Halogenation: types, kinetics and industrial case studies MCQs With Answer

Introduction:
This quiz collection on Halogenation — covering types, kinetics and industrial case studies — is tailored for M.Pharm students studying MPC 204T Pharmaceutical Process Chemistry. It emphasizes mechanistic distinctions (electrophilic, radical, and addition halogenation), kinetic features (rate‑determining steps, kinetic isotope effects, chain length), reagent selection and stereochemical outcomes. Industrial case studies focus on common processes (benzylic/alkane halogenation, production of chlorinated intermediates), environmental concerns (THMs, dioxins) and practical reagents (NBS, SOCl2, FeCl3). Questions reinforce problem solving and process understanding needed for designing safe, selective and scalable halogenation steps in pharmaceutical manufacturing.

Q1. Which type of halogenation of aromatic compounds proceeds through formation of an arenium (sigma) complex and typically requires a Lewis acid catalyst?

• Radical halogenation of alkanes
• Electrophilic aromatic halogenation (requires Lewis acid catalyst)
• Nucleophilic aromatic substitution (SNAr)
• Addition halogenation of alkenes

Correct Answer: Electrophilic aromatic halogenation (requires Lewis acid catalyst)

Q2. In free‑radical halogenation of alkanes, which elementary step is generally the rate‑determining step under typical chain propagation conditions?

• Initiation: homolysis of X2
• Propagation step: hydrogen abstraction by halogen radical
• Termination: radical–radical recombination
• Chain transfer to solvent

Correct Answer: Propagation step: hydrogen abstraction by halogen radical

Q3. Which halogenation process shows the highest selectivity for tertiary C–H abstraction over primary C–H abstraction?

• Fluorination
• Chlorination
• Bromination
• Iodination

Correct Answer: Bromination

Q4. Addition of molecular halogen (X2) to an alkene typically occurs with which stereochemical outcome and intermediate?

• Syn addition via carbocation intermediate
• Anti addition via bridged halonium ion intermediate
• Free‑radical addition with retention of configuration
• Concerted pericyclic addition with suprafacial approach

Correct Answer: Anti addition via bridged halonium ion intermediate

Q5. Which reagent is commonly employed for selective allylic or benzylic bromination under radical conditions?

• N‑Bromosuccinimide (NBS) in presence of light or peroxide
• N‑Chlorosuccinimide (NCS) under acidic conditions
• PCl5 with pyridine
• Br2 in aqueous medium at 0 °C

Correct Answer: N‑Bromosuccinimide (NBS) in presence of light or peroxide

Q6. Which catalyst is typically used to promote electrophilic chlorination of benzene in laboratory and industrial aromatic chlorination?

• FeCl3 (iron(III) chloride)
• Pd/C (palladium on carbon)
• NaBH4 (sodium borohydride)
• Alkali metal hydroxide

Correct Answer: FeCl3 (iron(III) chloride)

Q7. In Hammett correlation studies of electrophilic aromatic substitution (including halogenation), the Hammett ρ (rho) value is typically:

• Large positive, indicating increased rate with electron withdrawing groups
• Large negative, indicating increased rate with electron donating groups
• Zero, indicating no substituent effect
• Variable random sign depending on solvent

Correct Answer: Large negative, indicating increased rate with electron donating groups

Q8. The ‘peroxide effect’ (anti‑Markovnikov addition) is observed for addition of hydrogen halides to alkenes with which hydrogen halide?

• HCl
• HBr
• HI
• HF

Correct Answer: HBr

Q9. Industrial production of methyl chloride (CH3Cl) from methane typically uses which overall approach?

• Free‑radical chlorination of methane with Cl2 and ultraviolet light
• Electrophilic chlorination using FeCl3 catalyst at room temperature
• SN2 substitution of methanol with NaCl in acetone
• Direct chlorination with HCl in presence of Lewis acid

Correct Answer: Free‑radical chlorination of methane with Cl2 and ultraviolet light

Q10. Which set of reagents and solvent is classically used for the Finkelstein halide exchange (to convert an alkyl chloride or bromide to the iodide)?

