Halogenation of benzene – reactivity MCQs With Answer

Halogenation of benzene – reactivity MCQs With Answer

Understanding halogenation of benzene is essential for B.Pharm students studying aromatic chemistry and drug synthesis. This introduction covers electrophilic aromatic substitution (EAS), mechanism steps, formation of the arenium ion, and the role of Lewis acid catalysts (FeBr3, AlCl3). Key concepts include regioselectivity (ortho/para vs. meta), influence of activating/deactivating substituents, and practical reagents (Cl2/Br2 with catalysts, I2 with oxidants, Selectfluor for fluorination, Sandmeyer for aryl diazonium halogenation). These MCQs emphasize reactivity trends, laboratory conditions, and mechanistic reasoning to strengthen problem-solving for pharmaceutical applications. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which mechanism best describes the substitution of a hydrogen on benzene by a halogen?

• Free radical substitution
• Electrophilic aromatic substitution
• Nucleophilic aromatic substitution
• Pericyclic addition

Correct Answer: Electrophilic aromatic substitution

Q2. What is the role of FeBr3 in bromination of benzene?

• It acts as a solvent
• It acts as a Lewis base to deactivate bromine
• It acts as a Lewis acid to polarize Br2 and generate Br+
• It oxidizes benzene to a radical cation

Correct Answer: It acts as a Lewis acid to polarize Br2 and generate Br+

Q3. Which step is the rate-determining step in electrophilic aromatic halogenation?

• Formation of the halogen-Lewis acid complex
• Deprotonation to restore aromaticity
• Attack of benzene on the electrophile to form the arenium ion
• Separation of catalyst from product

Correct Answer: Attack of benzene on the electrophile to form the arenium ion

Q4. Which halogen most readily undergoes direct electrophilic aromatic substitution under mild, controlled conditions?

• Fluorine (F2)
• Chlorine (Cl2) with AlCl3
• Bromine (Br2) with FeBr3
• Iodine (I2) without oxidant

Correct Answer: Bromine (Br2) with FeBr3

Q5. Why is iodination of benzene less favorable than bromination or chlorination?

• Iodine is too electronegative to form an electrophile
• The C–I bond formation is endothermic and iodination is reversible without an oxidant
• Iodine cannot form a sigma complex
• Iodine forms radicals that destroy benzene

Correct Answer: The C–I bond formation is endothermic and iodination is reversible without an oxidant

Q6. Which reagent is commonly used for electrophilic fluorination of aromatic rings in a controlled manner?

• F2 gas
• BrF3
• Selectfluor
• NaF in water

Correct Answer: Selectfluor

Q7. How do electron-donating groups (EDGs) on benzene affect halogenation reactivity?

• They deactivate the ring and direct meta
• They activate the ring and direct ortho/para
• They prevent halogenation entirely
• They convert the mechanism to radical substitution

Correct Answer: They activate the ring and direct ortho/para

Q8. Which substituent directs incoming halogens to the meta position?

• –OH
• –CH3
• –NO2
• –NH2

Correct Answer: –NO2

Q9. In the bromination of nitrobenzene under standard conditions, what is the expected major product?

• Ortho-bromonitrobenzene
• Para-bromonitrobenzene
• Meta-bromonitrobenzene
• Unsubstituted benzene

Correct Answer: Meta-bromonitrobenzene

Q10. Formation of the arenium ion during halogenation is stabilized primarily by which effect?

• Hyperconjugation only
• Resonance delocalization of the positive charge
• Inductive donation from the halogen
• Hydrogen bonding with the catalyst

Correct Answer: Resonance delocalization of the positive charge

Q11. Which technique is commonly used to halogenate an aromatic amine selectively at the aryl position via diazonium chemistry?

• Nitration followed by reduction
• Sandmeyer reaction of the aryl diazonium salt
• Direct radical halogenation with NBS
• Friedel–Crafts halogenation

Correct Answer: Sandmeyer reaction of the aryl diazonium salt

Q12. What is the effect of a strong electron-withdrawing group on the rate of electrophilic halogenation?

• Significantly increases the rate
• Has no effect
• Decreases the rate by deactivating the ring
• Changes mechanism to nucleophilic substitution

Correct Answer: Decreases the rate by deactivating the ring

Q13. Why is chlorine less selective than bromine in aromatic halogenation?

