50 questions on this topic of Some Basic Principles of Organic Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.
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Answer: (D) NO₂⁻ and CN⁻
Ambident nucleophiles possess two nucleophilic centers through either of which they can attack an electrophile. Nitrite ion (:NO₂⁻) can attack through nitrogen or oxygen, and cyanide ion (:CN⁻) can attack through carbon or nitrogen.
Answer: (A) Both Statement I and Statement II are correct
The triphenylmethyl radical is remarkably stable due to resonance delocalization across the three aromatic rings, and bulky phenyl groups provide steric protection against dimerization. It exists in dynamic equilibrium with its quinoid dimer in solution. Both statements are factually correct.
Answer: (A) Its positive charge is delocalised over the benzene ring by resonance
Resonance spreads the positive charge over the ortho and para positions of the ring, stabilising benzyl (and allyl) carbocations.
NCERT Class 11
Answer: (A) Both Statement I and Statement II are true
The cyclopentadienyl anion has 6 π electrons (4 from two double bonds and 2 from the lone pair) in a planar, cyclic, fully conjugated ring system of five sp² carbons. According to Hückel's rule [4n + 2 where n = 1], it is aromatic and consequently exhibits remarkable thermodynamic stability. Thus, both statements are true.
Answer: (C) (I) > (II) > (III) > (IV)
The stability of a carbanion increases with an increase in the s-character of the carbon holding the negative charge because higher s-character leads to greater electronegativity. The hybridization states are sp (50% s) for HC≡C⁻, sp² (33.3% s) for CH₂=CH⁻, and sp³ (25% s) for alkyl carbanions; among alkyl carbanions, electron-donating methyl groups destabilize the anion via the +I effect, making primary more stable than secondary.
Answer: (B) CCl₃⁻ is more stable than CF₃⁻ because chlorine can delocalize the negative charge into its vacant 3d-orbitals via dπ-pπ back bonding.
Even though fluorine is more electronegative than chlorine, CCl₃⁻ is much more stable than CF₃⁻. In CCl₃⁻, the unshared pair of electrons on carbon is delocalized into the empty 3d-orbitals of the chlorine atoms (pπ-dπ bonding), which is impossible for fluorine as it lacks vacant d-orbitals in its valence shell.
Answer: (C) 9
Hyperconjugation in carbocations occurs via overlap of σ(C-H) bonds of α-carbons with the empty p-orbital of the carbocation center. The tert-butyl carbocation has three equivalent methyl groups attached to the C⁺ center, giving 3 × 3 = 9 α-hydrogen atoms, and therefore 9 hyperconjugative structures can be drawn.
Answer: (B) SO₃ and AlCl₃
Both SO₃ and AlCl₃ are electrophiles. In SO₃, the sulfur atom is bonded to three electronegative oxygen atoms and carries a strong partial positive charge. AlCl₃ has an electron-deficient aluminum atom with an incomplete octet (six electrons). NH₃ and CN⁻ are nucleophiles, and H₃O⁺ is already protonated and cannot accept an electron pair at oxygen without bond cleavage.
Answer: (C) p-Methoxybenzyl carbocation
The p-methoxy group (-OCH₃) exhibits a powerful electron-donating resonance effect (+R or +M) via its lone pair of electrons, which strongly disperses the positive charge on the benzylic carbon. Although -CH₃ donates electron density through hyperconjugation (+H) and inductive effect (+I), the +R effect of -OCH₃ is much stronger. The -NO₂ and -Cl groups are destabilizing relative to -OCH₃ due to electron withdrawal.
Answer: (C) Cyclopropenyl cation (C₃H₃⁺)
The cyclopropenyl cation (C₃H₃⁺) is planar, fully conjugated, cyclic, and possesses (4n + 2) π-electrons with n = 0 (2 π-electrons). Thus, it satisfies Hückel's rule and is aromatic, imparting extraordinary stability.
Answer: (D) 8
Hyperconjugation in a carbocation depends on the number of α-hydrogens directly attached to the carbons adjacent to the positively charged carbon (C⁺). In 2-methylbutan-2-yl cation, the C⁺ is attached to two methyl groups (3 + 3 = 6 α-H) and one methylene group (2 α-H), giving a total of 6 + 2 = 8 α-hydrogens.
Answer: (B) Statement I is correct but Statement II is incorrect
Statement I is correct because the central carbon of a carbanion carries a lone pair of electrons which it can donate, behaving as a Lewis base. Statement II is incorrect because simple alkyl carbanions are sp³-hybridized and possess a pyramidal (trigonal pyramidal) geometry, similar to ammonia.