Organic Chemistry is one of the most important and scoring parts of Class 12 Chemistry. In board exams, a significant number of questions are based on reasoning – the “why” type reasoning questions that test your conceptual understanding rather than just memory. These questions are often tricky because they require clear logic, not rote learning.
To help students prepare effectively, we’ve compiled a complete collection of important reasoning questions and answers from Class 12 Organic Chemistry. This covers all the major chapters – Haloalkanes & Haloarenes, Alcohols, Phenols & Ethers, Aldehydes & Ketones, Carboxylic Acids, and Amines.
Each question is answered in a concise, exam-ready format, making it easier for you to revise quickly before exams. Whether it’s about the polarity of C–X bonds, the acidity of phenols, the stability of carbocations, or the reactivity of diazonium salts – you’ll find everything explained with proper reasoning.
This blog will not only strengthen your concepts but also boost your confidence in tackling application-based questions in the CBSE/State Board exams. Mastering these will ensure you secure full marks in the reasoning-based section of organic chemistry.
Reasoning Questions : Haloalkanes and Haloarenes
1. Why is the C–X bond in alkyl halides polar?
Answer: The C–X bond in alkyl halides is polar because halogens are more electronegative than carbon, pulling electron density toward themselves.
2. Why are alkyl halides insoluble in water but soluble in organic solvents?
Answer: Alkyl halides are insoluble in water (cannot form strong H-bonds with H₂O) but dissolve in organic solvents due to similar van der Waals interactions.
3. Why do alkyl halides have higher boiling points than alkanes of comparable molecular mass?
Answer: Alkyl halides have higher boiling points than alkanes of similar mass because of stronger dipole–dipole and van der Waals interactions.
4. Why does the boiling point increase with increase in the size of the halogen atom in alkyl halides?
Answer: Boiling point increases with halogen size (Cl < Br < I) as larger atoms give stronger London dispersion forces.
5. Why is the C–Cl bond stronger than the C–I bond?
Answer: The C–Cl bond is stronger than C–I because Cl is smaller and overlaps better with carbon, forming a stronger bond.
6. Why is the C–F bond shorter and stronger than other C–X bonds?
Answer: The C–F bond is shortest and strongest due to the very small size and high electronegativity of fluorine.
7. Why are alkyl halides reactive towards nucleophilic substitution reactions?
Answer: Alkyl halides are reactive towards nucleophiles because the polarized C–X bond makes carbon electrophilic.
8. Why do tertiary alkyl halides undergo nucleophilic substitution (SN1) more readily than primary halides?
Answer: Tertiary halides undergo SN1 faster because they form stable tertiary carbocations, aided by inductive and hyperconjugation effects.
9. Why do primary alkyl halides prefer SN2 mechanism over SN1?
Answer: Primary halides prefer SN2 since primary carbocations are unstable; backside attack occurs more easily with less steric hindrance.
10. Why is the reactivity order of halides towards SN1: tertiary > secondary > primary?
Answer: SN1 reactivity: 3° > 2° > 1° due to carbocation stability.
11. Why is the reactivity order of halides towards SN2: methyl > primary > secondary > tertiary?
Answer: SN2 reactivity: CH₃X > 1° > 2° > 3° due to steric hindrance — least hindered reacts fastest.
12. Why is fluoroalkane less reactive than other haloalkanes towards nucleophilic substitution?
Answer: Fluoroalkanes are less reactive since the C–F bond is very strong and not easily broken.
13. Why is iodoalkane more reactive than chloroalkane towards nucleophilic substitution?
Answer: Iodoalkanes are most reactive since C–I bond is weakest, hence cleaved easily.
14. Why are vinyl halides and aryl halides less reactive towards nucleophilic substitution than alkyl halides?
Answer: Vinyl and aryl halides are less reactive because the C–X bond has partial double bond character due to resonance.
15. Why is the C–Cl bond in chlorobenzene shorter than the C–Cl bond in chloroethane?
Answer: In chlorobenzene, the C–Cl bond is shorter than in chloroethane due to resonance (C–Cl partial double bond).
16. Why does chlorobenzene not undergo nucleophilic substitution easily?
Answer: Chlorobenzene does not undergo SN easily because the C–Cl bond is strengthened by resonance and the C is less electrophilic.
17. Why does chlorobenzene give electrophilic substitution more easily than benzene?
Answer: Chlorobenzene undergoes electrophilic substitution more readily than benzene because –Cl donates electrons via resonance, activating the ring.