• Chlorine is less reactive toward benzene
• Chlorine forms a stronger Lewis-acid complex
• Chlorine electrophile is more reactive, giving less regioselectivity
• Chlorine cannot form an arenium ion

Correct Answer: Chlorine electrophile is more reactive, giving less regioselectivity

Q14. Which catalyst is typically used for chlorination of benzene in laboratory practice?

• FeBr3
• AlCl3
• H2SO4
• NaOH

Correct Answer: AlCl3

Q15. Which reagent is preferred for selective benzylic bromination (not aromatic ring) under radical conditions?

• Br2 with FeBr3
• N-bromosuccinimide (NBS) with peroxide or light
• Br2 with AlCl3
• Selectfluor

Correct Answer: N-bromosuccinimide (NBS) with peroxide or light

Q16. What happens to aromaticity in the sigma complex (arenium ion) intermediate?

• Aromaticity is fully preserved
• Aromaticity is temporarily lost in the ring where substitution occurs
• Aromaticity increases due to positive charge delocalization
• The ring becomes antiaromatic

Correct Answer: Aromaticity is temporarily lost in the ring where substitution occurs

Q17. Which of the following statements about fluorination of benzene is correct?

• Direct F2 halogenation is the mildest method
• Electrophilic aromatic fluorination often requires specialized reagents like Selectfluor
• Fluorination via Friedel–Crafts is common
• Ionic fluorination proceeds without catalysts in water

Correct Answer: Electrophilic aromatic fluorination often requires specialized reagents like Selectfluor

Q18. During bromination of anisole (methoxybenzene), what is the major regioisomer formed?

• Meta-bromoanisole
• Ortho/para-bromoanisole (major at para)
• Only para-bromoanisole with no ortho
• No reaction occurs

Correct Answer: Ortho/para-bromoanisole (major at para)

Q19. Which describes the reactivity order of halogenation for halogens in electrophilic aromatic substitution under catalyzed conditions?

• F2 > Cl2 > Br2 > I2
• Cl2 > Br2 > I2 > F2
• Br2 > Cl2 > F2 > I2
• Reactivity depends on conditions; generally Br2 and Cl2 are most practical

Correct Answer: Reactivity depends on conditions; generally Br2 and Cl2 are most practical

Q20. Which factor most strongly influences the regioselectivity (ortho/para vs. meta) in aromatic halogenation?

• Temperature only
• Sterics only
• Electronic effects of existing substituents
• The color of the halogen

Correct Answer: Electronic effects of existing substituents

Q21. In electrophilic aromatic substitution, what is the function of the Lewis acid catalyst after formation of the sigma complex?

• It protonates the ring
• It coordinates to the halogen and helps regenerate the catalyst while assisting deprotonation
• It abstracts electrons from the ring permanently
• It converts the halogen into a radical

Correct Answer: It coordinates to the halogen and helps regenerate the catalyst while assisting deprotonation

Q22. Which aromatic substrate is most reactive toward electrophilic bromination?

• Nitrobenzene
• Benzene
• Toluene
• Chlorobenzene

Correct Answer: Toluene

Q23. For Friedel–Crafts halogenation, which statement is correct?

• Friedel–Crafts directly adds halogen to benzene without catalysts
• Friedel–Crafts reactions refer to alkylation/acylation, not direct halogenation
• Friedel–Crafts is a radical halogenation process
• Friedel–Crafts is used to iodinate benzene

Correct Answer: Friedel–Crafts reactions refer to alkylation/acylation, not direct halogenation

Q24. Which of the following halogenating methods is commonly used to introduce chlorine into an aromatic ring via replacement of an amino group?

• Electrophilic chlorination with Cl2/AlCl3
• Sandmeyer reaction using CuCl on an aryl diazonium salt
• NBS benzylic chlorination
• Gattermann–Koch reaction

Correct Answer: Sandmeyer reaction using CuCl on an aryl diazonium salt

Q25. Which statement about the influence of steric hindrance on halogenation is true?

