Multiple Choice Questions (MCQs) for JEE Chemistry: Organic Chemistry Some Basic Principles and Techniques
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Practice Organic Chemistry Some Basic Principles and Techniques MCQs for JEE Chemistry
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Multiple Choice Questions
Assertion (A): Tertiary carbocations are generally formed more easily than primary carbocations.
Reason (R): Hyperconjugative as well as inductive effects of the additional alkyl groups stabilise tertiary carbocations.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true: stability order 3° > 2° > 1°, so 3° cations form more readily.
2. Reason is true: more alkyl groups mean more α C–H bonds (hyperconjugation) and more +I donation.
3. The reason explains the assertion.
Teacher's Note:
1. Stability of the cation decides ease of formation.
2. Both effects increase with the number of alkyl groups.
Assertion (A): Carbanions have an ammonia-like pyramidal shape.
Reason (R): The carbon atom carrying the negative charge has an octet of electrons.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Assertion is true: a carbanion carbon is sp³ with three bond pairs and one lone pair, like NH₃.
2. Reason is true: the carbon has 8 electrons.
3. But the octet is not what causes the pyramidal shape; the geometry follows from the four electron pairs (three bonding, one lone). So R does not explain A.
Teacher's Note:
1. Shape comes from electron-pair repulsion.
2. An octet alone does not fix the geometry.
Assertion (A): Pyrrole is a weaker base than its hydrogenated product, pyrrolidine.
Reason (R): The lone pair of electrons on nitrogen is delocalised in pyrrole.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true: pyrrolidine is a secondary aliphatic amine and strongly basic; pyrrole is almost non-basic.
2. Reason is true and is the cause: in pyrrole the nitrogen lone pair is part of the aromatic sextet and cannot be donated to a proton.
Teacher's Note:
1. A delocalised lone pair is not available for protonation.
2. Pyrrole is protonated on carbon rather than nitrogen.
Assertion (A): A mixture of o-nitrophenol and p-nitrophenol can be separated by steam distillation.
Reason (R): o-Nitrophenol is steam volatile but p-nitrophenol is not.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true.
2. Reason is true and explains it: o-nitrophenol has intramolecular hydrogen bonding, so it is less associated and volatile, whereas p-nitrophenol has intermolecular hydrogen bonds and is not steam volatile.
Teacher's Note:
1. Intramolecular H-bonding lowers boiling point.
2. Steam distillation carries over the volatile isomer.
Assertion (A): The carbocation CF₃–CH₂⁺ is less stable than CF₃⁺.
Reason (R): In CF₃–CH₂⁺ the strongly electron-withdrawing −CF₃ group intensifies the positive charge, but in CF₃⁺ the lone pairs on each of the three F atoms overlap with the empty p-orbital of the carbon and disperse the positive charge.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion: the −I effect of CF₃ strongly destabilises CF₃CH₂⁺, while in CF₃⁺ each F donates a lone pair (+M) to the empty p orbital; relative stabilities are as stated.
2. The reason describes exactly these two effects, so it explains the assertion.
Teacher's Note:
1. Compare −I destabilisation with +M stabilisation.
2. Halogen lone pairs can stabilise an adjacent cation centre.
Assertion (A): The acidity of the compounds decreases in the order HC≡CH > CH₂=CH₂ > CH₃–CH₃.
Reason (R): The acidity of the compounds increases as the electronegativity of the carbon to which H is attached increases.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true: pKₐ about 25, 44 and 50 respectively.
2. Reason is true: more s-character (sp > sp² > sp³) means greater electronegativity of carbon, which stabilises the carbanion and explains the order.
Teacher's Note:
1. s-character governs both electronegativity and acidity here.
2. Ethyne is the strongest acid of the three.
Assertion (A): The acidity of alcohols follows the order 1° > 2° > 3°.
Reason (R): The +I effect of the additional alkyl groups favours the cleavage of the O–H bond.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (c) A is true, but R is false.
Explanation:
1. Assertion is true: methanol > primary > secondary > tertiary in solution.
2. Reason is false: the +I effect of alkyl groups pushes electrons onto oxygen and makes the O–H bond harder to break (it destabilises the alkoxide).
Teacher's Note:
1. Alkyl groups reduce acidity.
2. Steric hindrance to solvation also lowers the acidity of tertiary alcohols.
Assertion (A): Phenol is less acidic than p-nitrophenol.
