Sulphonation of benzene MCQs With Answer

Sulphonation of benzene MCQs With Answer — Introduction

Sulphonation of benzene is a fundamental electrophilic aromatic substitution reaction that B. Pharm students must master. This process converts benzene into benzene sulfonic acid using reagents like concentrated sulfuric acid, fuming sulfuric acid (oleum) or sulfur trioxide (SO3). Key learning points include the reaction mechanism (arenium/sigma complex), role of reagents, reversible nature and desulfonation, directing and deactivating effects of the -SO3H group, and analytical identification (IR, NMR). Understanding reaction conditions, industrial sulfonation, and practical workup helps in pharmaceutical synthesis and impurity control. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary product when benzene undergoes sulphonation with fuming sulfuric acid?

• Benzenesulfonic acid
• Benzene oxide
• Benzyl alcohol
• Benzaldehyde

Correct Answer: Benzenesulfonic acid

Q2. Sulphonation of benzene is classified as which type of reaction?

• Electrophilic aromatic substitution
• Nucleophilic substitution
• Free radical substitution
• Addition reaction

Correct Answer: Electrophilic aromatic substitution

Q3. Which species is generally considered the electrophile in the sulphonation of benzene?

• Sulfur trioxide (SO3)
• Sulfate ion (SO4^2-)
• Sulfite ion (SO3^2-)
• Sulfide ion (S^2-)

Correct Answer: Sulfur trioxide (SO3)

Q4. What intermediate is formed during the rate-determining step of benzene sulphonation?

• Arenium ion (sigma complex)
• Free radical cation
• Carbanion intermediate
• Pericyclic transition state

Correct Answer: Arenium ion (sigma complex)

Q5. Which reagent mixture is commonly called oleum?

• SO3 dissolved in concentrated H2SO4
• HCl dissolved in H2O
• SO2 in H2O
• HNO3 mixed with H2SO4

Correct Answer: SO3 dissolved in concentrated H2SO4

Q6. How does the -SO3H group affect further electrophilic substitution on a benzene ring?

• It is strongly deactivating and meta-directing
• It is strongly activating and ortho/para-directing
• It is weakly activating and meta-directing
• It has no directing effect

Correct Answer: It is strongly deactivating and meta-directing

Q7. Which condition favors desulfonation of benzenesulfonic acid back to benzene?

• Heating with dilute aqueous acid or steam
• Cooling to low temperatures
• Treatment with dry HCl gas
• Exposure to strong bases at low temperature

Correct Answer: Heating with dilute aqueous acid or steam

Q8. For selective monosulfonation of benzene, which parameter is most critical?

• Controlling temperature and amount of SO3
• Using a radical initiator
• Adding a strong base
• Running the reaction under high vacuum

Correct Answer: Controlling temperature and amount of SO3

Q9. Compared to benzoic acid, benzenesulfonic acid is:

• Much stronger acid
• Much weaker acid
• Approximately the same acidity
• Non-acidic

Correct Answer: Much stronger acid

Q10. Which spectroscopic technique shows characteristic strong absorption for S=O stretches in sulfonic acids?

• IR spectroscopy
• UV-Vis spectroscopy
• Mass spectrometry
• Polarimetry

Correct Answer: IR spectroscopy

Q11. Which reagent introduces a sulfonyl chloride (-SO2Cl) group to an aromatic ring directly?

• Chlorosulfonic acid (ClSO3H)
• Concentrated nitric acid
• Sodium sulfite
• Hydrochloric acid

Correct Answer: Chlorosulfonic acid (ClSO3H)

Q12. Why is sulphonation considered reversible under certain conditions?

• Because the sulfonic acid can be removed by hydrolysis or heat
• Because SO3 polymerizes and reforms benzene
• Because the sulfonic group rearranges on the ring
• Because benzene decomposes back to benzene precursor

Correct Answer: Because the sulfonic acid can be removed by hydrolysis or heat

Q13. Which substrate is more reactive toward sulphonation: anisole (methoxybenzene) or nitrobenzene?

• Anisole
• Nitrobenzene
• Both equally reactive
• Neither reacts under normal conditions

Correct Answer: Anisole

Q14. In the mechanism of sulphonation, what role does concentrated sulfuric acid play?

• It protonates and generates the active electrophile (SO3/HOSO2+) and acts as solvent
• It acts as a nucleophile attacking the ring
• It provides free radicals to initiate the reaction
• It oxidizes benzene to phenol

Correct Answer: It protonates and generates the active electrophile (SO3/HOSO2+) and acts as solvent

Q15. Which description best fits the arenium ion intermediate in sulphonation?

• A resonance-stabilized carbocation formed after electrophile addition
• A free radical stabilized by resonance
• A negatively charged carbanion
• A neutral cyclic transition state

Correct Answer: A resonance-stabilized carbocation formed after electrophile addition

Q16. What is the likely orientation of sulfonation on toluene under typical conditions?

• Ortho and para positions predominantly
• Only meta position
• Only para exclusively
• No substitution occurs

Correct Answer: Ortho and para positions predominantly

Q17. Which substituent on benzene will strongly deactivate the ring toward sulphonation?

