Haloalkanes and Haloarenes: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Give an example of an allylic halide.
2. Define a chiral molecule.
3. Write the Finkelstein reaction.
4. What is a racemic mixture?
5. Name the reagent for Sandmeyer chlorination.
6. Give one use of iodoform.
7. Product of CH₃CH₂Br + KCN.
8. State Saytzeff's rule.
9. Why is SOCl₂ preferred for R–Cl?
10. Which is chiral: 1- or 2-chlorobutane?

D. Short answer I (2 marks each)

1. Distinguish SN1 and SN2 (two points).
2. Why are haloarenes less reactive than haloalkanes?
3. Product of CH₃CH=CH₂ + HBr with and without peroxide.
4. Why is chlorine o/p directing?
5. Arrange in increasing SN1 reactivity: 1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane.
6. Why must Grignard reagents be made under anhydrous conditions?
7. KCN gives R–CN but AgCN gives R–NC. Why?
8. Write the Wurtz–Fittig reaction.

Rough work

E. Short answer II (3 marks each)

1. Explain the SN2 mechanism with the reaction of CH₃Br and OH⁻.
2. Explain why hydrolysis of optically active 3-bromo-3-methylhexane gives a racemic product.
3. How will you convert: (a) propene to 1-iodopropane (b) benzene to biphenyl (c) aniline to chlorobenzene?
4. Write the products of 2-bromobutane with (a) aq KOH (b) alc KOH (c) Mg in dry ether.
5. Give the uses and harmful effects of freons and DDT.
6. Explain the effect of NO₂ at o/p on nucleophilic substitution of chlorobenzene.

F. Long answer (5 marks each)

1. (a) Give the mechanism of SN1 with (CH₃)₃CBr. (b) Why are allylic and benzylic halides very reactive in SN1? (c) Arrange C₆H₅CH₂Br, C₆H₅CH(CH₃)Br, C₆H₅C(CH₃)₂Br in SN1 order.
2. A compound C₄H₉Cl (A) is optically active. (a) Identify A. (b) Write its products with aq KOH, alc KOH and Na. (c) Which product is formed by Saytzeff's rule?

G. HOTS (higher-order thinking)

1. Why is the dipole moment of CH₃Cl larger than that of CH₃F though F is more electronegative?
2. Why does neopentyl bromide react very slowly by both SN1 and SN2?
3. Why can't vinyl chloride be hydrolysed easily?
4. Why is the ether solvent essential in preparing RMgX?

20 MCQs

1. Fastest SN1
   (a) CH₃Br (b) CH₃CH₂Br (c) (CH₃)₃CBr (d) (CH₃)₂CHBr
2. Fastest SN2
   (a) CH₃Br (b) (CH₃)₃CBr (c) (CH₃)₂CHBr (d) CH₃CH₂CHBrCH₃
3. Allylic halide
   (a) CH₂=CHCl (b) CH₃CH₂Cl (c) C₆H₅Cl (d) CH₂=CHCH₂Cl
4. Product of R–X + AgNO₂
   (a) R–ONO (b) R–NO₂ (c) R–OH (d) R–NH₂
5. Chiral molecule
   (a) 2-chloropropane (b) 2-chlorobutane (c) 1-chlorobutane (d) chloroethane
6. SN2 gives
   (a) racemisation (b) no product (c) retention (d) inversion
7. Major product of CH₃CH₂CHBrCH₃ + alc KOH
   (a) but-2-ene (b) but-1-ene (c) butan-2-ol (d) butane
8. Highest boiling point
   (a) CH₃Cl (b) CH₃Br (c) CH₃I (d) CH₃F
9. Reagent preferred for R–OH → R–Cl
   (a) PCl₃ (b) Cl₂ (c) HCl alone (d) SOCl₂
10. 2C₆H₅Cl + 2Na gives
    (a) toluene (b) biphenyl (c) benzene (d) phenol
11. Peroxide effect is seen with
    (a) HCl (b) HI (c) HBr (d) HF
12. Chloroform is stored in dark bottles to avoid formation of
    (a) COCl₂ (b) CCl₄ (c) Cl₂ (d) CH₂Cl₂
13. Enantiomers differ in
    (a) b.p. (b) sign of optical rotation (c) density (d) reactivity with achiral reagents
14. Ambident nucleophile
    (a) OH⁻ (b) Cl⁻ (c) CN⁻ (d) NH₃
15. Haloarenes are o/p directing because of
    (a) –I effect (b) steric effect (c) hyperconjugation (d) +R effect
16. Most reactive towards nucleophilic substitution
    (a) 2,4,6-trinitrochlorobenzene (b) chlorobenzene (c) 3-nitrochlorobenzene (d) toluene
17. Finkelstein reaction uses
    (a) NaI in acetone (b) AgF (c) Cl₂/FeCl₃ (d) Mg
18. SN1 rate depends on
    (a) [Nu⁻] only (b) neither (c) both (d) [RX] only
19. DDT is
    (a) a refrigerant (b) a non-biodegradable insecticide (c) an anaesthetic (d) a fertiliser
20. Lowest b.p. isomer of C₄H₉Br
    (a) 1-bromobutane (b) 2-bromobutane (c) 2-bromo-2-methylpropane (d) 1-bromo-2-methylpropane

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: Benzyl chloride undergoes SN1 easily. R: The benzyl carbocation is resonance stabilised.
2. A: Chlorobenzene does not undergo SN2. R: The ring hinders back-side attack and C–Cl has partial double-bond character.
3. A: SN1 of an optically active halide gives inversion only. R: The carbocation is planar.
4. A: AgCN gives isocyanides. R: AgCN is mainly ionic.
5. A: Alkyl iodides react faster than chlorides. R: The C–I bond is the weakest.
6. A: Iodoform is an antiseptic. R: It liberates free iodine.

Key: 1 (a) · 2 (a) · 3 (d) · 4 (c) · 5 (a) · 6 (a)

Answers to the practice set