Reactions of indole MCQs With Answer

Reactions of indole MCQs With Answer — This concise, Student-friendly post helps B.Pharm students master core indole chemistry concepts. Indole reactions are central in medicinal chemistry and drug design, covering electrophilic substitution at C3, N‑alkylation/acylation, Vilsmeier–Haack formylation, Mannich and Pictet–Spengler cyclizations, selective halogenation (NBS), oxidation to isatin, and directed metallation strategies for C2 functionalization. These Reactions of indole MCQs With Answer are designed to reinforce mechanism, regiochemistry, reagents, and synthesis applications relevant to pharmaceutical sciences. Clear explanations and targeted practice make this an ideal revision tool for exams and competitive tests. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which position on the indole ring is most reactive toward electrophilic aromatic substitution under normal conditions?

• C2 position
• C3 position
• N1 position
• C4 position

Correct Answer: C3 position

Q2. The Vilsmeier–Haack reaction (DMF + POCl3) on unsubstituted indole predominantly gives which product?

• 2‑Formylindole
• 3‑Formylindole
• N‑Formylindole
• No reaction due to indole deactivation

Correct Answer: 3‑Formylindole

Q3. Which reagent is commonly used for selective bromination of indole at the 3‑position?

• N‑Bromosuccinimide (NBS)

Correct Answer: N‑Bromosuccinimide (NBS)

Q4. Why are Friedel–Crafts acylations often problematic on indole without N‑protection?

• Indole is too deactivated to undergo Friedel–Crafts
• The indole nitrogen coordinates to Lewis acids and leads to polymerization
• Friedel–Crafts gives only N‑acylation
• Friedel–Crafts selectively gives C4 products instead

Correct Answer: The indole nitrogen coordinates to Lewis acids and leads to polymerization

Q5. The Mannich reaction on indole typically introduces a substituent at which position?

• N1 position
• C2 position
• C3 position
• C4 position

Correct Answer: C3 position

Q6. Protection of the indole nitrogen with a Boc group primarily helps to:

• Increase nucleophilicity of the ring
• Direct electrophiles exclusively to C4
• Prevent N‑alkylation and enable directed metallation at C2
• Make indole inert to all electrophiles

Correct Answer: Prevent N‑alkylation and enable directed metallation at C2

Q7. Oxidation of indole under controlled conditions often yields which heterocycle used in synthesis?

• Indoline
• Isatin
• Carbazole
• Anthranilic acid

Correct Answer: Isatin

Q8. Which oxidant is frequently used to convert indole to isatin in laboratory conditions?

• Pd/C hydrogenation
• m‑CPBA
• KMnO4 or oxone under controlled conditions
• LiAlH4

Correct Answer: KMnO4 or oxone under controlled conditions

Q9. N‑alkylation of indole nitrogen is typically achieved using which condition?

• Treatment with alkyl halide and strong base (e.g., NaH)
• Direct reaction with alcohol in absence of catalyst
• Oxidative coupling with CuCl2
• Photochemical N‑alkylation only

Correct Answer: Treatment with alkyl halide and strong base (e.g., NaH)

Q10. The Fischer indole synthesis forms indoles from which two main components?

• Aromatic amine and carboxylic acid
• Phenylhydrazine and carbonyl compound (aldehyde/ketone)
• Indole N‑oxide and alkene
• Nitroarene and Grignard reagent

Correct Answer: Phenylhydrazine and carbonyl compound (aldehyde/ketone)

Q11. In electrophilic substitution on indole, stabilization of the sigma complex is largely due to:

• Resonance donation from the nitrogen lone pair into the ring
• Inductive withdrawal by nitrogen
• Steric hindrance at C3
• Strong hydrogen bonding with solvent

Correct Answer: Resonance donation from the nitrogen lone pair into the ring

Q12. Which transformation converts tryptamine derivatives into tetrahydro-β-carbolines in biosynthetic or synthetic chemistry?

• Mannich reaction
• Pictet–Spengler reaction
• Vilsmeier–Haack formylation
• Friedel–Crafts acylation

Correct Answer: Pictet–Spengler reaction

Q13. Halogenation of indole at C2 is easier when:

• C3 is unsubstituted and highly electron rich
• N is protected and C3 is blocked (so electrophile attacks C2)
• The reaction is done under strongly acidic conditions only
• Only free indole undergoes C2 halogenation

Correct Answer: N is protected and C3 is blocked (so electrophile attacks C2)

Q14. Which reagent gives N‑oxide of indole by oxygen transfer to nitrogen?

