN2).
  37. 3° > 2° > 1° (reactivity: tertiary highest in SN1).
  38. Species that donates an electron pair to form a new covalent bond, e.g. OH⁻, CN⁻, NH₃.
  39. Group that departs with its bonding electrons; e.g. halide ion X⁻ (Cl⁻, Br⁻, I⁻).
  40. C–I bond is weaker and iodide ion is a better leaving group.
  41. Elimination of a molecule (e.g. HX) from adjacent β-carbon, yielding an alkene.
  42. Alcoholic KOH (or strong base like KOH/NaOH in alcohol).
  43. More substituted alkene is the major product in elimination reactions.
  44. Propene.
  45. Substitution reaction forming alcohol (R–OH).
  46. Elimination reaction forming alkene (C=C).
  47. β-Elimination (E2/E1) reaction.
  48. Nucleophiles having more than one nucleophilic site, e.g. CN⁻, NO₂⁻.
  49. Linkage isomerism.
  50. Example: R–O–N=O (alkyl nitrite) and R–NO₂ (nitroalkane).
  51. Haloarene: aromatic ring directly bonded to halogen (Ar–X).
  52. Due to partial double bond character of C–X bond (resonance) and sp² hybridisation, bond is stronger and less polar.
  53. Because C–Cl bond has partial double bond character and the benzene ring is stable; SN1/SN2 mechanisms are not favoured.
  54. Resonance gives C–Cl bond partial double bond character, making it stronger and shorter.
  55. sp² hybridisation of the carbon attached to halogen.
  56. Due to partial double bond character, C–Cl bond in chlorobenzene is shorter than in CH₃Cl.
  57. +M effect: electron-donating resonance effect which increases electron density through π-bond.
  58. Electron-withdrawing –I group activates ring towards nucleophilic substitution, especially at o/p positions relative to the group.
  59. Ortho and para positions.
  60. Dow’s process (conversion of chlorobenzene to phenol with NaOH at high T & P).
  61. Halide exchange reaction between alkyl chloride/bromide and NaI in acetone.
  62. Sodium iodide (NaI) in dry acetone.
  63. R–Cl + NaI (acetone) → R–I + NaCl (precipitate).
  64. Coupling of alkyl halides using sodium metal in dry ether to form alkanes.
  65. Because a mixture of products is formed, difficult to separate.
  66. Solvent for the reaction and stabilises sodium and the organic intermediates.
  67. Sandmeyer reaction.
  68. Ar–N₂⁺X⁻ (e.g. benzene diazonium chloride, C₆H₅N₂⁺Cl⁻).
  69. NaNO₂ + HCl (nitrous acid in situ) at 273–278 K.
  70. Low temperature, about 273–278 K (0–5 °C).
  71. As a solvent and as a methylating reagent (methyl chloride uses).
  72. Chloroform (as an anaesthetic) / ethyl chloride (local anaesthetic) – any suitable example.
  73. Causes liver and kidney damage; affects central nervous system (any one harmful effect).
  74. Freons (e.g. CCl₂F₂) used as refrigerants/propellants.
  75. Because they deplete the ozone layer and are environmentally harmful.
  76. Ozone layer depletion.
  77. DDT or BHC (e.g. dichloro-diphenyl-trichloroethane).
  78. Dichloro-diphenyl-trichloroethane.
  79. It is non-biodegradable and bio-accumulates, causing long-term environmental damage.
  80. It remains in the environment for a long time without degradation.
  81. Cl–CH₂–CH₂–Cl.
  82. Br–CH₂–CH₂–Cl.
  83. 1-Bromopropane.
  84. 2-Chlorobutane.
  85. 1,4-Dichlorobenzene.
  86. 3-Bromomethylbenzene (m-bromotoluene).
  87. CH₃–CHBr–CH₂–CH₃.
  88. 4-Nitrochlorobenzene.
  89. Benzyl chloride (chloromethylbenzene).
  90. 1-Bromo-1-phenylethane (1-bromo-1-phenylethane / α-bromoethylbenzene).
  91. Allylic halide: halogen on carbon adjacent to C=C; e.g. CH₂=CH–CH₂Cl.
  92. Benzylic halide: halogen on carbon attached directly to benzene ring; e.g. C₆H₅–CH₂Cl.
  93. Benzylic halide.
  94. Because the benzylic carbocation is resonance-stabilised.
  95. Vinylic halide: halogen directly attached to an sp² carbon of C=C; e.g. CH₂=CH–Cl.
  96. No, they are relatively inert to nucleophilic substitution because of strong Csp²–X bond and partial double bond character.
  97. Example: CH₂=CH–Cl.
  98. Both halogens on same carbon atom, e.g. CH₃–CHCl₂.
  99. Halogens on adjacent carbon atoms, e.g. CH₂Cl–CH₂Cl.
  100. Example: 1,2-dichloroethane, ClCH₂–CH₂Cl.
  101. It forms precipitate of silver halide with halide ion released; used to detect rate/type of substitution.
  102. 3° alkyl halides react faster, giving immediate precipitate; 1° very slow/ no precipitate at room temperature.
  103. Aryl halides do not undergo Friedel–Crafts with same ease as alkyl halides due to low reactivity of C–X bond.
  104. Lewis acid catalysts such as AlCl₃ / FeCl₃.
  105. Williamson ether synthesis.
  106. CH₃CH₂Br + KCN (alc.) → CH₃CH₂–C≡N + KBr.
  107. CH₃CH₂Br + AgCN → CH₃CH₂–NC + AgBr.
  108. In KCN, C is nucleophilic; in AgCN, N is nucleophilic due to covalent Ag–C bond, giving cyanide vs isocyanide.
  109. Used as refrigerants and aerosol propellants.
  110. Ethyl alcohol mixed with small amounts of methanol and other substances to make it unfit for drinking (not exactly halo, but contextually: denatured using poisonous additives, sometimes chloroform/carbon tetrachloride earlier).