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

Introduction: Organic Intermediates: Carbocations formation, stability and applications MCQs With Answer is designed for M.Pharm students studying MPC 102T Advanced Organic Chemistry I. This set of questions focuses on mechanistic aspects of carbocation generation, structural and electronic factors that control their stability (hyperconjugation, resonance, inductive effects, hybridization and neighboring group participation), common rearrangements, experimental detection and important synthetic applications such as SN1, E1, Friedel–Crafts and solvolysis reactions. The MCQs test both conceptual understanding and problem-solving ability required in pharmaceutical organic chemistry, reinforcing how carbocations influence reaction outcomes and drug-synthesis strategies.

Q1. Which fundamental process most commonly leads to formation of a carbocation intermediate in organic reactions?

• Heterolytic bond cleavage generating a cation and an anion
• Homolytic bond cleavage producing two radicals
• Nucleophilic attack on an electrophile
• Pericyclic concerted cycloaddition

Correct Answer: Heterolytic bond cleavage generating a cation and an anion

Q2. Which of the following carbocations is generally the most stable in simple alkyl series?

• Primary carbocation
• Secondary carbocation
• Tertiary carbocation
• Methyl carbocation

Correct Answer: Tertiary carbocation

Q3. Which interaction describes stabilization of a carbocation by overlap of adjacent C–H σ orbitals with the empty p orbital?

• Resonance delocalization
• Hyperconjugation
• Inductive electron withdrawal
• Hydrogen bonding

Correct Answer: Hyperconjugation

Q4. The 2-norbornyl cation is a classical example of which type of carbocation?

• Non-classical bridged carbocation with delocalized bonding
• Simple classical tertiary carbocation
• Free radical cation
• Carbanion stabilized by conjugation

Correct Answer: Non-classical bridged carbocation with delocalized bonding

Q5. How does carbocation stability affect the rate of an SN1 reaction?

• Greater carbocation stability decreases SN1 rate
• Greater carbocation stability increases SN1 rate
• Carbocation stability has no effect on SN1 rate
• SN1 rate depends only on nucleophile concentration

Correct Answer: Greater carbocation stability increases SN1 rate

Q6. According to the Hammond postulate, the transition state of an endothermic step most closely resembles which species?

• The reactants
• The products
• The intermediate of lowest energy
• An average of reactants and products irrespective of energetics

Correct Answer: The products

Q7. Which rearrangement commonly involves migration of an alkyl or hydride group to give a more stable carbocation?

• Wagner–Meerwein rearrangement (1,2-alkyl/hydride shift)
• Beckmann rearrangement
• Claisen rearrangement
• Buchner ring expansion

Correct Answer: Wagner–Meerwein rearrangement (1,2-alkyl/hydride shift)

Q8. Which experimental technique has been used to directly observe stabilized carbocations under strongly acidic conditions?

• Low-temperature NMR in superacid media
• Room-temperature IR spectroscopy in neutral solvents
• UV–Vis at ambient conditions
• Standard thin-layer chromatography (TLC)

Correct Answer: Low-temperature NMR in superacid media

Q9. What is the geometry and hybridization of the carbon bearing the positive charge in a typical planar carbocation?

• Tetrahedral, sp3
• Trigonal planar, sp2
• Linear, sp
• Square planar, dsp2

Correct Answer: Trigonal planar, sp2

Q10. Which of the following reaction mechanisms proceeds without formation of a discrete carbocation intermediate?

• E1 elimination
• SN1 nucleophilic substitution
• E2 elimination (concerted)
• Solvolysis via ionization

Correct Answer: E2 elimination (concerted)

Q11. The solvolysis of tert-butyl chloride in water follows which kinetic order, consistent with carbocation formation being rate-determining?

• Zero order
• First order
• Second order
• Pseudo-second order

Correct Answer: First order

Q12. Which substituent most strongly stabilizes a nearby carbocation by resonance donation when attached to an aromatic ring?

• Nitro group (–NO2)
• Fluoro substituent (–F)
• Methoxy group (–OCH3)
• Trifluoromethyl (–CF3)

Correct Answer: Methoxy group (–OCH3)

Q13. Arrange the following carbocations in order of increasing stability: methyl, primary, secondary, tertiary.

• tertiary < secondary < primary < methyl
• methyl < primary < secondary < tertiary
• primary < methyl < tertiary < secondary
• secondary < tertiary < primary < methyl

Correct Answer: methyl < primary < secondary < tertiary

Q14. In acid-catalyzed hydration of 3,3-dimethyl-1-butene, which major product is expected due to carbocation formation and rearrangement (Markovnikov addition)?

• Primary alcohol at the terminal carbon
• Secondary alcohol without rearrangement
• Tertiary alcohol resulting from carbocation stabilization
• Ether from intermolecular substitution

Correct Answer: Tertiary alcohol resulting from carbocation stabilization

Q15. Which synthetic transformation commonly proceeds via a carbocation generated by Lewis-acid activation and is widely used in aromatic substitution?

• Friedel–Crafts alkylation
• Wittig olefination
• Borane hydroboration
• Grignard addition to carbonyls

Correct Answer: Friedel–Crafts alkylation

Q16. Protonation of an alcohol followed by heterolytic C–O bond cleavage typically generates which intermediate?

• Carbanion
• Carbocation
• Radical anion
• Diradical

Correct Answer: Carbocation

Q17. What is the effect of neighboring group participation (anchimeric assistance) on the rate of solvolysis?

• It decreases the rate by stabilizing the leaving group
• It increases the rate by forming a stabilized bridged intermediate
• It has no effect because neighboring groups are too distant
• It converts the mechanism to a radical pathway

Correct Answer: It increases the rate by forming a stabilized bridged intermediate

Q18. Which type of 1,2-shift is most commonly observed to convert a less stable carbocation into a more stable one during rearrangement?

• Hydride shift (1,2-H shift)
• β-Elimination
• Radical coupling
• Pericyclic sigmatropic shift

Correct Answer: Hydride shift (1,2-H shift)

Q19. Consider carbocations at differently hybridized centers (sp3, sp2, sp). Which hybridization gives the most stabilized carbocation center?

• sp (most stable)
• sp2 (most stable)
• sp3 (most stable)
• All have equal stability

Correct Answer: sp3 (most stable)

Q20. Which description best characterizes the bonding in a non-classical carbocation like the bridged norbornyl ion?

• Localized two-center two-electron bond at the cationic carbon only
• Delocalization via a three-center two-electron (2e–3c) bonding over a bridge
• Completely ionic separation without any covalent character
• Diradical character with unpaired electrons

Correct Answer: Delocalization via a three-center two-electron (2e–3c) bonding over a bridge

Download