Organic Chemistry: Some Basic Principles and Techniques (GOC) Revision Notes

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Reaction Mechanism

104 Short Q&A • Basic Principles of Organic Chemistry

Class 11 JEE / NEET

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104 Short Q&A Facts

Part 1: Bond Fission and Reactive Intermediates
1. Q1: What is Homolytic Fission?
   A: The bond cleavage where each bonded atom takes one electron of the shared pair.
2. Q2: What is the electronic species formed during homolytic fission?
   A: Free Radical (R·).
3. Q3: What is Heterolytic Fission?
   A: The bond cleavage where the shared electron pair stays with one of the fragments.
4. Q4: What are the two types of charged species formed during heterolytic fission?
   A: Carbocation (R⁺) and Carbanion (R⁻).
5. Q5: What conditions favor homolytic fission?
   A: High temperature, light (UV), non-polar solvents, or the presence of peroxides.
6. Q6: What conditions favor heterolytic fission?
   A: Polar solvents (e.g., water) and a significant electronegativity difference in the bond.
7. Q7: Define a Carbocation.
   A: A species containing a carbon atom with a positive charge and only six valence electrons (electron deficient).
8. Q8: Define a Carbanion.
   A: A species containing a carbon atom with a negative charge and a full octet (eight valence electrons).
9. Q9: Define a Free Radical.
   A: A neutral species containing a carbon atom with an unpaired electron.
10. Q10: What is the geometry and hybridization of a simple Carbocation (CH₃⁺)?
    A: Trigonal Planar, sp² hybridization.
11. Q11: What is the geometry and hybridization of a simple Carbanion (CH₃⁻)?
    A: Pyramidal, sp³ hybridization.
12. Q12: What is the geometry and hybridization of a simple Alkyl Free Radical (CH₃·)?
    A: Planar or slightly Pyramidal, usually sp² or sp³.
13. Q13: Rank the stability of alkyl carbocations (1°, 2°, 3°).
    A: 3° > 2° > 1°.
14. Q14: Rank the stability of alkyl free radicals (1°, 2°, 3°).
    A: 3° > 2° > 1°.
15. Q15: Rank the stability of alkyl carbanions (1°, 2°, 3°).
    A: 1° > 2° > 3° (opposite to carbocations/radicals).
16. Q16: What electronic effects stabilize Carbocations?
    A: Inductive Effect (+I) and Hyperconjugation.
17. Q17: What electronic effects stabilize Carbanions?
    A: Inductive Effect (−I) and Resonance/Mesomeric Effect (−M).
18. Q18: Which is generally the most stable reactive intermediate?
    A: A species stabilized by Resonance (e.g., Benzyl or Allyl carbocation/radical).
19. Q19: What is the general name for the species that forms the new bond by attacking the substrate?
    A: Attacking Reagent.
Part 2: Attacking Reagents (Electrophiles and Nucleophiles)
20. Q20: Define an Electrophile.
    A: An electron-deficient species (electron-loving, acts as a Lewis Acid).
21. Q21: Give two examples of positively charged Electrophiles.
    A: H⁺, NO₂⁺, CH₃⁺ (Carbocation).
22. Q22: Give two examples of neutral Electrophiles.
    A: AlCl₃, BF₃, SO₃ (Lewis Acids with incomplete octet).
23. Q23: Define a Nucleophile.
    A: An electron-rich species (nucleus-loving, acts as a Lewis Base).
24. Q24: Give two examples of negatively charged Nucleophiles.
    A: OH⁻, CN⁻, R₃C⁻ (Carbanion).
25. Q25: Give two examples of neutral Nucleophiles.
    A: H₂O, NH₃, R-OH (species with lone pairs).
26. Q26: Where do Electrophiles typically attack the substrate molecule?
    A: At the site of high electron density (e.g., π-bond or negative charge).
27. Q27: Where do Nucleophiles typically attack the substrate molecule?
    A: At the site of low electron density (e.g., C atom of C=O or C⁺).
28. Q28: How is Nucleophilicity related to Basicity?
