Nucleophilic Substitution Revision Q&A – Haloalkanes & Haloarenes (Class 12 Chemistry)

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Nucleophilic Substitution

Haloalkanes & Haloarenes • NCERT Class 12 Chemistry

Class 12 JEE / NEET

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Nucleophilic Substitution Revision Q&A

1. Q: What is a nucleophilic substitution reaction?
   A: A reaction in which a nucleophile replaces a leaving group, typically a halide, from an organic substrate.
2. Q: Name two main mechanisms of nucleophilic substitution in haloalkanes.
   A: SN1 (unimolecular) and SN2 (bimolecular) mechanisms.
3. Q: Which carbon hybridization is most favorable for nucleophilic substitution?
   A: sp³-hybridized carbon.
4. Q: Why do haloarenes undergo nucleophilic substitution with difficulty?
   A: Due to resonance stabilization and partial double bond character in C–X bond.
5. Q: What is the order of reactivity for SN2 mechanism: methyl, 1°, 2°, or 3° alkyl halides?
   A: Methyl > 1° > 2° > 3°.
6. Q: What is the order of reactivity for SN1 mechanism: methyl, 1°, 2°, or 3° alkyl halides?
   A: 3° > 2° > 1° > methyl.
7. Q: What determines the rate of SN1 reaction?
   A: Concentration of only the substrate.
8. Q: What determines the rate of SN2 reaction?
   A: Concentrations of both the substrate and nucleophile.
9. Q: Name a good example of a nucleophile.
   A: Hydroxide ion (OH^–).
10. Q: Which halide is the best leaving group?
    A: Iodide (I^–).
11. Q: What is Walden inversion?
    A: Complete inversion of configuration at the reaction center in SN2.
12. Q: What is the product configuration from SN1 mechanism?
    A: Racemic mixture (both retention and inversion).
13. Q: What is the intermediate in SN1 mechanism?
    A: Carbocation.
14. Q: Why do tertiary haloalkanes typically undergo SN1?
    A: They readily form stable tertiary carbocations.
15. Q: Why is SN2 mechanism slow for tertiary alkyl halides?
    A: Due to high steric hindrance.
16. Q: What solvent favors SN1 reactions?
    A: Polar protic solvents (e.g., water, alcohol).
17. Q: What solvent favors SN2 reactions?
    A: Polar aprotic solvents (e.g., DMSO, acetone).
18. Q: What is the rate law for SN2 reaction?
    A: Rate = k [substrate][nucleophile].
19. Q: Which is more reactive in SN2: methyl chloride or methyl iodide?
    A: Methyl iodide, because iodide is a better leaving group.
20. Q: What is the nature of transition state in SN2 reaction?
    A: Pentavalent carbon with partial bonds to nucleophile and leaving group.
21. Q: What type of stereochemistry results from SN1 mechanism?
    A: Racemization.
22. Q: Is the rate of SN1 affected by nucleophile strength?
    A: No, only by substrate concentration.
23. Q: Which factor increases SN2 rate: strong or weak nucleophile?
    A: Strong nucleophile.
24. Q: What is solvolysis?
    A: Nucleophilic substitution by the solvent molecule (usually water or alcohol).
25. Q: How does branching affect SN2 rate?
    A: More branching decreases SN2 rate.
26. Q: Which carbon skeleton is most favorable for SN2?
    A: Methyl and primary alkyl.
27. Q: Which nucleophile can replace the halide in alkyl halides to form nitriles?
    A: Cyanide ion (CN^–).
28. Q: What is the product when bromoethane reacts with KCN?
    A: Propionitrile (C₂H₅CN).
29. Q: What is produced when haloalkane reacts with aqueous NH₃?
    A: Primary amine (RNH₂).
30. Q: What is Finkelstein reaction?
    A: Halide exchange: RX + NaI (acetone) → RI + NaX.
31. Q: Role of acetone in Finkelstein reaction?
    A: NaX precipitates out, driving reaction forward.
32. Q: What is the general equation for hydrolysis of alkyl halide by aqueous KOH?
    A: R–X + OH^– → R–OH + X^–.
33. Q: Can vinyl chloride undergo SN1 or SN2?
    A: No, due to double bond character in C–Cl.
34. Q: Name a reaction where nitrite ion (NO₂^–) acts as nucleophile.
    A: SN2: R–Br + NaNO₂ → R–ONO + NaBr.