• Steric hindrance has no effect on orientation
• Bulky substituents can reduce ortho substitution and favor para
• Steric hindrance always prevents halogenation
• Steric effects favor meta substitution

Correct Answer: Bulky substituents can reduce ortho substitution and favor para

Q26. Which intermediate is directly formed after benzene attacks the activated halogen in EAS?

• Phenyl radical
• Arenium ion (sigma complex)
• Aroyl chloride
• Carbocation on the halogen

Correct Answer: Arenium ion (sigma complex)

Q27. Which reagent combination is commonly used to iodinate an activated aromatic ring?

• I2 alone at room temperature
• I2 with an oxidant like HNO3 or CuCl2
• Br2 with FeBr3
• Selectfluor and base

Correct Answer: I2 with an oxidant like HNO3 or CuCl2

Q28. For nitrated benzene derivatives, why is further electrophilic halogenation slow?

• Nitro groups are strong EDGs activating the ring too much
• Nitro groups are strong EWGs deactivating the ring toward EAS
• Nitro groups scavenge halogen electrophiles
• Nitro groups induce radical halogenation instead

Correct Answer: Nitro groups are strong EWGs deactivating the ring toward EAS

Q29. Which method gives an aryl fluoride from an aniline derivative?

• Direct fluorination with F2
• Diazotization followed by replacement with fluoride (Balz–Schiemann reaction)
• Sandmeyer with CuF
• NBS substitution

Correct Answer: Diazotization followed by replacement with fluoride (Balz–Schiemann reaction)

Q30. In electrophilic aromatic halogenation, what is the typical oxidation state change of the halogen?

• Halogen is reduced from 0 to -1 when forming C–X
• Halogen is oxidized from 0 to +1
• Halogen remains at 0 throughout
• Halogen is converted to peroxide

Correct Answer: Halogen is reduced from 0 to -1 when forming C–X

Q31. Which substrate would undergo fastest bromination under identical EAS conditions?

• Chlorobenzene
• Methoxybenzene (anisole)
• Nitrobenzene
• Benzene

Correct Answer: Methoxybenzene (anisole)

Q32. What is a common side reaction when using excess halogen or very reactive halogenation conditions on benzene?

• Formation of aryl diazonium salts
• Polysubstitution (multiple halogenation) and ring deactivation
• Hydrogenation of the ring
• Conversion to phenol

Correct Answer: Polysubstitution (multiple halogenation) and ring deactivation

Q33. Which catalyst is used to facilitate bromination by forming a bromonium-type electrophile?

• FeBr3
• NaBr
• HCl
• CuI

Correct Answer: FeBr3

Q34. Which reagent pair is suitable for converting an aniline to an aryl chloride via diazonium intermediate?

• NaNO2, HCl then CuCl (Sandmeyer)
• NBS then FeBr3
• AlCl3 then Cl2
• H2, Pd/C

Correct Answer: NaNO2, HCl then CuCl (Sandmeyer)

Q35. How does resonance effect of an electron-withdrawing substituent influence the sigma complex during halogenation?

• It stabilizes the positive charge in all resonance forms
• It destabilizes the sigma complex by withdrawing electron density, raising activation energy
• It converts the mechanism to radical pathway
• It has no influence

Correct Answer: It destabilizes the sigma complex by withdrawing electron density, raising activation energy

Q36. Which halogenation approach is used in medicinal chemistry to introduce a halogen that improves lipophilicity and metabolic stability at aromatic positions?

• Random radical halogenation
• Directed electrophilic aromatic substitution with appropriate catalysts
• Hydrogenation followed by halide exchange
• Oxidative cleavage then halogenation

Correct Answer: Directed electrophilic aromatic substitution with appropriate catalysts

Q37. Which aromatic functional group strongly activates the ring and often requires protection before halogenation to avoid overreaction?

• –NO2
• –SO3H
• –NH2 (free aniline)
• –CF3

Correct Answer: –NH2 (free aniline)

Q38. In the context of EAS halogenation, what is meant by “regenerating the catalyst”?

• The catalyst is consumed and not recovered
• After deprotonation the Lewis acid is released and becomes available for another cycle
• The catalyst is permanently bound to product
• The catalyst becomes a radical

Correct Answer: After deprotonation the Lewis acid is released and becomes available for another cycle

Q39. Which experimental condition can favor para substitution over ortho during halogenation?