    A: They are similar but not identical; nucleophilicity is a kinetic concept (rate of attack), basicity is a thermodynamic concept (equilibrium of proton acceptance).
29. Q29: Which is a stronger nucleophile in polar protic solvents: F⁻ or I⁻?
    A: I⁻ (F⁻ is strongly solvated, reducing its nucleophilicity).
30. Q30: Which is a stronger base: F⁻ or I⁻?
    A: F⁻ (based on pK_a of conjugate acid HF).
Part 3: Electronic Effects (Permanent Effects)
31. Q31: What is the Inductive Effect (I-effect)?
    A: The polarization of σ-bonds caused by the unequal sharing of electrons due to a difference in electronegativity.
32. Q32: Is the Inductive Effect a temporary or permanent effect?
    A: Permanent effect.
33. Q33: How does the magnitude of the Inductive Effect change with distance?
    A: It decreases rapidly with distance and becomes negligible after 3-4 carbon atoms.
34. Q34: Define the +I Effect (Positive Inductive Effect).
    A: The electron-releasing or electron-donating tendency of a group (e.g., alkyl groups).
35. Q35: Define the −I Effect (Negative Inductive Effect).
    A: The electron-withdrawing tendency of a group (e.g., NO₂, CN).
36. Q36: Give the order of stability for Carboxylic Acids (Acidity) based on the −I effect.
    A: −I group increases acidity by stabilizing the conjugate base.
37. Q37: What is the Resonance Effect (M-effect) or Mesomeric Effect?
    A: The delocalization of π-electrons (or lone pairs) within a conjugated system.
38. Q38: Is the Resonance Effect a temporary or permanent effect?
    A: Permanent effect.
39. Q39: Define the +M Effect (Positive Mesomeric Effect).
    A: The group releases electrons into the conjugated system (e.g., OH, NH₂).
40. Q40: Define the −M Effect (Negative Mesomeric Effect).
    A: The group withdraws electrons from the conjugated system (e.g., NO₂, CHO).
41. Q41: What are Canonical Forms (or Resonating Structures)?
    A: Hypothetical structures used to represent the true structure (resonance hybrid) of a molecule with delocalized electrons.
42. Q42: Which is more stable: a resonance hybrid or its canonical forms?
    A: The Resonance Hybrid (due to resonance energy).
43. Q43: What is the condition necessary for a molecule to show resonance?
    A: It must have a conjugated system (π-bonds alternating with single bonds or lone pairs).
44. Q44: Which effect stabilizes the negative charge on the carboxylate ion (RCOO⁻)?
    A: Resonance (two equivalent resonating structures).
45. Q45: What is Hyperconjugation (or the No-Bond Resonance/Baker-Nathan Effect)?
    A: The delocalization of σ-electrons of a C-H bond of an alkyl group adjacent to an unsaturated system.
46. Q46: What electronic species does Hyperconjugation involve?
    A: σ electrons (specifically C-H bond electrons).
47. Q47: How many α-hydrogens are in the tert-butyl carbocation?
    A: Nine α-hydrogens (three CH₃ groups × 3H).
48. Q48: How does the number of α-hydrogens relate to stability?
    A: More α-hydrogens means greater stability (due to more hyperconjugative structures).
49. Q49: Rank the stabilizing effects in terms of magnitude (excluding E-effect).
    A: Resonance (M) > Hyperconjugation (H) > Inductive (I).
Part 4: Electronic Effects (Temporary Effect) and Reaction Types
50. Q50: What is the Electromeric Effect (E-effect)?
    A: The complete transfer of π-electrons of a multiple bond to one of the bonded atoms in the presence of an attacking reagent.
51. Q51: Is the Electromeric Effect a temporary or permanent effect?
    A: Temporary effect (only active when the reagent is present).
52. Q52: Define the +E Effect (Positive Electromeric Effect).
    A: The π-electrons are transferred towards the atom to which the attacking reagent gets attached (e.g., addition of H⁺ to C=C).
53. Q53: Define the −E Effect (Negative Electromeric Effect).