35. Q: What is the product when methyl bromide reacts with AgNO₂?
    A: CH₃NO₂ (nitro compound).
36. Q: Define ambident nucleophile.
    A: A nucleophile that can attack via two different atoms (e.g., CN^–, NO₂^–).
37. Q: Distinguish between SN1 and SN2 using an optically active substrate.
    A: SN2 gives inversion; SN1 leads to racemic mixture.
38. Q: Why is p-nitrochlorobenzene more reactive towards nucleophilic substitution than chlorobenzene?
    A: NO₂ group is electron-withdrawing and stabilizes the intermediate.
39. Q: What is the intermediate in nucleophilic aromatic substitution (SNAr)?
    A: Meisenheimer complex (anionic intermediate).
40. Q: What is the benzyne mechanism?
    A: A mechanism in which aryl halide undergoes elimination-addition under strong base to form benzyne.
41. Q: What happens when chlorobenzene is heated with NaNH₂ at high temperature?
    A: Aniline (C₆H₅NH₂) forms via benzyne intermediate.
42. Q: What increases the rate of nucleophilic substitution in aryl halides?
    A: Electron-withdrawing groups (NO₂, CN) at ortho and para positions.
43. Q: Do alkyl fluorides undergo nucleophilic substitution readily?
    A: No, F^– is a poor leaving group.
44. Q: What kind of product is formed by reaction of haloalkane with NaOR?
    A: Ether (R–OR).
45. Q: Williamson ether synthesis is an example of which substitution mechanism?
    A: SN2 mechanism.
46. Q: Excess ammonia used with alkyl halide gives what byproduct?
    A: Secondary and tertiary amines by further alkylation.
47. Q: Which solvent increases rate of SN2 reaction?
    A: Polar aprotic solvent.
48. Q: Give one industrial utility of nucleophilic substitution.
    A: Synthesis of pharmaceuticals and dyes.
49. Q: What is the role of steric hindrance in SN2?
    A: More steric hindrance decreases SN2 rate.
50. Q: What is the most common nucleophilic substitution reaction of haloalkanes in daily life?
    A: Saponification (soap making) and preparation of alcohols from alkyl halides.
51. Q: Why are aryl halides less reactive in SN1 and SN2 mechanisms?
    A: Resonance and poor stabilization of carbocation in SN1 or steric/electronic effects in SN2.
52. Q: What is meant by a “good leaving group”?
    A: A group that can stabilize the negative charge upon departure (e.g., I^–).
53. Q: What type of nucleophilic substitution occurs on benzyl chloride?
    A: Both SN1 and SN2 (due to resonance-stabilized benzyl carbocation).
54. Q: What happens to optical activity in SN2 reactions if the substrate is optically active?
    A: Optical inversion occurs.
55. Q: Which is a better nucleophile: OH^– or H₂O?
    A: OH^–.
56. Q: What is the effect of a polar protic solvent on nucleophile strength?
    A: Weakens nucleophile through H-bonding.
57. Q: Are allylic halides reactive via SN1 or SN2?
    A: Yes; allylic carbocation is resonance stabilized.
58. Q: What is the mechanism for conversion of 2-bromobutane to 2-butanol?
    A: SN1 for 2° or 3° halide, SN2 for 1° and methyl halides.
59. Q: What happens when bromoethane is heated with NaOH (aq)?
    A: Ethanol is formed.
60. Q: What is the stereochemical effect in SN2 reaction of chiral alkyl halide?
    A: Inversion of configuration.
61. Q: Why is neopentyl bromide slow in SN2?
    A: Bulky group hinders nucleophile approach.
62. Q: What is unique about the SNAr mechanism in aryl halides?
    A: Requires strongly electron-withdrawing groups for activation.
63. Q: What is the main utility of SN1 reactions in organic synthesis?
    A: Formation of tertiary alcohols and rearrangements.
64. Q: What is the product of 1-bromopropane with KCN?
    A: Butyronitrile (C₃H₇CN).
65. Q: What is the effect of NO₂ group at ortho/para positions in aryl halides?
    A: Strongly increases reactivity towards nucleophilic substitution.
66. Q: Why does SN2 not occur with tertiary alkyl halides?
    A: Steric hindrance prevents nucleophile approach.