• LiAlH4
• m‑CPBA (meta‑chloroperbenzoic acid)
• PCl5
• NBS

Correct Answer: m‑CPBA (meta‑chloroperbenzoic acid)

Q15. Which statement about indole sulfonation is correct?

• Sulfonation occurs preferentially at N1
• Sulfonation at C3 is common with fuming sulfuric acid
• Indoles cannot be sulfonated
• Sulfonation gives exclusively C2 products

Correct Answer: Sulfonation at C3 is common with fuming sulfuric acid

Q16. Which pathway is commonly used to introduce an aldehyde at C3 of indole for further functionalization?

• Vilsmeier–Haack reaction
• Reimer–Tiemann reaction
• Birch reduction
• Kolbe electrolysis

Correct Answer: Vilsmeier–Haack reaction

Q17. For selective lithiation at the 2‑position of indole, which tactic is commonly used?

• Use of bulky electrophiles only
• N‑protection (e.g., Boc) followed by strong base (e.g., LDA) at low temperature
• Photochemical activation
• Direct lithiation without protection always gives C2 selectivity

Correct Answer: N‑protection (e.g., Boc) followed by strong base (e.g., LDA) at low temperature

Q18. What is the primary reason indoles undergo electrophilic substitution more rapidly than benzene?

• Higher ring strain in indole
• Electron donation from the pyrrole‑like nitrogen increases ring electron density
• Indole is less aromatic than benzene
• Indole has fewer resonance structures

Correct Answer: Electron donation from the pyrrole‑like nitrogen increases ring electron density

Q19. Which transformation yields 3‑(aminomethyl)indole derivatives commonly found in pharmaceutical scaffolds?

• Mannich reaction
• Friedel–Crafts acylation
• Sandmeyer reaction
• Wittig reaction

Correct Answer: Mannich reaction

Q20. When indole is treated with strong electrophiles without protection, polymerization often occurs because:

• The electrophile is too weak
• Indole forms highly reactive intermediates that couple intermolecularly
• Solvent prevents reaction
• Only N‑substitution is possible

Correct Answer: Indole forms highly reactive intermediates that couple intermolecularly

Q21. Which method is widely used to synthesize substituted indoles from nitroarenes and vinylmetal reagents?

• Bartoli indole synthesis
• Schiemann reaction
• Gattermann–Koch formylation
• Kolbe–Schmitt reaction

Correct Answer: Bartoli indole synthesis

Q22. Electrophilic substitution at C3 of indole proceeds via which key intermediate?

• Benzenonium ion localized at C4
• Sigma complex (Wheland intermediate) stabilized by nitrogen
• Carbanion intermediate at C3
• Free radical at N1

Correct Answer: Sigma complex (Wheland intermediate) stabilized by nitrogen

Q23. Which reagent pair is typical for Vilsmeier–Haack formylation of indole?

• SOCl2 + DMF
• DMF + POCl3
• DMF + HCl
• POCl3 + H2O

Correct Answer: DMF + POCl3

Q24. Which protective group on indole nitrogen is commonly removed under mild acidic conditions?

• Boc (tert‑butoxycarbonyl)
• TBDMS (tert‑butyldimethylsilyl)
• Cbz (benzyloxycarbonyl)
• Aryl sulfonyl (e.g., Ts)

Correct Answer: Boc (tert‑butoxycarbonyl)

Q25. Indole N‑acylation (formation of N‑acetamide) typically requires which reagent?

• Acetic anhydride or acetyl chloride with base
• Acidic hydrolysis
• Birch reduction conditions
• Oxidative coupling

Correct Answer: Acetic anhydride or acetyl chloride with base

Q26. Which strategy is often used to selectively functionalize C2 of indole when C3 is more reactive?

