Hydrocarbons (Class 11 NCERT) Revision Notes

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112 Short Q&A • Alkanes, Alkenes, Alkynes & Benzene

Class 11 JEE / NEET

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112 Short Q&A Facts

Part 1: Alkanes (Saturated Hydrocarbons)
1. Q1: Define a Hydrocarbon.
   A: Organic compounds composed only of Carbon and Hydrogen atoms.
2. Q2: What is the general formula for Alkanes?
   A: C_nH_2n+2.
3. Q3: What is the hybridization of carbon atoms in alkanes?
   A: sp³.
4. Q4: What is the geometry around each carbon atom in alkanes?
   A: Tetrahedral (≈109.5° bond angle).
5. Q5: What is Wurtz Reaction used for?
   A: The synthesis of higher alkanes by coupling two alkyl halides with Na metal in dry ether.
6. Q6: What is the limitation of the Wurtz reaction?
   A: It fails for synthesizing unsymmetrical alkanes (forms a mixture of three products).
7. Q7: What are the products when sodium acetate is heated with soda lime (Decarboxylation)?
   A: Methane (CH₄) and Na₂CO₃.
8. Q8: What is the process of forming methane from aluminum carbide and water?
   A: Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄ (Hydrolysis).
9. Q9: What is the Sabatier-Senderens Reaction?
   A: Hydrogenation of alkenes or alkynes over Ni catalyst to form alkanes.
10. Q10: Why are alkanes often called paraffins?
    A: Because of their low affinity towards chemical reagents (least reactive).
11. Q11: What type of reactions do alkanes primarily undergo?
    A: Free Radical Substitution reactions (e.g., halogenation).
12. Q12: Write the three steps of the free radical substitution mechanism.
    A: Initiation, Propagation, and Termination.
13. Q13: Which is the most reactive halogen in the halogenation of alkanes?
    A: F₂ (Fluorination is explosive).
14. Q14: What is Isomerisation?
    A: Conversion of n-alkanes to branched alkanes using AlCl₃/HCl at high T.
15. Q15: What is the product of controlled oxidation of methane with Cu/Ag catalyst at high T, P?
    A: Methanol (CH₃OH).
16. Q16: What is the main characteristic of the cracking of alkanes?
    A: Breaking large alkanes into smaller alkanes and alkenes at high temperature.
17. Q17: What is the main product of the complete combustion of alkanes?
    A: CO₂ and H₂O.
18. Q18: What is the configuration of the preferred conformation of ethane?
    A: Staggered conformation (most stable).
19. Q19: What is the energy difference between the staggered and eclipsed conformations of ethane called?
    A: Torsional Strain.
Part 2: Alkenes (Unsaturated Hydrocarbons)
20. Q20: What is the general formula for Alkenes?
    A: C_nH_2n (containing one double bond).
21. Q21: What is the hybridization of carbon atoms involved in the double bond?
    A: sp².
22. Q22: What is the geometry around the double-bonded carbons?
    A: Trigonal Planar (≈120° bond angle).
23. Q23: How many σ and π bonds are in the double bond?
    A: One σ and one π bond.
24. Q24: Why are alkenes more reactive than alkanes?
    A: Due to the presence of the weak π-bond, which is easily cleaved.
25. Q25: What type of reaction is characteristic of alkenes?
    A: Electrophilic Addition Reactions.
26. Q26: What is the product of the addition of H₂O to ethene (in the presence of H⁺)?
    A: Ethanol (CH₃CH₂OH).
27. Q27: State the rule governing the addition of unsymmetrical reagents to unsymmetrical alkenes.
    A: Markownikoff's Rule.
28. Q28: What is the major product of HBr addition to propene?
    A: 2-Bromopropane (CH₃CH(Br)CH₃).
29. Q29: What type of intermediate is formed during the electrophilic addition of HBr?
    A: Carbocation (R⁺).
