Organic Intermediates: Carbanions formation, stability and applications MCQs With Answer

Introduction: Organic Intermediates: Carbanions formation, stability and applications MCQs With Answer is designed for M.Pharm students preparing for MPC 102T Advanced Organic Chemistry I. This quiz-focused blog emphasizes key concepts: how carbanions are generated, factors controlling their stability (hybridization, resonance, inductive effects, hyperconjugation), and practical applications in synthesis (enolate chemistry, nucleophilic additions, organometallic reagents, E1cb pathways). Questions address mechanistic details, reagent selection (LDA, n-BuLi, NaNH2, Grignard), acidity trends and special stabilized carbanions like cyclopentadienyl. Each MCQ reinforces theoretical understanding and problem-solving skills required for advanced organic transformations encountered in pharmaceutical research and formulation chemistry.

Q1. Which base and conditions are most suitable for forming the kinetic enolate (carbanion) from a non-enolizable ketone at low temperature?

• LDA in THF at -78°C
• Sodium ethoxide in ethanol at room temperature
• Pyridine at 0°C
• HCl in water

Correct Answer: LDA in THF at -78°C

Q2. Which of the following factors most strongly stabilizes a carbanion?

• Electron-withdrawing groups conjugated to the carbanionic center
• Increased alkyl substitution via hyperconjugation
• Greater alkene character (sp2) compared with alkyne (sp)
• Presence of bulky steric groups adjacent to the anionic center

Correct Answer: Electron-withdrawing groups conjugated to the carbanionic center

Q3. Which carbanion is generally most stabilized?

• Benzyl carbanion (Ph-CH2–)
• Primary alkyl carbanion (e.g., CH3CH2–)
• Vinyl carbanion (H2C=CH–)
• Alkynyl carbanion (acetylide, HC≡C–)

Correct Answer: Benzyl carbanion (Ph-CH2–)

Q4. Which parent C–H bond is the most acidic, reflecting formation of the most stable carbanion?

• Terminal alkyne (ethyne, pKa ≈ 25)
• Alkene (ethene, pKa ≈ 44)
• Alkane (ethane, pKa ≈ 50)
• Alcohol (ethanol, pKa ≈ 16)

Correct Answer: Terminal alkyne (ethyne, pKa ≈ 25)

Q5. The reaction mechanism that explicitly proceeds via a discrete carbanion intermediate and is common in β-elimination of poor leaving groups is:

• E1cb mechanism
• E1 mechanism
• E2 mechanism
• SN1 mechanism

Correct Answer: E1cb mechanism

Q6. Which description best fits a phosphonium ylide used in the Wittig reaction?

• A neutral species with carbanion character adjacent to a positively charged phosphorus
• A stable free radical centered on carbon
• A neutral carbocation equivalent
• An anion localized entirely on phosphorus

Correct Answer: A neutral species with carbanion character adjacent to a positively charged phosphorus

Q7. Which reagent is commonly used to deprotonate a less acidic α-hydrogen selectively at low temperature to form the kinetic enolate?

• LDA (lithium diisopropylamide)
• Sodium hydroxide
• Piperidine
• Ammonium chloride

Correct Answer: LDA (lithium diisopropylamide)

Q8. Which substituent effect generally destabilizes a carbanion?

• Electron-donating alkyl groups via hyperconjugation
• Electron-withdrawing carbonyl conjugation
• Resonance delocalization into an adjacent aromatic ring
• Presence of adjacent nitro or cyano groups

Correct Answer: Electron-donating alkyl groups via hyperconjugation

Q9. Which reagent functions as a carbanion equivalent for nucleophilic addition to carbonyl compounds in laboratory synthesis?

• Grignard reagent (RMgX)
• Tertiary amine (e.g., triethylamine)
• Hydrochloric acid
• Azobisisobutyronitrile (AIBN)

Correct Answer: Grignard reagent (RMgX)

Q10. Which statement about the cyclopentadienyl anion (C5H5–) is correct?