67. Q: Which is faster, SN1 or SN2 mechanism for primary alkyl halide?
    A: SN2.
68. Q: Is the SN1 mechanism possible for primary or methyl alkyl halides?
    A: No, unstable carbocation.
69. Q: What type of reaction is typically seen with vinyl halides?
    A: Not nucleophilic substitution; resistant due to resonance.
70. Q: Example of nucleophilic substitution forming an ether?
    A: Williamson synthesis: R–X + R′O^– → R–O–R′.
71. Q: What occurs when an alkyl halide reacts with NaNO₂?
    A: Alkyl nitrite (R–ONO) forms.
72. Q: For SN2, does the rate increase or decrease with a strong nucleophile?
    A: Increase.
73. Q: Is resonance important in SNAr reactions?
    A: Yes, it stabilizes Meisenheimer intermediate.
74. Q: Give an example of a polar aprotic solvent.
    A: Dimethyl sulfoxide (DMSO).
75. Q: State one use of nucleophilic substitution in biotech industry.
    A: Synthesis of pharmaceuticals like local anesthetics.
76. Q: Why is SN1 not favored in non-polar solvent?
    A: Carbocation formation is not stabilized.
77. Q: Give the mechanism for the hydrolysis of tert-butyl bromide.
    A: SN1 (forms tert-butyl carbocation, then attacked by water).
78. Q: Why are primary haloalkanes more reactive in SN2?
    A: Less steric hindrance for nucleophile approach.
79. Q: What is the major product when chlorobenzene is heated with NaOH at 623K, 300 atm?
    A: Phenol (C₆H₅OH).
80. Q: Which alkyl halides generally react via SN2 mechanism?
    A: Methyl and primary alkyl halides.
81. Q: What is the product when ethyl chloride reacts with aqueous ammonia?
    A: Ethylamine (C₂H₅NH₂).
82. Q: What is the stereochemistry of the product in SN2 substitution of chiral bromobutane?
    A: Inverted configuration at reaction center.
83. Q: State the main factor controlling SN2 rate.
    A: Steric accessibility of reaction center.
84. Q: What is the product on hydrolysis of 2-bromopentane in aqueous KOH?
    A: 2-pentanol.
85. Q: Name a nucleophile that can substitute halide in haloalkane to form an amine.
    A: NH₃ (ammonia).
86. Q: Why does increasing alkyl group size decrease SN2 rate?
    A: Increases steric hindrance for nucleophile approach.
87. Q: What is a basic requirement for SN1 reaction to occur?
    A: Ability to form stable carbocation intermediate.
88. Q: What is the main utility of SN2 mechanism in industry?
    A: Synthesis of alcohols, ethers, and nitriles.
89. Q: Name one commercial substance prepared by nucleophilic aromatic substitution.
    A: Aniline dyes.
90. Q: What is the typical leaving group in nucleophilic substitution of haloalkanes?
    A: Halide ion (Cl^–, Br^–, I^–).
91. Q: What is the effect of electron-donating groups on aryl halide reactivity in SNAr?
    A: Decreases reactivity.
92. Q: Which nucleophilic substitution leads to racemization in the product?
    A: SN1 mechanism.
93. Q: Can haloarenes undergo nucleophilic substitution via SN2?
    A: No, due to resonance and double bond character.
94. Q: What is the common use of SN2 reaction in laboratory synthesis?
    A: Preparation of primary alcohols from alkyl halides.
95. Q: Give the product of SN2 reaction of methyl bromide with NaOH.
    A: Methanol (CH₃OH).
96. Q: What is the rate-determining step in SN1 mechanism?
    A: Ionization of alkyl halide to form carbocation.
97. Q: Name an example where SN2 mechanism is used to form ether in industry.
    A: Williamson’s ether synthesis.
98. Q: What is the basicity trend of halide leaving groups?
    A: I^– < Br^– < Cl^– < F^–.
99. Q: What role do crown ethers play in nucleophilic substitution reactions?
    A: Enhance solubility of salts, increase reaction rate in SN2.
100. Q: Why do aryl halides require harsh conditions for substitution?
     A: Strong C–X bond due to resonance, so strong bases/high temperature needed.

Nucleophilic Substitution Quiz (30 MCQs)

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