18. Why does KCN give cyanide with alkyl halide but AgCN gives isocyanide?
Answer: KCN (ionic) attacks via C atom → cyanides (R–C≡N), while AgCN (covalent) attacks via N atom → isocyanides (R–N≡C).
19. Why is the dipole moment of chlorobenzene less than that of cyclohexyl chloride?
Answer: Dipole moment of chlorobenzene is less than cyclohexyl chloride because of cancellation of bond dipole due to resonance.
20. Why do alkyl halides undergo elimination reactions in the presence of strong bases?
Answer: Strong bases abstract β-H from alkyl halides, leading to elimination (β-elimination → alkene).
21. Why does the ease of dehydrohalogenation follow the order: tertiary > secondary > primary?
Answer: Ease of dehydrohalogenation: 3° > 2° > 1° due to carbocation stability in E1 and hyperconjugation stabilizing alkenes.
22. Why do allyl halides and benzyl halides undergo nucleophilic substitution faster than alkyl halides?
Answer: Allyl and benzyl halides undergo substitution faster because resonance stabilizes transition states and carbocations.
23. Why is the C–X bond in vinyl chloride stronger than that in alkyl chloride?
Answer: C–X bond in vinyl chloride is stronger due to sp²–C–X bond (greater s-character → stronger bond).
24. Why are alkyl halides used as solvents and propellants (though many are now banned)?
Answer: Alkyl halides are used as solvents/propellants due to volatility and ability to dissolve non-polar compounds (though many banned due to toxicity/ozone damage).
25. Why is chlorofluorocarbon (CFC) environmentally harmful?
Answer: CFCs are harmful because UV light breaks them, releasing Cl· radicals that destroy ozone in the stratosphere.
Reasoning Questions : Alcohols, Phenols and Ethers
26. Why do alcohols have higher boiling points than alkanes and haloalkanes of comparable molecular mass?
Answer: Alcohols have higher boiling points than alkanes/haloalkanes due to intermolecular hydrogen bonding.
27. Why are lower alcohols soluble in water but solubility decreases with increase in molecular mass?
Answer: Lower alcohols are soluble in water (H-bonding with H₂O), but solubility decreases as hydrophobic alkyl chain increases.
28. Why is phenol more acidic than ethanol?
Answer: Phenol is more acidic than ethanol because the phenoxide ion is resonance-stabilized, whereas ethoxide is not.
29. Why is phenol less acidic than carboxylic acid?
Answer: Phenol is less acidic than carboxylic acids because carboxylate ion has two resonance structures with charge delocalized over two O atoms.
30. Why does electron-withdrawing substituents (–NO₂, –CN) increase the acidity of phenols?
Answer: –NO₂/–CN withdraw electrons, stabilizing the phenoxide ion, hence increasing acidity.
31. Why does electron-donating substituents (–CH₃, –OCH₃) decrease the acidity of phenols?
Answer: –CH₃/–OCH₃ donate electrons, destabilizing phenoxide ion, decreasing acidity.
32. Why is the acidity order of nitrophenols: p-nitrophenol > o-nitrophenol > m-nitrophenol?
Answer: Acidity order: o-nitrophenol > p-nitrophenol > m-nitrophenol due to stronger –I and –R effects at ortho/para vs only –I at meta. But that is incorrect because of intermolecular H-bonding in o-nitrophenol, proton cannot be donated easily, making it less acidic than p-nitrophenol. So the correct acidic order is: p-nitrophenol > o-nitrophenol > m-nitrophenol
33. Why is ortho-nitrophenol less soluble in water than p-nitrophenol?
Answer: o-nitrophenol is less soluble in water than p-nitrophenol because of intramolecular H-bonding (reduces H-bonding with water).
34. Why does phenol undergo electrophilic substitution more readily than benzene?
Answer: Phenol undergoes electrophilic substitution faster than benzene due to +R effect of –OH increasing electron density in the ring.
35. Why does phenol undergo bromination with aqueous bromine water easily, while benzene requires a catalyst?
Answer: Phenol reacts with Br₂ water directly (gives 2,4,6-tribromophenol) because –OH strongly activates the ring, unlike benzene which needs a catalyst.
36. Why does phenol give a violet colour with neutral FeCl₃ solution?
Answer: Phenol gives violet colour with FeCl₃ due to complex formation between Fe³⁺ and the phenoxide ion.
