Birch reduction MCQs With Answer

Birch reduction MCQs With Answer

by

Birch reduction MCQs With Answer offers B.Pharm students a focused, exam-oriented review of the Birch reduction — a key method for partial aromatic ring hydrogenation using alkali metals in liquid ammonia and a proton source. This SEO-friendly introduction covers mechanism steps (electron transfer, radical anion formation, protonation), common reagents (sodium, lithium, tert-butanol), regiochemical trends, stereochemical outcomes and pharmaceutical applications such as selective dearomatization in drug synthesis. Clear explanations and targeted practice strengthen understanding of reaction scope, safety, workup and analytical identification of products. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary outcome of a Birch reduction on benzene?

  • Formation of cyclohexane
  • Formation of 1,4-cyclohexadiene
  • Formation of 1,3-cyclohexadiene
  • Formation of benzylic alcohol

Correct Answer: Formation of 1,4-cyclohexadiene

Q2. Which combination of reagents is characteristic of a classical Birch reduction?

  • Pd/C and H2
  • Sodium or lithium in liquid ammonia with an alcohol as proton source
  • Hydrazine and base
  • BCl3 and LiAlH4

Correct Answer: Sodium or lithium in liquid ammonia with an alcohol as proton source

Q3. What is the role of liquid ammonia in Birch reduction?

  • It acts as an oxidant
  • It serves as a low-temperature solvent and stabilizes solvated electrons
  • It provides protons for protonation steps
  • It acts as a strong acid catalyst

Correct Answer: It serves as a low-temperature solvent and stabilizes solvated electrons

Q4. Which species is the immediate result of the first electron transfer to an aromatic ring in the Birch mechanism?

  • Carbocation
  • Radical cation
  • Radical anion
  • Carbanion

Correct Answer: Radical anion

Q5. In the Birch reduction mechanism, what happens after the formation of the radical anion?

  • Direct elimination to form alkene
  • Protonation to give a neutral radical
  • Immediate oxidation to reform aromatic ring
  • Nucleophilic substitution at substituent

Correct Answer: Protonation to give a neutral radical

Q6. Which proton source is commonly used in Birch reductions?

  • Concentrated sulfuric acid
  • tert-Butanol or ethanol
  • Water at high temperature
  • Strong bases like NaOH

Correct Answer: tert-Butanol or ethanol

Q7. Which metal is frequently preferred for Birch reductions in laboratory practice?

  • Magnesium
  • Iron
  • Sodium or lithium
  • Copper

Correct Answer: Sodium or lithium

Q8. How many electrons are transferred overall per aromatic ring reduced to a 1,4-diene in a Birch reduction?

  • One electron
  • Two electrons
  • Three electrons
  • Four electrons

Correct Answer: Two electrons

Q9. Which intermediate receives the second electron in the Birch sequence before the final protonation?

  • Radical intermediate
  • Carbocation intermediate
  • Carbanion (anion) intermediate
  • Persistent radical cation

Correct Answer: Carbanion (anion) intermediate

Q10. How does an electron-donating substituent (e.g., OMe) typically influence the regioselectivity of a Birch reduction?

  • Directs reduction to give double bonds adjacent to the substituent
  • Prevents any reduction from occurring
  • Leads to reduction at positions remote from the substituent
  • Converts substituent to a leaving group

Correct Answer: Directs reduction to give double bonds adjacent to the substituent

Q11. How does an electron-withdrawing group (e.g., CO2Me) affect product distribution in Birch reduction?

  • It favors formation of a fully saturated ring
  • It stabilizes negative charge at the position para to the group, directing protonation accordingly
  • It prevents electron transfer altogether
  • It is reduced to an alcohol preferentially

Correct Answer: It stabilizes negative charge at the position para to the group, directing protonation accordingly

Q12. Which statement correctly contrasts Birch reduction with catalytic hydrogenation?