30. Q30: What is the rule that explains the minor product of HBr addition in the presence of peroxide?
    A: Anti-Markownikoff's Rule (or Peroxide Effect).
31. Q31: What is the intermediate in the addition of HBr under peroxide conditions?
    A: Free Radical.
32. Q32: What is the test used to distinguish between saturated and unsaturated hydrocarbons?
    A: Bromine water test (Unsaturated decolorizes the brown color).
33. Q33: What is the product when an alkene reacts with Br₂ in CCl₄?
    A: A Vicinal Dibromide (1,2-dibromoalkane).
34. Q34: Name the reagent used for the hydroxylation of alkenes (cis-addition).
    A: Cold, dilute, aqueous KMnO₄ (Bayer's reagent).
35. Q35: What is the final product of the ozonolysis of but-2-ene?
    A: Ethanal (CH₃CHO).
36. Q36: What is Ozonolysis used for in organic chemistry?
    A: To locate the position of the double bond.
37. Q37: What is Geometrical Isomerism (Cis-Trans isomerism)?
    A: Isomerism arising due to restricted rotation around a C=C bond.
38. Q38: Which is generally more stable, Cis or Trans isomer?
    A: Trans isomer (due to reduced steric repulsion).
39. Q39: What is the type of elimination reaction used to prepare alkenes from alkyl halides?
    A: β-Elimination or Dehydrohalogenation.
Part 3: Alkynes (Triply Bonded Hydrocarbons)
40. Q40: What is the general formula for Alkynes?
    A: C_nH_2n−2 (containing one triple bond).
41. Q41: What is the hybridization of carbon atoms involved in the triple bond?
    A: sp.
42. Q42: What is the geometry around the triple-bonded carbons?
    A: Linear (180° bond angle).
43. Q43: How many σ and π bonds are in the triple bond?
    A: One σ and two π bonds.
44. Q44: Why are terminal alkynes (R−C≡C−H) acidic?
    A: The sp hybrid C atom is highly electronegative (50% s-character), making the C-H bond highly polar.
45. Q45: Name the reaction used to test for terminal alkynes.
    A: Reaction with Tollen's reagent (Ag⁺) or ammoniacal Cu₂Cl₂ (Cu⁺) to form metal acetylides.
46. Q46: Are non-terminal alkynes (e.g., but-2-yne) acidic?
    A: No, they lack the terminal C-H bond.
47. Q47: What is the product of the addition of one mole of HBr to ethyne?
    A: Vinyl Bromide (CH₂=CHBr).
48. Q48: What is the final product of the hydration (addition of H₂O) of ethyne?
    A: Ethanal (Acetaldehyde, CH₃CHO).
49. Q49: What is the intermediate formed during the hydration of alkynes?
    A: An Enol (which rapidly tautomerizes to a ketone or aldehyde).
50. Q50: What is the product when ethyne is passed through a red-hot iron tube?
    A: Benzene (C₆H₆) (Cyclic Polymerisation).
51. Q51: What is the reagent used to convert an alkyne to a Cis-Alkene?
    A: Lindlar's Catalyst (Pd/BaSO₄ poisoned with sulfur or quinoline).
52. Q52: What is the reagent used to convert an alkyne to a Trans-Alkene?
    A: Na/liquid NH₃ (Birch Reduction).
53. Q53: Which is generally less reactive toward addition, Alkenes or Alkynes?
    A: Alkynes (initial addition is harder due to high C-C bond energy).
54. Q54: What is the product of the addition of 2 moles of HBr to ethyne?
    A: 1,1-Dibromoethane (CH₃CHBr₂).
Part 4: Arenes (Aromatic Hydrocarbons)
55. Q55: What is the simplest aromatic hydrocarbon?
    A: Benzene (C₆H₆).
56. Q56: What is the hybridization of all carbon atoms in benzene?
    A: sp².
57. Q57: What is the geometry around each carbon atom in benzene?
    A: Trigonal Planar (forming a hexagonal ring).