    A: The π-electrons are transferred away from the atom to which the attacking reagent gets attached (e.g., attack of CN⁻ on C=O).
54. Q54: What is the main characteristic difference between the I-effect and the E-effect?
    A: I-effect involves σ-electrons (partial shift); E-effect involves π-electrons (complete transfer).
55. Q55: What are the four major types of organic reactions?
    A: Substitution, Addition, Elimination, and Rearrangement.
56. Q56: Define an Addition Reaction.
    A: Two molecules combine to form a single product (characteristic of unsaturated compounds).
57. Q57: Define a Substitution Reaction.
    A: An atom or group of atoms in a molecule is replaced by another atom or group.
58. Q58: Define an Elimination Reaction.
    A: Two atoms or groups are removed from a molecule, resulting in the formation of a multiple bond.
59. Q59: What type of reagents typically initiate reactions in Alkanes (e.g., halogenation)?
    A: Free Radicals (Free Radical Substitution).
60. Q60: What type of reagents typically initiate reactions in Alkenes (C=C)?
    A: Electrophiles (Electrophilic Addition).
61. Q61: What type of reagents typically initiate reactions in Alkyl Halides?
    A: Nucleophiles (Nucleophilic Substitution).
62. Q62: What is Saytzeff's Rule (or Zaitsev's Rule)?
    A: In elimination reactions, the more substituted (more stable) alkene product is the major product.
63. Q63: What is Markownikoff's Rule?
    A: In the addition of HX to an unsymmetrical alkene, the negative part (X) goes to the carbon atom with fewer hydrogen atoms.
64. Q64: What mechanism is associated with Markownikoff's Rule?
    A: Electrophilic Addition (involving carbocation intermediate).
65. Q65: What is the Anti-Markownikoff's Rule (or Peroxide Effect)?
    A: Addition of HBr (only) to an unsymmetrical alkene in the presence of peroxides occurs via a free radical mechanism.
Part 5: Stereochemistry and Reaction Mechanism
66. Q66: What is Chirality?
    A: The property of an object (or molecule) being non-superimposable on its mirror image.
67. Q67: What is the most common feature that causes chirality in organic molecules?
    A: The presence of a chiral center (carbon atom bonded to four different groups).
68. Q68: What are Enantiomers?
    A: Non-superimposable mirror images of each other (optically active).
69. Q69: What is Racemic Mixture?
    A: An equimolar mixture of dextrorotatory (+−) and laevorotatory (−) enantiomers (optically inactive).
70. Q70: What is Retention of Configuration?
    A: The spatial arrangement of groups around the chiral center remains the same during a reaction.
71. Q71: What is Inversion of Configuration (Walden Inversion)?
    A: The spatial arrangement of groups around the chiral center is reversed during a reaction.
72. Q72: What is Racemisation?
    A: The conversion of an optically active compound into an optically inactive racemic mixture.
73. Q73: Which substitution mechanism (SN1 or SN2) results in Racemisation?
    A: SN1 (due to the planar carbocation intermediate).
74. Q74: Which substitution mechanism (SN1 or SN2) results in Inversion of Configuration?
    A: SN2 (due to backside attack).
75. Q75: In an SN2 reaction, how does the transition state look?
    A: A single transition state where the nucleophile and the leaving group are partially bonded to the carbon.
76. Q76: In an SN1 reaction, how many steps are involved?
    A: Two steps (formation of carbocation followed by nucleophilic attack).
77. Q77: Which step is the rate-determining step in the SN1 mechanism?
    A: The first step (formation of the carbocation).
78. Q78: What is the overall order of the SN2 reaction?
    A: Second Order (rate depends on [Substrate]¹ and [Nucleophile]¹).