Reason (R): The phenoxide ion is more stable than the p-nitrophenoxide ion.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Assertion is true: the −M and −I effects of NO₂ stabilise the p-nitrophenoxide ion (pKₐ 7.15 against 10.0 for phenol).
2. Reason is false: it is the p-nitrophenoxide ion that is more stable.
Teacher's Note:
1. Electron-withdrawing groups strengthen phenols.
2. A more stable conjugate base means a stronger acid.
Assertion (A): Pyrrole is more basic than pyridine.
Reason (R): In pyrrole, nitrogen is sp³-hybridised.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. Assertion is false: pyridine (pKₐ of its conjugate acid 5.2) is basic, but the pyrrole lone pair is part of the aromatic sextet and pyrrole is essentially non-basic.
2. Reason is false: nitrogen in pyrrole is sp²-hybridised.
Teacher's Note:
1. Check the hybridisation and the lone pair's role.
2. Pyridine's lone pair lies in the plane and is free.
Assertion (A): Aniline is a weaker base than benzylamine.
Reason (R): In aniline, mesomeric interaction occurs between the benzene ring and the amino group.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true.
2. Reason is true and explains it: the lone pair on nitrogen is delocalised into the ring in aniline, whereas in benzylamine the CH₂ group blocks conjugation.
Teacher's Note:
1. Mesomeric delocalisation reduces availability of the lone pair.
2. Benzylamine behaves like an alkylamine.
Assertion (A): The cyclopentadienyl anion is much more stable than the allyl anion.
Reason (R): The cyclopentadienyl anion is aromatic in character.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true.
2. Reason is true and is the cause: the anion is planar, cyclic, fully conjugated with 6 π electrons (4n + 2, n = 1), so it gets aromatic stabilisation.
Teacher's Note:
1. Hückel's rule: 4n + 2 π electrons.
2. This is why cyclopentadiene (pKₐ about 16) is unusually acidic.
Assertion (A): Acetic acid is a stronger acid than ethyl alcohol.
Reason (R): In the acetate ion both C–O bond lengths are equal.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true (pKₐ 4.76 against about 16).
2. Reason is true and explains it: equal C–O bonds show the negative charge is delocalised over both oxygens, which stabilises the acetate ion; the ethoxide charge is localised on one oxygen.
Teacher's Note:
1. Resonance in the conjugate base increases acidity.
2. Equal bond lengths are evidence of delocalisation.
Assertion (A): The tropylium ion is more stable than the benzyl cation.
Reason (R): The tropylium ion is anti-aromatic in nature.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Assertion is true: tropylium has 6 π electrons in a cyclic, planar, conjugated system, so it is aromatic.
2. Reason is false: it is aromatic, not anti-aromatic.
Teacher's Note:
1. Aromatic means 4n + 2 π electrons.
2. Anti-aromatic systems are destabilised.
Assertion (A): Phenol is more acidic than benzyl alcohol.
Reason (R): The phenoxide ion (C₆H₅O⁻) is more stable than the benzyloxide ion (C₆H₅CH₂O⁻).
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true (pKₐ about 10 against about 15).
2. Reason is true and explains it: the charge on phenoxide is delocalised into the ring, but in benzyloxide the CH₂ group isolates the oxygen from the ring.
Teacher's Note:
1. A resonance-stabilised conjugate base means a stronger acid.
2. Benzyl alcohol behaves like an ordinary alcohol.
Assertion (A): Picric acid is a stronger acid than phenol.
Reason (R): Picric acid has a –COOH group, while phenol has an –OH group.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Assertion is true: three nitro groups stabilise the 2,4,6-trinitrophenoxide ion and the pKₐ is about 0.4.
2. Reason is false: picric acid (2,4,6-trinitrophenol) has an OH group, not a COOH group.
Teacher's Note:
1. Do not confuse the name 'picric acid' with a carboxylic acid.
2. The acidity comes from three NO₂ groups.
Assertion (A): Pyridine is less basic than trimethylamine.
Reason (R): The nitrogen atom in pyridine is sp²-hybridised, while in trimethylamine it is sp³-hybridised.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true (conjugate acid pKₐ about 5.2 for pyridine and 9.8 for trimethylamine).
2. Reason is true and explains it: the lone pair in an sp² orbital (33% s) is held more tightly than in an sp³ orbital (25% s).
Teacher's Note:
1. More s-character means a less available lone pair.
2. Also the +I groups raise the basicity of trimethylamine.
Assertion (A): The enol form of cyclohexane-1,3,5-trione is more stable than its keto form.