• It is aromatic and stabilized by 6 π electrons
• It is non-aromatic and highly unstable
• It contains an sp-hybridized carbanion center only
• It lacks resonance and is less stable than a simple alkyl carbanion

Correct Answer: It is aromatic and stabilized by 6 π electrons

Q11. Considering hybridization only (ignoring resonance and inductive effects), which carbanion is most stable?

• sp-hybridized carbanion (acetylide)
• sp2-hybridized carbanion (vinylic)
• sp3-hybridized carbanion (alkyl)
• All have equal stability

Correct Answer: sp-hybridized carbanion (acetylide)

Q12. Directed ortho metalation (DoM) to form an aryl carbanion equivalent typically uses which strong base?

• n-Butyllithium (n-BuLi)
• Sodium bicarbonate
• Palladium catalyst with H2
• Tert-butanol

Correct Answer: n-Butyllithium (n-BuLi)

Q13. Which named reaction involves formation of an enolate (carbanion equivalent) that attacks another carbonyl compound to give a β-hydroxy carbonyl product?

• Aldol condensation
• Birch reduction
• Sandmeyer reaction
• Hofmann elimination

Correct Answer: Aldol condensation

Q14. Which statement about benzylic carbanions is true?

• The negative charge is delocalized into the aromatic ring, increasing stability
• The benzylic carbanion is always less stable than a primary alkyl carbanion
• Electron-donating groups on the ring always increase benzylic carbanion stability
• Benzylic carbanions cannot be formed by deprotonation

Correct Answer: The negative charge is delocalized into the aromatic ring, increasing stability

Q15. E1cb elimination is favored under which of the following conditions?

• Presence of a poor leaving group and an acidic α-hydrogen leading to a carbanion intermediate
• Excellent leaving group that departs first to form a carbocation
• Concerted removal of proton and leaving group without intermediate
• Very weak base and high dilution

Correct Answer: Presence of a poor leaving group and an acidic α-hydrogen leading to a carbanion intermediate

Q16. Which organometallic reagent is generally the most ionic and therefore most “carbanion-like” in reactivity?

• Organolithium (e.g., n-BuLi)
• Organocopper (Gilman reagent)
• Organotin reagent (e.g., R3SnLi)
• Neutral organosilicon reagent

Correct Answer: Organolithium (e.g., n-BuLi)

Q17. Which reaction pathway specifically uses enolates (carbanion equivalents) to form a carbon–carbon bond between two carbonyl compounds?

• Claisen condensation
• Nitration of benzene
• Hydrogenation of alkenes
• Benzoin condensation (uses acyl anion equivalent via cyanide, not enolate)

Correct Answer: Claisen condensation

Q18. Which structural feature most increases the acidity of an α-hydrogen (thus stabilizing the corresponding carbanion)?

• Presence of two adjacent electron-withdrawing carbonyl groups (1,3-dicarbonyl)
• Multiple adjacent alkyl substituents
• Alkoxy groups donating electrons by induction
• Absence of resonance stabilization

Correct Answer: Presence of two adjacent electron-withdrawing carbonyl groups (1,3-dicarbonyl)

Q19. Between allylic and vinylic carbanions, which is more stabilized and why?

• Allylic carbanion is more stabilized due to resonance delocalization
• Vinylic carbanion is more stabilized because of greater s-character
• Both have equal stability
• Neither can be stabilized by resonance

Correct Answer: Allylic carbanion is more stabilized due to resonance delocalization

Q20. Which reagent is commonly used to deprotonate a terminal alkyne to give the acetylide carbanion in synthetic practice?

• Sodium amide (NaNH2)
• Sodium chloride (NaCl)
• Potassium carbonate (K2CO3) in water
• Sodium acetate in ethanol

Correct Answer: Sodium amide (NaNH2)

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