Organic Intermediates: Carbenes and nitrenes formation and synthetic uses MCQs With Answer

Organic Intermediates: Carbenes and Nitrenes — MCQs With Answer

Introduction: This quiz set focuses on carbenes and nitrenes — two highly reactive intermediates central to advanced synthetic organic chemistry. Aimed at M.Pharm students studying MPC 102T Advanced Organic Chemistry I, the questions probe formation methods (diazo compounds, diazirines, azides, iminoiodinanes, Simmons–Smith carbenoids), spin states (singlet vs triplet), mechanistic behavior (concerted vs stepwise), and key synthetic applications (cyclopropanation, C–H insertion, Wolff, Curtius, Schmidt rearrangements, aziridine formation). The MCQs are designed to deepen conceptual understanding, link structure/reactivity relationships, and reinforce safe laboratory handling of these energetic species.

Q1. What best describes a carbene?

• A divalent carbon species with six valence electrons; singlet carbenes have paired electrons and triplet carbenes have two unpaired electrons
• A tetravalent carbon cation stabilized by resonance
• A carbon species with eight valence electrons and a filled octet
• A radical cation formed by one-electron oxidation of methane

Correct Answer: A divalent carbon species with six valence electrons; singlet carbenes have paired electrons and triplet carbenes have two unpaired electrons

Q2. Which of the following is a common laboratory method for generating free carbenes or metal-carbenes?

• Thermolysis or photolysis of diazo compounds (loss of N2)
• Reduction of alkyl halides with sodium borohydride
• Hydrolysis of esters under neutral conditions
• Electrophilic aromatic substitution of benzene

Correct Answer: Thermolysis or photolysis of diazo compounds (loss of N2)

Q3. The Simmons–Smith reagent used for cyclopropanation is best described as:

• Iodomethylzinc iodide, a carbenoid reagent generated from CH2I2 and Zn(Cu)
• A free singlet :CH2 carbene generated in situ from diazomethane
• A radical initiator that forms cyclopropyl radicals
• A nucleophilic methylating agent similar to MeLi

Correct Answer: Iodomethylzinc iodide, a carbenoid reagent generated from CH2I2 and Zn(Cu)

Q4. Which catalyst is especially noted for generating metal-carbenes that perform selective C–H insertion reactions from diazo precursors?

• Dirhodium(II) carboxylates such as Rh2(OAc)4
• Pd/C under hydrogen atmosphere
• FeCl3 in aqueous solution
• AlCl3 as a Lewis acid at low temperature

Correct Answer: Dirhodium(II) carboxylates such as Rh2(OAc)4

Q5. Which experimental outcome is diagnostic of a singlet carbene participating in cyclopropanation of an alkene?

• Stereospecific cyclopropane formation with retention of the alkene stereochemistry
• Complete loss of stereochemical information and formation of racemate only
• Formation of a carbocation intermediate followed by rearrangement
• Exclusive formation of polymeric byproducts without cyclopropanes

Correct Answer: Stereospecific cyclopropane formation with retention of the alkene stereochemistry

Q6. Which substituent pattern most stabilizes the singlet state of a carbene?

• Heteroatoms with lone pairs (e.g., OR, NR2) that can donate into the empty p-orbital
• Two alkyl groups only, which always favor singlet over triplet
• Strong electron-withdrawing groups exclusively, which always favor singlet
• Bulky tert-butyl groups that prevent orbital overlap

Correct Answer: Heteroatoms with lone pairs (e.g., OR, NR2) that can donate into the empty p-orbital

Q7. Which reagent is the classic precursor for generating organic nitrenes by loss of nitrogen gas?

• Organic azides (R–N3)
• Alkyl halides (R–X) under basic conditions
• Hydrazones under acidic workup
• Peroxides under thermal initiation

Correct Answer: Organic azides (R–N3)

Q8. Which statement correctly contrasts the reactivity of singlet and triplet nitrenes?

• Singlet nitrenes are electrophilic and tend to insert into C–H or add to π-bonds; triplet nitrenes behave like diradicals and often give H-abstraction or stepwise radical additions
• Triplet nitrenes are more electrophilic and always give concerted insertions, whereas singlets form radicals
• Both singlet and triplet nitrenes are nucleophilic and prefer substitution reactions
• Nitrenes do not show spin-dependent behavior and react identically in all cases

Correct Answer: Singlet nitrenes are electrophilic and tend to insert into C–H or add to π-bonds; triplet nitrenes behave like diradicals and often give H-abstraction or stepwise radical additions

Q9. Reaction of a nitrene with an alkene typically furnishes which functional group under concerted (singlet) conditions?