Reason (R): Cyclohexane-1,3,5-trione contains α-hydrogen atoms.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Assertion is true: the enol form is phloroglucinol (1,3,5-trihydroxybenzene), which is aromatic.
2. Reason is true, since the CH₂ groups are α to the carbonyls, but α-hydrogens only make enolisation possible; they do not make the enol more stable. The aromaticity does.
Teacher's Note:
1. Enolisation requires α-H.
2. Stability of the enol comes from aromatic stabilisation.
Assertion (A): Tautomers have different functional groups.
Reason (R): Tautomers have the same molecular formula.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Assertion is true: for example the keto form has C=O and the enol form has C=C and OH.
2. Reason is true: tautomers are isomers, so the molecular formula is the same.
3. But same formula does not explain why the functional groups differ; it is the migration of a hydrogen that does so.
Teacher's Note:
1. Tautomers are functional-group isomers in equilibrium.
2. Same formula is true of all isomers.
Assertion (A): The compounds CH₃CH₂NO₂, (CH₃)₂CHNO₂ and CH₃CH₂CH(NO₂)CH₃ are all soluble in NaOH.
Reason (R): All the above compounds have lower boiling points than CH₃NO₂.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Assertion is true: each has an α-hydrogen, can form the acidic aci-nitro form and dissolves in alkali.
2. Reason is false: these larger molecules have higher boiling points than nitromethane.
Teacher's Note:
1. Boiling point rises with molar mass in a homologous series.
2. Solubility in NaOH depends on α-H.
Assertion (A): Formic acid is a stronger acid than benzoic acid.
Reason (R): The pKₐ of formic acid is higher than that of benzoic acid.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Assertion is true: pKₐ of formic acid is 3.75 against 4.20 for benzoic acid.
2. Reason is false: a stronger acid has a lower pKₐ, so formic acid's pKₐ is lower.
Teacher's Note:
1. Stronger acid means lower pKₐ.
2. The phenyl group in benzoic acid releases electrons by resonance and weakens it slightly.
Assertion (A): Benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane) is an aromatic compound.
Reason (R): It contains three π bonds in the ring.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. Assertion is false: the ring is saturated (cyclohexane skeleton); benzene hexachloride is made by adding 3 Cl₂ to benzene.
2. Reason is false: all three π bonds of benzene were used up in the addition, so none remain.
Teacher's Note:
1. Do not confuse benzene hexachloride with hexachlorobenzene.
2. Addition destroys the aromatic ring.
Assertion (A): Dimethylamine is more basic than trimethylamine in aqueous solution.
Reason (R): It is due to the solvation effect.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true: pKb is 3.27 for dimethylamine and 4.19 for trimethylamine.
2. Reason is true and explains it: the ammonium ion of the secondary amine has an N–H hydrogen-bonded to water, but the tertiary one has only one N–H and is solvated less, so its stability does not offset the +I gain.
Teacher's Note:
1. In water, 2° amines are stronger bases than 3° amines.
2. Solvation of the cation is the key.
Assertion (A): Benzylamine is more basic than ethylamine.
Reason (R): The phenyl group has a +I effect.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. Assertion is false: the phenyl group is −I (sp² carbons are more electronegative), so benzylamine (pKb 4.7) is a weaker base than ethylamine (pKb 3.3).
2. Reason is false for the same reason.
Teacher's Note:
1. Phenyl withdraws electrons by induction.
2. Alkyl groups donate, aryl groups withdraw.
Assertion (A): Triethylamine is more basic than trimethylamine in aqueous solution.
Reason (R): The +I effect of the ethyl group is more than that of the methyl group.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true (pKb about 3.25 against 4.19).
2. Reason is true and explains it: the larger +I effect of three ethyl groups makes the nitrogen lone pair more available and stabilises the ammonium ion.
Teacher's Note:
1. Larger alkyl group means stronger +I effect.
2. Steric hindrance only becomes limiting for much larger groups.
Assertion (A): In the compound o-nitrophenol, intramolecular hydrogen bonding is present, so its boiling point is lower than that of p-nitrophenol.
Reason (R): Intramolecular hydrogen bonding lowers the molecular association.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Assertion is true: o-nitrophenol boils at a lower temperature (it is steam volatile) than p-nitrophenol.
2. Reason is true and explains it: chelation uses the O–H for an internal bond, so fewer intermolecular bonds form.
Teacher's Note:
1. Intermolecular bonds raise boiling points.
2. Intramolecular bonds do not.
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