58. Q58: What is the Resonance Energy of benzene?
    A: The difference in energy between the resonance hybrid and the most stable canonical form (≈150 kJ/mol).
59. Q59: What does the high resonance energy indicate about the stability of benzene?
    A: Benzene is highly stable (thermodynamic stability).
60. Q60: What is the characteristic reaction type of aromatic hydrocarbons?
    A: Electrophilic Substitution Reactions.
61. Q61: Give the name of the intermediate formed in electrophilic substitution reactions of benzene.
    A: σ-complex or Arenium ion.
62. Q62: What is the purpose of FeCl₃ or AlCl₃ in the halogenation of benzene?
    A: To generate the stronger electrophile (Cl⁺).
63. Q63: Name the electrophile in the nitration of benzene.
    A: Nitronium ion (NO₂⁺).
64. Q64: Name the electrophile in the sulphonation of benzene.
    A: Sulphur Trioxide (SO₃).
65. Q65: Name the reaction where an alkyl group is introduced onto the benzene ring.
    A: Friedel-Crafts Alkylation.
66. Q66: Name the reaction where an acyl group is introduced onto the benzene ring.
    A: Friedel-Crafts Acylation.
67. Q67: What is the major limitation of Friedel-Crafts Alkylation?
    A: Polyalkylation (the product is more reactive than the reactant).
68. Q68: What is the product of the complete catalytic hydrogenation of benzene?
    A: Cyclohexane (C₆H₁₂).
69. Q69: What is the product of the reaction of benzene with Cl₂ under UV light?
    A: Benzene Hexachloride (BHC or Lindane), an addition product.
70. Q70: What type of groups are ortho and para directing?
    A: Electron-donating groups (+M or +I groups, e.g., -OH, -CH₃).
71. Q71: What type of groups are meta directing?
    A: Electron-withdrawing groups (−M or −I groups, e.g., -NO₂, -COOH).
72. Q72: What is the directing effect of the −CH₃ group on the benzene ring?
    A: Ortho and Para directing (weakly activating).
73. Q73: What is the directing effect of the −NO₂ group on the benzene ring?
    A: Meta directing (strongly deactivating).
74. Q74: Are halogens (-X) activating or deactivating toward electrophilic substitution?
    A: Deactivating (due to strong −I effect), but ortho-para directing (+M effect).
75. Q75: How is Benzene prepared from Phenol?
    A: By reduction using Zinc Dust (Zn).
76. Q76: How is Toluene oxidized to form Benzoic Acid?
    A: Using acidified KMnO₄ (Side chain oxidation).
Part 5: Preparation Methods & Miscellaneous
77. Q77: Name a method to prepare alkanes from alkyl halides using zinc.
    A: Reduction (R−X + Zn + H⁺ → R−H).
78. Q78: What is the catalyst used for the Kolbe's Electrolysis?
    A: No catalyst; it is an electrolytic reaction (Free Radical mechanism).
79. Q79: What is the limitation of Kolbe's electrolysis?
    A: It cannot be used to prepare methane (the product is always R−R).
80. Q80: How is Ethene prepared from Ethanol?
    A: Dehydration with conc. H₂SO₄ at 443 K.
81. Q81: How is Propene prepared from 1-Bromopropane?
    A: By Dehydrohalogenation using alcoholic KOH.
82. Q82: What is the type of reagent used for the dehydrohalogenation reaction?
    A: Alcoholic/ethanolic KOH (a strong base).
83. Q83: How is Ethyne prepared from Calcium Carbide (CaC₂)?
    A: Hydrolysis (CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂).
84. Q84: How is an alkyne prepared from a vicinal dihalide?
    A: By Double Dehydrohalogenation using NaNH₂ (sodamide).
85. Q85: What is the use of Decarboxylation in preparation?
    A: Reducing the number of C atoms in the product alkane by one.
86. Q86: How is Benzene prepared from ethyne?
    A: Cyclic Polymerization in a red-hot iron tube.