• Perform reactions at high temperature only
• Block or protect C3 or use directing/protecting groups on N1
• Use only radical reagents
• Oxidize the ring first to deactivate it

Correct Answer: Block or protect C3 or use directing/protecting groups on N1

Q27. The Sandmeyer reaction applied to an aniline precursor of indole chemistry often serves to:

• Introduce halogen substituents via diazonium intermediate
• Form aldehydes on the indole ring
• Hydrogenate the indole ring
• Directly form indole from aniline

Correct Answer: Introduce halogen substituents via diazonium intermediate

Q28. Which synthetic transformation converts indole into indoline (reduced indole)?

• Hydrogenation over Pd/C
• Peracid oxidation
• NBS bromination
• Electrophilic formylation

Correct Answer: Hydrogenation over Pd/C

Q29. In medicinal chemistry, why are 3‑substituted indoles particularly valuable?

• They are more water soluble than parent indole
• Substitution at C3 often modulates biological activity and binding to targets
• C3 substitution always reduces metabolic stability
• They are easier to synthesize than other isomers

Correct Answer: Substitution at C3 often modulates biological activity and binding to targets

Q30. Which reagent is commonly used to introduce a methyl group selectively at C3 of indole via electrophilic alkylation?

• CH3I and strong base for N‑alkylation
• CH3Cl with AlCl3 (Friedel–Crafts) without precautions
• Use of methanol and acid (acidic alkylation) or preformed carbocations in controlled Mannich‑type conditions
• Direct methylation with diazomethane on the ring

Correct Answer: Use of methanol and acid (acidic alkylation) or preformed carbocations in controlled Mannich‑type conditions

Q31. Which reaction would you choose to install a nitrile group onto an indole ring indirectly?

• Direct cyanation with NaCN under neutral conditions
• Sandmeyer reaction on an amino‑derived precursor
• Hydrogenation of a nitro group directly to nitrile
• Oxidation with KMnO4

Correct Answer: Sandmeyer reaction on an amino‑derived precursor

Q32. N‑sulfonylation (e.g., N‑Ts) of indole is useful because it:

• Increases nucleophilicity of the ring
• Makes the nitrogen a better leaving group for cross‑coupling
• Protects nitrogen and directs electrophiles to the ring while being stable to many conditions
• Has no practical synthetic use

Correct Answer: Protects nitrogen and directs electrophiles to the ring while being stable to many conditions

Q33. Which reagent set can convert a 3‑formylindole into a 3‑methylindole (reductive transformation)?

• Wittig reagent to give alkene then hydrogenation
• Direct acid catalysis gives methylation
• Oxidation with KMnO4
• Photochemical rearrangement

Correct Answer: Wittig reagent to give alkene then hydrogenation

Q34. Why is NBS preferred over Br2 for many indole bromination reactions?

• NBS is more acidic
• NBS offers milder, more controlled bromination and often gives selective 3‑bromination
• NBS does not brominate aromatic rings
• NBS only brominates nitrogen

Correct Answer: NBS offers milder, more controlled bromination and often gives selective 3‑bromination

Q35. Which catalyst or condition is typically avoided when attempting Friedel–Crafts on unprotected indole?

• Use of Lewis acids like AlCl3 without protection
• Use of mild Brønsted acids in Mannich reactions
• Use of DMF/POCl3 for Vilsmeier formylation
• NBS bromination conditions

Correct Answer: Use of Lewis acids like AlCl3 without protection

Q36. Conversion of indole to 2‑substituted indoles often involves which general tactic?

• Direct electrophilic attack at C2 without any modification
• Block C3 or use directed metallation at C2 after N‑protection
• Oxidize the ring then perform nucleophilic substitution
• Use photochemical C2 activation exclusively

Correct Answer: Block C3 or use directed metallation at C2 after N‑protection

Q37. In the Pictet–Spengler reaction involving tryptamine, the indole ring attacks the iminium ion at which position to form the new ring?

• N1 position
• C2 position
• C3 position
• C4 position

Correct Answer: C2 position

Q38. What is a common outcome when attempting strong electrophilic nitration on indole without protection?

• Clean mononitration at C3
• Oxidation to isatin exclusively
• Decomposition and polymerization due to harsh conditions
• Formation of N‑nitroindole as the major product

Correct Answer: Decomposition and polymerization due to harsh conditions

Q39. Which reaction is characteristic for converting indole into substituted indolones or oxindoles (C2 oxidation products)?

