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Haloalkanes and Haloarenes

Chemistry · Class 12NCERT pp. 159–1882 sets × 25 questions

Key facts and formulas for Haloalkanes and Haloarenes, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.

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Key facts for this chapter

IdeaRule or fact
Haloalkane vs haloareneHaloalkane: X on sp³ carbon. Aryl and vinylic halides: X on sp² carbon. Allylic and benzylic halides keep X on sp³ carbon.
SN2One step, backside attack, inversion, rate = k[RX][Nu]. Order 1° > 2° > 3°. Polar aprotic solvents (DMSO) speed it up.
SN1Two steps, planar carbocation, racemisation, rate = k[RX]. Order 3° > 2° > 1°; benzyl and allyl are fast.
EliminationAlcoholic KOH and heat give dehydrohalogenation. Saytzeff: the more substituted alkene is major. Bulky strong base on 3° halide gives alkene.
Name reactionsFinkelstein: RX + NaI. Swarts: RX + AgF. Sandmeyer: ArN₂⁺ + Cu₂Cl₂. Wurtz–Fittig: ArX + RX + Na.
Ambident nucleophilesKCN gives RCN, AgCN gives RNC. KNO₂ gives R–ONO, AgNO₂ gives R–NO₂.
HaloarenesX is o/p directing but deactivating. Nucleophilic substitution needs o/p nitro groups; more nitro groups, easier reaction. C–Cl is shorter due to sp² and resonance.
Radical halogenationLight and Cl₂: side chain (toluene gives benzyl chloride). FeCl₃ in the dark: ring o/p. Br₂ is highly selective for 3° C–H.
Chloroform and othersCarbylamine test gives RNC. Reimer–Tiemann uses :CCl₂. CHCl₃ + Ag gives ethyne. Iodoform test needs CH₃CH(OH)– or CH₃CO–.
Environment and usesDDT is stable and fat soluble. CFCs give Cl• radicals in UV. Teflon monomer is CF₂=CF₂. Iodoform acts through released iodine. Peroxide effect: HBr only.

Three sample questions

Q1. In which compound is the halogen attached to an sp² carbon?
  1. C₆H₅CH₂Br
  2. C₆H₅Br
  3. CH₂=CHCH₂Br
  4. CH₃CH₂Br
Hint and solution

Hint: Only vinylic and aryl halides have the halogen on a carbon that is part of a double bond or a ring. Check which carbon holds Br in each.

  1. X on sp² C → vinylic or aryl halide — Ask which carbon carries the halogen and what its hybridisation is. Only carbons in a C=C or in the benzene ring are sp².
  2. C₆H₅CH₂Br, CH₂=CHCH₂Br, CH₃CH₂Br → X on sp³ C — In these three the C–Br carbon is a CH₂ group with four single bonds, so it is sp³.
  3. C₆H₅Br → Br on ring carbon (sp²) — In bromobenzene Br is attached directly to a carbon of the benzene ring, which is sp² hybridised.
  4. Answer: C₆H₅Br — So the halogen is on an sp² carbon only in bromobenzene. Option B.
Answer: B. Only aryl and vinylic halides have X on an sp² carbon.
Q2. The number of structural isomers of C₅H₁₁Br is
  1. 6
  2. 7
  3. 9
  4. 8
Hint and solution

Hint: Write the three skeletons of C₅ (pentane, 2-methylbutane, 2,2-dimethylpropane) and count the different positions of Br on each.

  1. C₅H₁₁Br: skeletons = 3 — Find the carbon skeletons of C₅: n-pentane, 2-methylbutane and 2,2-dimethylpropane.
  2. n-pentane: 1-, 2-, 3-bromo → 3 — Position 1 and 5 are the same, 2 and 4 are the same, and 3 is the middle. So three isomers.
  3. 2-methylbutane: 4 kinds of H → 4 — It has four different types of hydrogen, giving four monobromides. 2,2-dimethylpropane has all H equivalent, giving 1.
  4. Total = 3 + 4 + 1 = 8 — Adding gives eight structural isomers: four primary, three secondary and one tertiary. Option D.
Answer: D. Four primary, three secondary and one tertiary bromide.
Q3. Chlorobenzene is obtained from benzenediazonium chloride using
  1. Cu₂Cl₂
  2. KCl
  3. NaCl
  4. CuI
Hint and solution

Hint: Replacing the diazonium group by Cl with a copper(I) salt is a named reaction. Which one?

  1. ArN₂⁺Cl⁻ + Cu₂Cl₂ → ArCl + N₂ — Replacing N₂⁺ by Cl using copper(I) chloride is the Sandmeyer reaction.
  2. Halogen introduced = halide of the copper salt — The halogen that enters the ring is the same as the halide in the copper salt. We want Cl, so we need CuCl.
  3. KCl, NaCl have no copper; CuI gives iodobenzene — Simple ionic chlorides cannot do the replacement and CuI would give the wrong halogen.
  4. Answer: Cu₂Cl₂ — So chlorobenzene is obtained using Cu₂Cl₂. Option A.
Answer: A. The Sandmeyer reaction uses CuCl for chlorine.

How to use this chapter

Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.

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