  • Birch reduction fully hydrogenates aromatics; catalytic hydrogenation gives dienes
  • Birch reduction partially reduces to 1,4-dienes; catalytic hydrogenation usually produces cyclohexane under H2/Pd
  • Both methods give identical products under all conditions
  • Catalytic hydrogenation uses liquid ammonia as solvent

Correct Answer: Birch reduction partially reduces to 1,4-dienes; catalytic hydrogenation usually produces cyclohexane under H2/Pd

Q13. Which of the following is a major safety concern when performing a Birch reduction in the lab?

  • Handling of flammable hydrogen gas at high pressure
  • Use of toxic organomercury reagents
  • Use of liquid ammonia and reactive alkali metals which can be pyrophoric
  • Generation of chlorine gas as byproduct

Correct Answer: Use of liquid ammonia and reactive alkali metals which can be pyrophoric

Q14. For medicinal chemistry, why is Birch reduction valuable in drug synthesis?

  • It always improves water solubility of drugs
  • It allows selective partial dearomatization to access saturated or partially saturated scaffolds used in drug design
  • It is a high-yield method for introducing halogens
  • It converts amines to nitriles efficiently

Correct Answer: It allows selective partial dearomatization to access saturated or partially saturated scaffolds used in drug design

Q15. Which product results from Birch reduction of phenol derivatives under typical conditions?

  • Phenol is oxidized to quinone
  • 1,4-cyclohexadienol (partially reduced phenol)
  • Full hydrogenation to cyclohexanol in a single step
  • Substitution of OH by H

Correct Answer: 1,4-cyclohexadienol (partially reduced phenol)

Q16. What determines the site of initial protonation in the Birch mechanism?

  • The acidity of the solvent only
  • The electron density distribution in the radical anion influenced by substituents
  • The molecular weight of the substrate
  • The concentration of metal alone

Correct Answer: The electron density distribution in the radical anion influenced by substituents

Q17. Which spectroscopy method is most diagnostic to confirm formation of a 1,4-cyclohexadiene product?

  • UV-Vis only
  • IR showing absence of aromatic C–H and 1H NMR showing characteristic olefinic signals around 5–6 ppm
  • Mass spectrometry alone without NMR
  • Flame photometry

Correct Answer: IR showing absence of aromatic C–H and 1H NMR showing characteristic olefinic signals around 5–6 ppm

Q18. In Birch reductions, which solvent is sometimes used as an alternative to liquid ammonia for safety and practicality?

  • Dimethyl sulfoxide (DMSO)
  • Ether solutions of sodium in ammonia are always required; no alternatives exist
  • Ethylamine or ammonia surrogates and specialized electron-donor solvents under controlled conditions
  • Conc. HCl

Correct Answer: Ethylamine or ammonia surrogates and specialized electron-donor solvents under controlled conditions

Q19. Which substituent generally slows down or inhibits Birch reduction of an aromatic ring?

  • Strong electron-donating groups like NR2
  • Strong electron-withdrawing groups like nitro (NO2)
  • Small alkyl groups like methyl
  • Hydrogen only

Correct Answer: Strong electron-withdrawing groups like nitro (NO2)

Q20. What is the stereochemical outcome at positions that become sp3 during Birch reduction?

  • Always racemic mixture with no stereocontrol
  • Often forms planar carbons only
  • Configuration is set at protonation steps and can lead to defined stereochemistry depending on substrate and proton source
  • Always gives a single enantiomer

Correct Answer: Configuration is set at protonation steps and can lead to defined stereochemistry depending on substrate and proton source

Q21. During Birch reduction, why is the reaction typically run at low temperature?

  • To prevent evaporation of benzene
  • To stabilize solvated electrons and control reactivity of alkali metals in liquid ammonia
  • To speed up the reaction dramatically
  • To crystallize the product directly

Correct Answer: To stabilize solvated electrons and control reactivity of alkali metals in liquid ammonia

Q22. What is the effect of using excess proton source in Birch reduction?