• Aziridine
• Epoxide
• Imine
• Ketone

Correct Answer: Aziridine

Q10. The Curtius rearrangement proceeds via which key intermediate?

• An acyl nitrene (from acyl azide) that rearranges to an isocyanate
• A carbenium ion stabilized by neighboring group participation
• A free carbene that inserts into solvent molecules
• An iminium ion formed by protonation of an amide

Correct Answer: An acyl nitrene (from acyl azide) that rearranges to an isocyanate

Q11. The Wolff rearrangement of α-diazoketones typically gives which product via a carbene intermediate?

• Ketenes (which can be trapped to give esters or amides)
• Alkenes via elimination
• Amines via reductive amination
• Carboxylic acids directly without trapping

Correct Answer: Ketenes (which can be trapped to give esters or amides)

Q12. Which transformation is least characteristic of free carbenes?

• Nucleophilic substitution of an sp3-hybridized carbon center (SN2)
• Cyclopropanation of alkenes
• C–H insertion to form C–C bonds
• Addition to multiple bonds to give ylides or cyclopropanes

Correct Answer: Nucleophilic substitution of an sp3-hybridized carbon center (SN2)

Q13. Which reagent is commonly used to generate electrophilic nitrenes for metal-catalyzed aziridination or insertion reactions?

• Iminoiodinanes such as PhI=NTs (formed from PhI(III) reagents and sulfonamides)
• Grignard reagents like RMgX
• Hydrazine hydrate under neutral conditions
• Alkyl lithium reagents at −78 °C

Correct Answer: Iminoiodinanes such as PhI=NTs (formed from PhI(III) reagents and sulfonamides)

Q14. Metal carbenoids such as the Simmons–Smith reagent differ from free carbenes primarily because they:

• Exhibit enhanced stereospecific (concerted) cyclopropanation with reduced free carbene character
• Always form radical intermediates that scramble stereochemistry
• Act as strong nucleophiles towards carbonyls under neutral conditions
• Are identical to free triplet carbenes in every reaction

Correct Answer: Exhibit enhanced stereospecific (concerted) cyclopropanation with reduced free carbene character

Q15. In metal-carbene C–H insertion reactions, which C–H bond is generally most reactive toward insertion?

• Tertiary C–H bonds (due to greater stabilization of the developing partial radical character in the transition state)
• Primary C–H bonds exclusively
• Aromatic C–H bonds only
• All C–H bonds react with equal facility

Correct Answer: Tertiary C–H bonds (due to greater stabilization of the developing partial radical character in the transition state)

Q16. Which of the following is a stable photochemical precursor to carbenes and is useful in photoaffinity labeling?

• Diazirines
• Hydroperoxides
• Alkyl chlorides
• Carboxylic acids

Correct Answer: Diazirines

Q17. Triplet carbenes are best characterized by which of the following features?

• They are diradical species with two unpaired electrons and typically undergo stepwise radical-type reactions
• They always undergo concerted pericyclic reactions without radical character
• They are closed-shell electrophiles identical to singlet carbenes
• They cannot be detected by spectroscopic methods

Correct Answer: They are diradical species with two unpaired electrons and typically undergo stepwise radical-type reactions

Q18. Which spectroscopic technique is particularly suitable for detecting triplet carbenes due to their unpaired electrons?

• Electron paramagnetic resonance (EPR / ESR)
• Infrared spectroscopy only
• Polarimetry
• Thin-layer chromatography (TLC)

Correct Answer: Electron paramagnetic resonance (EPR / ESR)

Q19. The Schmidt reaction of a ketone with hydrazoic acid under strongly acidic conditions typically gives which type of product via a nitrene-like intermediate?

• An amide derived from migration of an alkyl/aryl group to nitrogen (rearranged amide)
• An alcohol by direct reduction of the carbonyl
• A diene by elimination of N2
• A free carbene that inserts into solvent

Correct Answer: An amide derived from migration of an alkyl/aryl group to nitrogen (rearranged amide)

Q20. Which safety practice is most important when handling diazomethane, a common carbene precursor?

• Use dilute solutions at low temperature in a fume hood behind a blast shield and avoid strong acids or metals; treat with caution due to toxicity and explosive potential
• Heat neat diazomethane to speed reactions and evaporate solvents on the bench top
• Store large quantities of dry diazomethane in open containers near ignition sources
• Neutralize by adding concentrated mineral acid directly to neat diazomethane

Correct Answer: Use dilute solutions at low temperature in a fume hood behind a blast shield and avoid strong acids or metals; treat with caution due to toxicity and explosive potential

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