87. Q87: How is Toluene prepared from Benzene?
    A: Friedel-Crafts Alkylation using CH₃Cl/AlCl₃.
88. Q88: What is the Wurtz-Fittig reaction used for?
    A: To prepare alkyl-substituted benzenes (e.g., toluene) from aryl and alkyl halides.
89. Q89: Define Aromaticity (Hückel's Rule).
    A: The molecule must be planar, cyclic, fully conjugated, and have (4n+2)π electrons (where n=0, 1, 2, …).
90. Q90: Does cyclopropenyl cation follow Hückel's Rule?
    A: Yes (n=0, 2π electrons).
91. Q91: Does cyclobutadiene follow Hückel's Rule?
    A: No (it has 4π electrons, 4n system, thus anti-aromatic).
92. Q92: What is the term for a compound that does not follow Hückel's Rule but is fully conjugated and cyclic?
    A: Anti-aromatic.
93. Q93: What is the stability order: Aromatic, Anti-aromatic, Non-aromatic?
    A: Aromatic > Non-aromatic > Anti-aromatic.
94. Q94: What is the reason for the high boiling point of alkenes compared to alkanes of comparable mass?
    A: Alkenes are slightly polar, leading to stronger dipole-dipole forces.
95. Q95: How does the boiling point of alkanes change with branching?
    A: Decreases (due to decreased surface area and weaker van der Waals forces).
96. Q96: What is the role of the H⁺ ion in the electrophilic addition of HBr to alkenes?
    A: It acts as the electrophile that initiates the reaction by attacking the π-bond.
97. Q97: What is the main cause of the formation of 2-Bromopropane (major product) from propene and HBr?
    A: The formation of the more stable 2° carbocation intermediate.
98. Q98: What is the term for the replacement of a hydrogen atom in an aromatic ring by a halogen atom?
    A: Halogenation.
99. Q99: Name a reagent that causes hydroxylation of alkenes through anti-addition.
    A: Peroxyacids (e.g., RCO₃H) followed by hydrolysis.
100. Q100: What is the difference between elimination and substitution in alkyl halides?
     A: Elimination forms a π-bond; Substitution replaces the halogen with another group.
101. Q101: What is the relationship between E1 and SN¹ reaction mechanisms?
     A: Both share the same carbocation intermediate.
102. Q102: What is the effect of heat on the major product formation between E2 and SN² competition?
     A: Higher temperature favors E2 (Elimination).
103. Q103: What is the product of propene reacting with Cl₂ at high T (773 K)?
     A: Allyl Chloride (Substitution, not Addition).
104. Q104: What is the common name for the reaction of alkenes with O₂ in the presence of Ag at 523 K?
     A: Epoxidation (forms an epoxide).
105. Q105: Which isomer is formed when propene is oxidized with KMnO₄ in hot, acidic solution?
     A: CH₃COOH (Ethanoic acid) and CO₂ (from the CH₂ group).
106. Q106: Why does Cyclopentadiene act as a strong acid?
     A: Its conjugate base, Cyclopentadienyl anion, is aromatic (highly stable).
107. Q107: What is the shape of the Cyclohexane ring that is most stable?
     A: Chair form.
108. Q108: In the Kolbe's Electrolysis of sodium succinate, what is the product?
     A: Ethene (C₂H₄).
109. Q109: What is the product of heating n-hexane with Cr₂O₃/Al₂O₃ at 773 K?
     A: Benzene (Aromatization).
110. Q110: What is the difference between a meta-directing group's effect on reactivity and an ortho-para directing group's effect?
     A: Meta groups deactivate the ring; O/P groups usually activate (except halogens).
111. Q111: What is the primary cause of the low reactivity of alkanes?
     A: The presence of only strong σ bonds (C-C and C-H) and their non-polar nature.
112. Q112: What is the formula used to calculate the degree of unsaturation (Double Bond Equivalent)?
     A: DBE = C − H/2 − X/2 + N/2 + 1.

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