  • It prevents electron transfer entirely
  • It can quench solvated electrons and reduce reaction efficiency or lead to over-protonation
  • It increases the oxidation potential of the metal
  • It transforms ammonia into ammonium nitrate

Correct Answer: It can quench solvated electrons and reduce reaction efficiency or lead to over-protonation

Q23. Which of the following best describes the electron flow in the Birch reduction?

  • Nucleophilic attack followed by elimination
  • Successive one-electron transfers from metal to aromatic ring forming radical anion then carbanion
  • Concerted two-electron pericyclic rearrangement
  • Hydride transfer from metal hydride species

Correct Answer: Successive one-electron transfers from metal to aromatic ring forming radical anion then carbanion

Q24. Which pharmaceutical transformation might employ a Birch reduction?

  • Selective partial dearomatization to synthesize cyclohexadiene intermediates for steroid analogs
  • Direct formation of peptide bonds
  • Oxidative coupling of phenols to biaryls
  • Direct N-dealkylation of tertiary amines

Correct Answer: Selective partial dearomatization to synthesize cyclohexadiene intermediates for steroid analogs

Q25. What happens to a nitro-substituted aromatic ring under standard Birch conditions?

  • Nitro group is completely stable and ring is fully reduced to cyclohexane
  • Nitro group is often reduced or causes complex outcomes; nitro groups can be reduced by the strongly reducing conditions
  • Nitro group converts to peroxide
  • No reaction occurs at all

Correct Answer: Nitro group is often reduced or causes complex outcomes; nitro groups can be reduced by the strongly reducing conditions

Q26. Which is a common workup step after a Birch reduction?

  • Heating to reflux in concentrated acid
  • Careful quench with protic solvent followed by extraction and neutralization
  • Direct distillation without quench
  • Titration with strong base to pH 14

Correct Answer: Careful quench with protic solvent followed by extraction and neutralization

Q27. In mechanism lectures, which term best describes the electrons in liquid ammonia that enable Birch reductions?

  • Solvated electrons
  • Localized electron pairs
  • Conduction band electrons
  • Photogenerated electrons

Correct Answer: Solvated electrons

Q28. Which aromatic substrate is most straightforward for obtaining predictable Birch reduction products?

  • Polysubstituted heteroaromatics with multiple electron-withdrawing groups
  • Simple monosubstituted benzenes with well-characterized EDG or EWG
  • Highly congested fused polyaromatics only
  • Compounds containing only aliphatic chains

Correct Answer: Simple monosubstituted benzenes with well-characterized EDG or EWG

Q29. Which factor does NOT significantly influence Birch reduction regiochemistry?

  • Nature of ring substituents (EDG vs EWG)
  • Choice of proton source and solvent
  • Temperature and metal used
  • Molecular weight of the counterion in unrelated salts present in trace amounts

Correct Answer: Molecular weight of the counterion in unrelated salts present in trace amounts

Q30. In a Birch reduction, what is the likely product when anisole (methoxybenzene) is reduced?

  • Unchanged anisole
  • 1,4-cyclohexadiene with the methoxy-substituted carbon typically saturated (partially reduced ring)
  • Full hydrogenation to methoxycyclohexane exclusively
  • Cleavage of methoxy group to give phenol

Correct Answer: 1,4-cyclohexadiene with the methoxy-substituted carbon typically saturated (partially reduced ring)

Q31. Which of the following is true about reduction potential in Birch reductions?

  • Aromatic rings have very high reduction potentials making reduction difficult without solvated electrons
  • Reduction potentials are irrelevant to reactivity
  • Only metals that form hydrides can effect Birch reductions
  • Reduction occurs by thermal rearrangement

Correct Answer: Aromatic rings have very high reduction potentials making reduction difficult without solvated electrons

Q32. Why might lithium be chosen over sodium in a Birch reduction?

  • Lithium cannot provide solvated electrons
  • Lithium forms more soluble electron carriers and can give different reactivity or selectivity in some cases
  • Lithium is cheaper and safer than sodium
  • Lithium always yields aromatic substitution instead of reduction

Correct Answer: Lithium forms more soluble electron carriers and can give different reactivity or selectivity in some cases

Q33. What is a common side reaction in Birch reductions that impacts yield?