37. Why does dehydration of alcohols give different products under mild vs. drastic conditions?
Answer: Dehydration of alcohols: mild conditions → more stable alkene (Zaitsev’s rule), drastic conditions → rearrangements/mixtures possible.
38. Why is the reactivity order of alcohols towards Lucas reagent: tertiary > secondary > primary?
Answer: Lucas test: 3° > 2° > 1° because carbocation formation is fastest for tertiary alcohols.
39. Why does tertiary carbocation form more easily than primary carbocation during dehydration of alcohols?
Answer: Tertiary carbocation forms more easily than primary due to inductive and hyperconjugation stabilization.
40. Why are allyl and benzyl alcohols unusually reactive towards oxidation and substitution?
Answer: Allyl and benzyl alcohols are unusually reactive because resonance stabilizes carbocation and radicals formed during reactions.
41. Why does phenol not undergo nucleophilic substitution under normal conditions like haloarenes?
Answer: Phenol does not undergo nucleophilic substitution like haloarenes because C–O bond is very strong and stabilized by resonance.
42. Why is the C–O bond in phenol stronger than the C–O bond in alcohols?
Answer: C–O bond in phenol is stronger than in alcohols due to partial double bond character from resonance.
43. Why is methanol highly poisonous for humans?
Answer: Methanol is poisonous as it metabolizes to formaldehyde and formic acid, which damage optic nerves and cause acidosis.
44. Why does Williamson synthesis fail for preparing aryl ethers directly?
Answer: Williamson synthesis fails for aryl ethers because aryl halides do not undergo SN2 (C–X bond has partial double bond character).
45. Why is Williamson ether synthesis carried out with primary halides, not tertiary halides?
Answer: Williamson synthesis uses primary halides since tertiary halides undergo elimination instead of substitution.
46. Why are the ethers less reactive compared to alcohols and phenols?
Answer: Ethers are less reactive than alcohols/phenols because they lack O–H bond and cannot show strong H-bonding or acidity.
47. Why do ethers form explosive peroxides on standing in air?
Answer: Ethers form explosive peroxides on standing in air because oxygen reacts with them at α-hydrogen position forming unstable peroxides.
48. Why is the boiling point of ethers lower than isomeric alcohols?
Answer: Ethers have lower boiling points than isomeric alcohols because they cannot form intermolecular hydrogen bonds.
49. Why is anisole more reactive than benzene towards electrophilic substitution?
Answer: Anisole is more reactive than benzene towards electrophilic substitution because –OCH₃ group is strongly activating by +R effect.
50. Why is the electrophilic substitution in anisole directed to ortho and para positions?
Answer: In anisole, electrophilic substitution occurs at ortho and para positions due to resonance donation of –OCH₃.
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Reasoning Questions : Aldehydes, Ketones and Carboxylic Acids
51. Why is the carbonyl carbon in aldehydes and ketones electrophilic in nature?
Answer: The carbonyl carbon is electrophilic because C=O bond is polarized (C δ⁺, O δ⁻).
52. Why are aldehydes more reactive than ketones towards nucleophilic addition reactions?
Answer: Aldehydes are more reactive than ketones since they are less hindered and their carbocation intermediate is less destabilized.
53. Why does formaldehyde react faster than acetaldehyde in nucleophilic addition?
Answer: Formaldehyde reacts faster than acetaldehyde because it has no electron-donating alkyl group, making carbonyl C more electrophilic.
54. Why do aromatic aldehydes react more slowly than aliphatic aldehydes in nucleophilic addition reactions?
Answer: Aromatic aldehydes react slower than aliphatic aldehydes because resonance in Ar–CHO reduces electrophilicity.
55. Why does benzaldehyde not undergo aldol condensation easily?
Answer: Benzaldehyde does not undergo aldol easily because the α-hydrogen is not present. So, it undergo Cannizaro reaction.
56. Why do aldehydes and ketones undergo addition reactions, whereas alkenes undergo electrophilic addition?
Answer: Aldehydes/ketones undergo nucleophilic addition, whereas alkenes undergo electrophilic addition due to C=O polarization.
57. Why does the boiling point of aldehydes/ketones lie between alkanes and alcohols?
Answer: Boiling points of aldehydes/ketones lie between alkanes (weak London forces) and alcohols (strong H-bonding) because they have dipole–dipole interactions but no H-bonding.
58. Why are lower aldehydes and ketones soluble in water but higher members are not?
Answer: Lower aldehydes/ketones are soluble in water (H-bonding with water), but solubility decreases with size as hydrophobic chain increases.