• Very low temperature
• Use of bulky substituents or bulky catalysts/solvents to hinder ortho approach
• Excess electrophile
• Using radical initiators

Correct Answer: Use of bulky substituents or bulky catalysts/solvents to hinder ortho approach

Q40. Which statement is true about chlorination of benzene in presence of strong electron-donating substituents?

• EDGs will block chlorination completely
• EDGs will direct chlorination to ortho/para and increase reaction rate
• EDGs convert chlorination to nucleophilic aromatic substitution
• Chlorination only occurs at benzylic positions

Correct Answer: EDGs will direct chlorination to ortho/para and increase reaction rate

Q41. Why is bromination of benzene often performed at low temperature with a catalyst?

• To avoid formation of bromobenzene
• To control reactivity, reduce polysubstitution, and maintain selectivity
• To make bromine less soluble
• To convert mechanism to SNAr

Correct Answer: To control reactivity, reduce polysubstitution, and maintain selectivity

Q42. Which directing effect does a halogen substituent (e.g., -Cl) exert on further electrophilic substitution?

• Strongly activating and ortho/para directing
• Weakly deactivating but ortho/para directing
• Strongly deactivating and meta directing
• Has no directing effect

Correct Answer: Weakly deactivating but ortho/para directing

Q43. Which of these is a common laboratory precaution when performing aromatic halogenation?

• Use excess oxygen to speed the reaction
• Perform reactions behind a fume hood and control stoichiometry to prevent over-halogenation
• Add water to quench the Lewis acid immediately
• Run the reaction in brightly lit areas to activate halogens

Correct Answer: Perform reactions behind a fume hood and control stoichiometry to prevent over-halogenation

Q44. What is the expected product when benzene reacts with Br2 in absence of catalyst under dark conditions?

• Bromobenzene via EAS
• No significant reaction; benzene is inert without catalyst
• Polybrominated benzene
• Benzylic bromide formation

Correct Answer: No significant reaction; benzene is inert without catalyst

Q45. Which reaction uses a diazonium intermediate to introduce bromine specifically to an aromatic ring?

• Benzylic radical bromination with NBS
• Sandmeyer reaction with CuBr
• Direct Br2/FeBr3 bromination
• Friedel–Crafts bromination

Correct Answer: Sandmeyer reaction with CuBr

Q46. When comparing benzene to anisole, how much more reactive is anisole toward electrophilic halogenation qualitatively?

• Much less reactive than benzene
• Comparable reactivity
• Significantly more reactive due to resonance donation from –OMe
• Reactive only under radical conditions

Correct Answer: Significantly more reactive due to resonance donation from –OMe

Q47. Which observation supports that the rate-determining step in aromatic halogenation is formation of the sigma complex?

• Product formation is instantaneous
• Rate correlates with electron density of the ring; activating groups increase rate
• Rate is independent of substituents
• Reaction proceeds by radical chain mechanism

Correct Answer: Rate correlates with electron density of the ring; activating groups increase rate

Q48. Which reagent would you choose for selective monobromination of activated aromatic ring where over-bromination must be minimized?

• Excess Br2 without catalyst
• Br2 with controlled amount of FeBr3 at low temperature
• NBS under radical conditions in presence of benzene
• I2 with oxidant

Correct Answer: Br2 with controlled amount of FeBr3 at low temperature

Q49. Which of the following is NOT a characteristic of the sigma complex in EAS?

• It is a positively charged intermediate
• Aromaticity is temporarily lost at the attacked carbon
• It is fully aromatic and stabilized by all resonance forms
• It undergoes deprotonation to restore aromaticity

Correct Answer: It is fully aromatic and stabilized by all resonance forms

Q50. How can substitution pattern be controlled when performing electrophilic halogenation on a disubstituted benzene?

• Substitution is random and cannot be predicted
• Consider combined directing effects (activating/deactivating, ortho/para vs meta) and steric factors to predict major product
• Always yields meta product regardless of substituents
• Only temperature controls pattern, not substituents

Correct Answer: Consider combined directing effects (activating/deactivating, ortho/para vs meta) and steric factors to predict major product

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