  • Polymerization of solvent
  • Overreduction to saturated cyclohexane or reduction of functional groups (e.g., nitro to amine)
  • Formation of stable epoxides
  • Nitration of the aromatic ring

Correct Answer: Overreduction to saturated cyclohexane or reduction of functional groups (e.g., nitro to amine)

Q34. How does conjugation of substituents to the ring affect Birch reduction?

  • Conjugated substituents have no effect on the reaction
  • Conjugated electron-withdrawing groups can delocalize charge and change regioselectivity markedly
  • Conjugation accelerates oxidation only
  • They prevent protonation from occurring

Correct Answer: Conjugated electron-withdrawing groups can delocalize charge and change regioselectivity markedly

Q35. When designing a synthetic route, why is Birch reduction chosen for certain heteroaromatic compounds?

  • Birch reduction always leaves heteroatoms untouched and is universally applicable
  • It can selectively reduce aromaticity while preserving functional groups and creating useful partially saturated heterocycles
  • Heteroaromatics cannot undergo Birch reduction
  • It converts heterocycles to aliphatic alcohols exclusively

Correct Answer: It can selectively reduce aromaticity while preserving functional groups and creating useful partially saturated heterocycles

Q36. Which experimental precaution is essential when adding alkali metal to liquid ammonia?

  • Adding metal rapidly in large chunks for faster reaction
  • Ensuring the system is dry and nitrogen- or inert-gas-purged to prevent moisture and oxygen contact
  • Performing the addition at high temperature to accelerate solvated electron formation
  • Mixing with strong acid immediately

Correct Answer: Ensuring the system is dry and nitrogen- or inert-gas-purged to prevent moisture and oxygen contact

Q37. Which product arises from Birch reduction of naphthalene preferentially?

  • Selective partial reduction of one ring to give dihydronaphthalene isomers (1,4- or 1,2-dihydronaphthalene depending on conditions)
  • Complete oxidation to phthalic anhydride
  • No reaction at all
  • Full hydrogenation to decalin only

Correct Answer: Selective partial reduction of one ring to give dihydronaphthalene isomers (1,4- or 1,2-dihydronaphthalene depending on conditions)

Q38. Which analytical sign suggests incomplete quench after Birch reduction?

  • Stable clear solution with no solids
  • Persistent deep blue or green color indicating solvated electrons remain
  • Strong pleasant odor of ammonia only
  • Complete disappearance of all reagents

Correct Answer: Persistent deep blue or green color indicating solvated electrons remain

Q39. What is the effect of a para-carbonyl substituent on the Birch reduction pattern?

  • Carbonyls have no effect on reduction
  • They often lead to reduction generating double bonds adjacent to the carbonyl due to stabilization of anion at ortho/para positions
  • They make the ring inert to any reduction
  • They are converted to alcohols selectively by solvated electrons without affecting the ring

Correct Answer: They often lead to reduction generating double bonds adjacent to the carbonyl due to stabilization of anion at ortho/para positions

Q40. Which observation in 1H NMR indicates conversion of an aromatic ring to a 1,4-diene?

  • Loss of aromatic multiplets around 7–8 ppm and appearance of olefinic signals around 5–6 ppm with aliphatic signals for saturated carbons
  • Increase in signals at 9–10 ppm exclusively
  • Complete disappearance of all proton signals
  • Only a single sharp singlet at 0 ppm

Correct Answer: Loss of aromatic multiplets around 7–8 ppm and appearance of olefinic signals around 5–6 ppm with aliphatic signals for saturated carbons

Q41. Which reagent pair can be used to facilitate a Birch-like reduction under non-ammonia conditions in modern methods?