59. Why does formaldehyde polymerise easily to form trioxane/parafomaldehyde?
Answer: Formaldehyde polymerises easily to trioxane/parafomaldehyde due to high reactivity of its carbonyl group (no steric hindrance, strong electrophilicity).
60. Why does acetone undergo haloform reaction but acetaldehyde does not?
Answer: Acetone undergoes haloform reaction (methyl ketone test) while acetaldehyde does not, because acetone has –COCH₃ group that forms CHI₃.
61. Why is the α-hydrogen atom of aldehydes and ketones acidic?
Answer: The α-hydrogen in aldehydes/ketones is acidic due to resonance stabilization of the enolate ion formed after deprotonation.
62. Why is the enol form of acetylacetone particularly stable?
Answer: Enol form of acetylacetone is unusually stable due to intramolecular H-bonding and conjugation (keto–enol tautomerism).
63. Why is the carboxylic group planar and highly polar?
Answer: The carboxyl group (–COOH) is planar and highly polar because of resonance between C=O and C–OH bonds, delocalizing charge.
64. Why do carboxylic acids have higher boiling points than alcohols of comparable mass?
Answer: Carboxylic acids have higher boiling points than alcohols due to stronger intermolecular H-bonding and dimer formation.
65. Why are carboxylic acids stronger acids than phenols and alcohols?
Answer: They are stronger acids than phenols/alcohols because the carboxylate ion is resonance-stabilized with charge delocalized over two O atoms.
66. Why is formic acid stronger than acetic acid?
Answer: Formic acid is stronger than acetic acid because it lacks an electron-donating –CH₃ group, so the negative charge on the carboxylate ion is less destabilized.
67. Why is chloroacetic acid stronger than acetic acid?
Answer: Chloroacetic acid is stronger than acetic acid because the –Cl atom withdraws electrons by –I effect, stabilizing the carboxylate ion.
68. Why is trifluoroacetic acid one of the strongest simple carboxylic acids?
Answer: Trifluoroacetic acid is very strong because three –F atoms strongly withdraw electrons, stabilizing the conjugate base greatly.
69. Why does benzoic acid not undergo Friedel–Crafts reaction?
Answer: Benzoic acid does not undergo Friedel–Crafts because the –COOH group is strongly deactivating and also complexes with the Lewis acid catalyst.
70. Why is benzoic acid less acidic than formic acid?
Answer: Benzoic acid is less acidic than formic acid because the benzene ring donates electrons slightly (+R effect), reducing acidity.
71. Why does ortho-hydroxybenzoic acid (salicylic acid) have lower solubility in water than para-hydroxybenzoic acid?
Answer: o-hydroxybenzoic acid (salicylic acid) is less soluble than p-hydroxybenzoic acid because of intramolecular H-bonding in the ortho form, reducing interaction with water.
72. Why do dicarboxylic acids show higher acidity than monocarboxylic acids?
Answer: Dicarboxylic acids are more acidic than monocarboxylic acids because one –COOH group withdraws electrons (–I effect), stabilizing the conjugate base of the other.
73. Why does decarboxylation occur more readily in β-keto acids?
Answer: β-keto acids decarboxylate easily because the transition state is stabilized by keto–enol tautomerism and cyclic transition structure.
74. Why do carboxylic acids form dimers in vapour phase or in non-polar solvents?
Answer: Carboxylic acids form dimers in vapour phase/non-polar solvents via intermolecular hydrogen bonding.
75. Why does acetic acid show a sharp increase in boiling point due to hydrogen bonding dimer formation?
Answer: Acetic acid shows abnormally high boiling point due to strong H-bonded dimer formation, doubling molecular mass effectively.
Reasoning Questions : Amines
76. Why are amines basic in nature?
Answer: Amines are basic because nitrogen has a lone pair of electrons available for protonation.
77. Why is the basicity of amines attributed to the availability of a lone pair of electrons on nitrogen?
Answer: Their basicity arises from the availability of the nitrogen lone pair to accept H⁺.
78. Why are aliphatic amines stronger bases than ammonia?
Answer: Aliphatic amines are stronger bases than NH₃ because alkyl groups donate electrons (+I effect), increasing electron density on N.
79. Why are aromatic amines weaker bases than aliphatic amines?
Answer: Aromatic amines are weaker bases than aliphatic amines because resonance delocalizes the lone pair into the ring, reducing availability.