79. Q79: What is the overall order of the SN1 reaction?
    A: First Order (rate depends only on [Substrate]¹).
Part 6: Miscellaneous and Hybridization
80. Q80: What is the use of Curved Arrows in reaction mechanisms?
    A: To show the movement of electron pairs (or single electrons for radicals).
81. Q81: What does a Double-Headed Curved Arrow (⤵) show?
    A: Movement of a pair of electrons (ionic reactions).
82. Q82: What does a Single-Headed Curved Arrow (Fishhook Arrow, ⇀) show?
    A: Movement of a single electron (free radical reactions).
83. Q83: What is the hybridization of the C atom in CH₃OH?
    A: sp³.
84. Q84: What is the hybridization of the C atom in CH₃COCH₃ (Acetone)?
    A: sp² (for the carbonyl C) and sp³ (for the CH₃ groups).
85. Q85: Why is the C atom in a C=O bond electrophilic?
    A: Because O is more electronegative, it creates a partial positive charge on the C atom.
86. Q86: How does increasing the s-character affect the stability of a carbanion?
    A: Increases the stability (due to greater electron-withdrawing power).
87. Q87: Which is more stable: Vinyl carbocation (CH₂=CH⁺) or Ethyl carbocation (CH₃CH₂⁺)?
    A: Ethyl Carbocation (Vinyl is unstable due to electron-withdrawing sp² C).
88. Q88: What is the stability order of carbanions based on hybridization?
    A: sp > sp² > sp³.
89. Q89: What type of electronic displacement occurs in phenol (C₆H₅OH)?
    A: +M effect of OH group and Resonance.
90. Q90: What are the positions in the benzene ring activated by a +M group?
    A: Ortho and Para positions (electron density increases here).
91. Q91: What are the positions in the benzene ring deactivated by a −M group?
    A: Ortho and Para positions (electron density decreases here).
92. Q92: What is the geometry of the intermediate in an E2 reaction?
    A: Single transition state (not an isolable intermediate).
93. Q93: What is the geometry of the intermediate in an E1 reaction?
    A: Carbocation (sp², Trigonal Planar).
94. Q94: What is the term for the process of converting an optically active isomer into a racemic mixture?
    A: Racemisation.
95. Q95: How does the stability of a carbocation affect the rate of the SN1 reaction?
    A: Higher stability leads to a faster rate.
96. Q96: Why does CHCl₃ not give the characteristic test for chlorine with AgNO₃?
    A: The C-Cl bond is covalent and does not ionize to release Cl⁻ ions.
97. Q97: Which effect is responsible for the activation of the ortho and para positions in toluene (C₆H₅CH₃)?
    A: Hyperconjugation (of the CH₃ group).
98. Q98: Define Chromatography (briefly).
    A: A technique for separating components of a mixture based on their differential partitioning between a stationary and a mobile phase.
99. Q99: What is the purpose of Lassaigne's Test?
    A: To convert elements like N, S, X in organic compounds from covalent to ionic form for easy detection.
100. Q100: What is the Prussian blue color test used to confirm in Lassaigne's extract?
     A: The presence of Nitrogen (CN⁻ converted to Fe₄[Fe(CN)₆]₃).
101. Q101: Which method is suitable for the purification of aniline?
     A: Steam Distillation (aniline is steam volatile and immiscible with water).
102. Q102: What is the principle of Distillation?
     A: Separation based on the difference in boiling points of components.
103. Q103: What is the key difference between Distillation and Fractional Distillation?
     A: Fractional is used when boiling points are close; simple is when they are far apart.
104. Q104: What is the purpose of using a Separating Funnel?
     A: To separate two immiscible liquids (e.g., oil and water).

Reaction Mechanism Practice Quiz (30 MCQs)

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