• NaI in acetone
• KI in water
• LiBr in hexane
• AgNO3 in benzene

Correct Answer: NaI in acetone

Q11. Observation of a large primary kinetic isotope effect (KIE) for a halogenation reaction most directly indicates which mechanistic feature?

• The rate‑determining step involves breaking of a C–H bond
• The reaction is concerted with no discrete bond breaking
• The rate‑determining step is radical recombination
• The halogen attack is purely electrophilic with no C–H involvement

Correct Answer: The rate‑determining step involves breaking of a C–H bond

Q12. Which reagent is commonly used to convert carboxylic acids to the corresponding acid chlorides in process chemistry?

• Thionyl chloride (SOCl2)
• Sodium chloride (NaCl)
• Hydrogen chloride gas alone
• Nitric acid (HNO3)

Correct Answer: Thionyl chloride (SOCl2)

Q13. Addition of Br2 to cyclohexene gives which stereochemical relationship between the two bromine substituents?

• Cis‑1,2‑dibromide due to syn addition
• Trans‑1,2‑dibromide due to anti addition via bromonium ion
• Mixture of cis and trans via radical pathway
• Retention of starting stereochemistry without addition

Correct Answer: Trans‑1,2‑dibromide due to anti addition via bromonium ion

Q14. In hydrogen‑abstraction reactivity of halogen radicals (X•), which radical is the most reactive toward C–H bond abstraction?

• Fluorine radical (F•)
• Chlorine radical (Cl•)
• Bromine radical (Br•)
• Iodine radical (I•)

Correct Answer: Fluorine radical (F•)

Q15. A major undesired class of by‑products formed during chlorination of drinking water that raises regulatory concern is:

• Trihalomethanes (THMs) such as chloroform
• Simple alkanes like methane and ethane
• Inert noble gases
• Carboxylic acids exclusively

Correct Answer: Trihalomethanes (THMs) such as chloroform

Q16. The key intermediate in electrophilic aromatic halogenation (bromination/chlorination) is commonly named:

• Carbocationic rearrangement complex
• Wheland intermediate (sigma complex / arenium ion)
• Free radical cation
• Radical chain carrier

Correct Answer: Wheland intermediate (sigma complex / arenium ion)

Q17. Which initiation method is typically employed to generate halogen radicals for free‑radical halogenation reactions?

• Photochemical homolysis of X2 using UV light
• Thermal generation of carbocations at low temperature
• Electrophilic activation with Lewis acid
• Metal‑catalyzed nucleophilic substitution

Correct Answer: Photochemical homolysis of X2 using UV light

Q18. For selective benzylic bromination of toluene to give benzyl bromide in laboratory practice, which reagent/condition is often chosen to maximize selectivity?

• N‑Bromosuccinimide (NBS) with radical initiator or light
• Excess Br2 in water at elevated temperature
• HBr addition under acidic conditions
• Direct chlorination with Cl2/hv

Correct Answer: N‑Bromosuccinimide (NBS) with radical initiator or light

Q19. In radical chain halogenation, the term ‘chain length’ (ν) is defined as:

• The number of initiation steps per termination event
• The number of propagation cycles (substrate molecules transformed) per initiation event
• The number of radicals present at steady state
• The order of the reaction with respect to halogen

Correct Answer: The number of propagation cycles (substrate molecules transformed) per initiation event

Q20. In a Finkelstein reaction converting R–Cl to R–I using NaI in acetone, what is the principal thermodynamic driving force for the reaction to proceed?

• Formation and precipitation of NaCl (insoluble in acetone)
• Generation of molecular chlorine gas
• Strong acid formation driving equilibrium
• Complexation of iodide by acetone

Correct Answer: Formation and precipitation of NaCl (insoluble in acetone)

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