80. Why is aniline less basic than cyclohexylamine?
Answer: Aniline is less basic than cyclohexylamine because in aniline the lone pair is delocalized into the benzene ring.
81. Why does the basic strength of aliphatic amines decrease in aqueous solution compared to the gaseous state?
Answer: Basic strength of aliphatic amines decreases in aqueous solution compared to gas phase due to solvation and hydrogen bonding effects.
82. Why is the basicity order in gas phase: tertiary amine > secondary amine > primary amine > ammonia, but in aqueous solution it is different?
Answer: Gas phase order: 3° > 2° > 1° > NH₃ (inductive effect dominates).
Aqueous phase order differs because of solvation: 2° > 1° > 3° > NH₃.
83. Why is the basicity of amines affected by hydrogen bonding and solvation?
Answer: Basicity is affected by H-bonding and solvation: more H-bonding stabilizes the conjugate acid, enhancing basicity in solution.
84. Why is pKb of aniline higher than that of ammonia?
Answer: pKb of aniline is higher than NH₃ because its lone pair is less available due to resonance with the aromatic ring.
85. Why is aniline soluble in hydrochloric acid but not in water?
Answer: Aniline dissolves in HCl (forms soluble anilinium salt) but not in water (insufficient polarity for H-bonding).
86. Why does acylation of amines occur?
Answer: Amines undergo acylation because the lone pair on N attacks acyl groups, forming amides.
87. Why does aniline not undergo Friedel–Crafts reaction under normal conditions?
Answer: Aniline does not undergo Friedel–Crafts normally because it reacts with Lewis acid catalyst (AlCl₃), forming inactive complexes.
88. Why does aniline give a coloured product with bromine water directly, but not benzene?
Answer: Aniline reacts with bromine water directly, giving coloured tribromoaniline, whereas benzene requires a catalyst due to lack of activation.
89. Why does acetanilide give a monosubstituted product with bromine water, unlike aniline?
Answer: Acetanilide gives mono-bromo product because the –NHCOCH₃ group moderates the activating effect of –NH₂, preventing poly-substitution.
90. Why does diazotisation occur only with primary aromatic amines and not secondary or tertiary?
Answer: Diazotisation occurs only with primary aromatic amines because they form stable diazonium salts, whereas secondary/tertiary amines do not.
91. Why are diazonium salts stable only in cold aqueous solution (0–5 °C)?
Answer: Diazonium salts are stable only at 0–5 °C in aqueous solution; above this temperature they decompose to give phenols or other products.
92. Why do tertiary amines not give nitrous acid test?
Answer: Tertiary amines do not give nitrous acid test because they cannot form diazonium salts (no NH₂ group).
93. Why does the order of boiling points of isomeric amines change with branching?
Answer: Boiling points of isomeric amines change with branching because branching reduces surface area and weakens van der Waals forces.
94. Why do primary amines have higher boiling points than tertiary amines of comparable molecular mass?
Answer: Primary amines have higher boiling points than tertiary amines of similar mass due to stronger intermolecular hydrogen bonding.
95. Why does the alkylation of amines often lead to a mixture of products instead of a single alkylated amine?
Answer: Alkylation of amines gives mixtures because the product amines are also nucleophilic and undergo further alkylation.
96. Why do aromatic primary amines undergo diazotisation but aliphatic primary amines do not give stable diazonium salts?
Answer: Aromatic primary amines undergo diazotisation (stable aryl diazonium salts), whereas aliphatic primary amines give unstable diazonium salts that decompose.
97. Why are aryl diazonium salts useful intermediates in organic synthesis?
Answer: Aryl diazonium salts are useful intermediates because they can be converted to a wide variety of compounds (halides, phenols, azo dyes, etc.).
Conclusion
Reasoning questions in Class 12 Organic Chemistry are designed to test your conceptual clarity and logical thinking. By practicing and revising the questions given here, you’ll develop a deeper understanding of reaction mechanisms, bond strengths, acidity/basicity trends, and the role of substituents.
For board exams, always remember:
- Understand the “why” behind every reaction, not just the “how.”
- Write answers in short, precise statements – examiners prefer clarity over long explanations.
- Revise regularly so these concepts stay fresh in your mind before the exam.
Mastering these reasoning questions will not only help you score full marks in the exam but also give you a strong foundation for competitive exams like JEE, NEET, and other entrance tests.
So, keep practicing, focus on concepts, and Organic Chemistry will become one of your strongest scoring areas. 🚀
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