• Reductive amination
• Peracid oxidation or specific oxidative rearrangements
• Friedel–Crafts acylation
• Grignard addition

Correct Answer: Peracid oxidation or specific oxidative rearrangements

Q40. Which coupling method is commonly used to form C–C bonds on halogenated indoles for medicinal chemistry?

• Buchwald–Hartwig amination
• Pd‑catalyzed Suzuki or Negishi cross‑coupling on 3‑ or 2‑halogenated indoles
• SN1 substitution on halogenated indoles
• Hydrolysis

Correct Answer: Pd‑catalyzed Suzuki or Negishi cross‑coupling on 3‑ or 2‑halogenated indoles

Q41. Which structural feature of indole makes the 3‑position especially nucleophilic?

• The carbonyl group at C3
• Resonance stabilization of positive charge when electrophile attacks C3 via nitrogen participation
• Presence of a permanently positive charge
• Steric shielding of other positions

Correct Answer: Resonance stabilization of positive charge when electrophile attacks C3 via nitrogen participation

Q42. What is the typical fate of indole under strong acidic conditions at elevated temperatures during electrophilic reactions?

• Cleansly forms mono‑substituted products only
• Undergoes polymerization or overreaction unless controlled
• Is converted to benzene derivatives
• Becomes completely inert

Correct Answer: Undergoes polymerization or overreaction unless controlled

Q43. Which functional group introduction is efficiently performed by the Vilsmeier–Haack reaction on indoles and useful for further transformations?

• Nitrile group
• Formyl (aldehyde) group
• Carboxylic acid group
• Sulfonyl group

Correct Answer: Formyl (aldehyde) group

Q44. When synthesizing drug‑like 3‑substituted indoles, which of the following sequences is a common approach?

• Halogenation at C3, then Pd‑catalyzed cross‑coupling to install diverse substituents
• Direct SN2 substitution at C3
• Oxidation to isatin then reduction to indole
• Use of only photochemical methods

Correct Answer: Halogenation at C3, then Pd‑catalyzed cross‑coupling to install diverse substituents

Q45. Which reagent is suitable for converting an indole N‑Boc derivative back to the free indole?

• TFA (trifluoroacetic acid) in dichloromethane
• Strong base such as NaH
• Hydrogenation with Raney nickel
• KMnO4 oxidation

Correct Answer: TFA (trifluoroacetic acid) in dichloromethane

Q46. A protected indole is metalated using s‑BuLi/TMEDA at low temperature to install a substituent. Metalation typically occurs at which position when N is protected?

• N1
• C2 (directed metalation)
• C5
• Random mixture of positions

Correct Answer: C2 (directed metalation)

Q47. Which of the following best describes the outcome of nitration of indole under controlled, mild nitrating conditions?

• Selective nitration at C3
• Exclusive nitration at N1
• Preferential nitration at C5/C6 depending on directing effects if C3 is blocked
• No nitration possible

Correct Answer: Preferential nitration at C5/C6 depending on directing effects if C3 is blocked

Q48. Which process is used industrially or in synthesis to modify indole frameworks to improve drug‑like properties by introducing polar groups?

• Halogenation only
• Functionalization at C3 (e.g., formylation then further elaboration) and N‑modification (acylation, alkylation)
• Only N‑oxidation
• Burning and reformation

Correct Answer: Functionalization at C3 (e.g., formylation then further elaboration) and N‑modification (acylation, alkylation)

Q49. In designing a synthetic route to a 2,3‑disubstituted indole, which sequence is often practical?

• Introduce C2 substituent first by directed lithiation, then introduce C3 substituent via electrophilic substitution or cross‑coupling
• Always introduce C3 first then C2 by random methods
• Only carry out N‑alkylation and expect rearrangement
• Oxidize to isatin and then reinstall substituents

Correct Answer: Introduce C2 substituent first by directed lithiation, then introduce C3 substituent via electrophilic substitution or cross‑coupling

Q50. Which general precaution is most important when performing electrophilic reactions on indole substrates in the lab?

• Always use aqueous base
• Control temperature, protect the nitrogen if necessary, and use mild reagents to avoid polymerization
• Perform reactions at highest possible temperature for speed
• No special precautions are needed compared to benzene

Correct Answer: Control temperature, protect the nitrogen if necessary, and use mild reagents to avoid polymerization

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