  • H2 and Pd/C in ethanol
  • Electron donor reagents like alkali metals with crown ethers or dissolving metal alternatives in ethereal solvents
  • KMnO4 and base
  • Br2 and light

Correct Answer: Electron donor reagents like alkali metals with crown ethers or dissolving metal alternatives in ethereal solvents

Q42. In a substrate bearing both an electron-donating and an electron-withdrawing substituent, which typically governs the Birch reduction regiochemistry?

  • The weaker of the two substituents
  • Steric hindrance only
  • The electronic influence that best stabilizes the intermediate radical anion/carbanion (often the stronger resonance effect)
  • The solvent dielectric constant alone

Correct Answer: The electronic influence that best stabilizes the intermediate radical anion/carbanion (often the stronger resonance effect)

Q43. Why might deuterated alcohol be used as proton source in a Birch reduction experiment?

  • To slow the reaction irreversibly
  • To incorporate deuterium labels at positions that get protonated, useful for mechanistic and metabolic studies
  • Because it reacts faster than normal alcohol
  • To prevent formation of any diene products

Correct Answer: To incorporate deuterium labels at positions that get protonated, useful for mechanistic and metabolic studies

Q44. Which biological or drug-related advantage can partial dearomatization via Birch reduction confer?

  • Always increases metabolic stability dramatically
  • Introduces 3D shape and reduces planarity, potentially improving target binding and pharmacokinetics
  • Removes all functional group diversity
  • Guarantees reduced toxicity in all drugs

Correct Answer: Introduces 3D shape and reduces planarity, potentially improving target binding and pharmacokinetics

Q45. Which of these functional groups is typically tolerant under standard Birch conditions?

  • Unprotected aldehydes are always tolerated without change
  • Some ether and alkyl substituents (e.g., methoxy) are often tolerated and direct regiochemistry
  • Nitro groups are fully stable without transformation
  • All epoxides remain unchanged

Correct Answer: Some ether and alkyl substituents (e.g., methoxy) are often tolerated and direct regiochemistry

Q46. What effect does adding a chelating ligand to the metal have in modified Birch-type reductions?

  • It prevents the metal from donating electrons
  • It can modulate reactivity and selectivity by changing the metal’s reduction potential and solubility
  • It always accelerates the reaction tenfold
  • It converts the reaction into an oxidation

Correct Answer: It can modulate reactivity and selectivity by changing the metal’s reduction potential and solubility

Q47. Which factor is most important to control to avoid overreduction to cyclohexane in Birch reactions?

  • Ambient room light intensity
  • Stoichiometry of electrons (amount of metal) and careful addition of proton source
  • Molecular sieves present in the solvent
  • Using very high pressure of ammonia

Correct Answer: Stoichiometry of electrons (amount of metal) and careful addition of proton source

Q48. What mechanistic evidence supports the radical nature of the Birch reduction?

  • Observation of carbocation rearrangements
  • Detection of radical intermediates by electron paramagnetic resonance (EPR) and trapping experiments
  • Isolation of stable hydride intermediates resembling metal hydrides
  • Formation of ionic salts exclusively

Correct Answer: Detection of radical intermediates by electron paramagnetic resonance (EPR) and trapping experiments

Q49. Which modification can convert a Birch reduction into a chemoselective method for reducing particular rings in polyaromatic substrates?

  • Using strong acids only
  • Tuning metal choice, solvent, proton source and temperature to favor one ring over another
  • Adding excess oxygen to the reaction mixture
  • Performing the reaction at extremely high temperatures

Correct Answer: Tuning metal choice, solvent, proton source and temperature to favor one ring over another

Q50. Which practical tip helps scale up Birch reductions in a medicinal chemistry lab?

  • Always use open-air conditions to speed the process
  • Implement rigorous inert atmosphere techniques, controlled addition of metal, and flow or specialized setups to safely manage ammonia and reactive metals
  • Avoid cooling to keep ammonia gaseous
  • Use excess ammonium salts to stabilize solvated electrons

Correct Answer: Implement rigorous inert atmosphere techniques, controlled addition of metal, and flow or specialized setups to safely manage ammonia and reactive metals

Leave a Comment