• Nitro (-NO2)
• Methyl (-CH3)
• Methoxy (-OCH3)
• Hydroxyl (-OH)

Correct Answer: Nitro (-NO2)

Q18. Which of the following best explains why aniline is difficult to sulfonate directly using concentrated sulfuric acid?

• The amino group is protonated under strongly acidic conditions, changing reactivity and causing polymerization
• Aniline is too inert to react
• Aniline forms a stable sulfonic acid salt that cannot be isolated
• Concentrated sulfuric acid oxidizes aniline to nitrobenzene

Correct Answer: The amino group is protonated under strongly acidic conditions, changing reactivity and causing polymerization

Q19. Industrially, aromatic sulfonation is widely used to make intermediates for which application?

• Detergents and surfactants
• Polymer cross-linking agents only
• Explosives exclusively
• Metal extraction

Correct Answer: Detergents and surfactants

Q20. Which statement about the temperature effect on sulfonation equilibrium is correct?

• Lower temperatures favor sulfonation (formation of sulfonic acid) while higher temperatures favor desulfonation
• Higher temperatures always favor sulfonation exclusively
• Temperature has no effect on sulfonation equilibrium
• Lower temperatures promote desulfonation

Correct Answer: Lower temperatures favor sulfonation (formation of sulfonic acid) while higher temperatures favor desulfonation

Q21. Which analytical change indicates successful sulfonation in 1H NMR of a substituted benzene?

• Downfield shifts of aromatic protons and disappearance or change of original patterns due to new substituent
• Appearance of an intense peak at 7.27 ppm only
• Complete loss of all aromatic signals
• Only aliphatic signals appear

Correct Answer: Downfield shifts of aromatic protons and disappearance or change of original patterns due to new substituent

Q22. Which bond vibration region in IR is characteristic of sulfonic acid S=O stretching?

• Approximately 1000–1350 cm^-1 (strong bands)
• Approximately 3000–3500 cm^-1 only
• Near 1600 cm^-1 only
• No characteristic bands in IR

Correct Answer: Approximately 1000–1350 cm^-1 (strong bands)

Q23. In aromatic sulfonation, which species serves as the nucleophile?

• The pi electrons of the aromatic ring
• Sulfur trioxide
• The sulfate anion
• Water molecule

Correct Answer: The pi electrons of the aromatic ring

Q24. Which pathway is typically used to reverse sulfonation (remove -SO3H) in synthetic chemistry?

• Hydrolysis/desulfonation by heating with dilute acid or steam
• Treatment with strong bases at low temperature
• Oxidation with KMnO4
• Hydrogenation with Pd/C

Correct Answer: Hydrolysis/desulfonation by heating with dilute acid or steam

Q25. What is the effect of an electron-donating substituent on the rate of benzene sulfonation?

• It increases the reaction rate
• It decreases the reaction rate
• It has no effect
• It always prevents the reaction

Correct Answer: It increases the reaction rate

Q26. Which of the following is NOT typically required for aromatic sulfonation?

• A Lewis acid catalyst like AlCl3
• Strong acid medium (H2SO4 or oleum)
• SO3 or equivalent sulfonating agent
• Control of temperature for selectivity

Correct Answer: A Lewis acid catalyst like AlCl3

Q27. Which product results when benzene reacts with chlorosulfonic acid followed by hydrolysis?

• Benzenesulfonic acid
• Benzene sulfonyl chloride without further reaction
• Benzene diol
• Benzoic acid

Correct Answer: Benzenesulfonic acid

Q28. Which statement best describes regioselectivity in sulfonation of phenol under mild conditions?

• Phenol is strongly activated and gives ortho and para sulfonation
• Phenol is deactivated and gives only meta product
• Phenol does not undergo sulfonation
• Phenol gives only para at all temperatures

Correct Answer: Phenol is strongly activated and gives ortho and para sulfonation

Q29. When sulfonation is used as a blocking group in synthetic chemistry, what is the main advantage?

• It temporarily deactivates positions to direct subsequent substitution elsewhere and can be removed later
• It permanently converts the molecule to an inactive form
• It increases volatility for distillation
• It protects the ring from any further reactions permanently

Correct Answer: It temporarily deactivates positions to direct subsequent substitution elsewhere and can be removed later

Q30. Which observation indicates over-sulfonation (polysulfonation) has occurred?

• Multiple new sulfonic acid signals and reduced aromatic proton count in NMR
• Only a single unchanged aromatic peak
• No change in IR spectrum
• Complete disappearance of carbon signals in 13C NMR

Correct Answer: Multiple new sulfonic acid signals and reduced aromatic proton count in NMR

Q31. What safety concern is most relevant when handling fuming sulfuric acid (oleum)?

• It releases SO3 fumes and is highly corrosive, requiring strict PPE
• It is a weak irritant only
• It is flammable and easily ignites
• It produces toxic carbon monoxide

Correct Answer: It releases SO3 fumes and is highly corrosive, requiring strict PPE

Q32. Which is a common laboratory method to monitor progress of sulfonation?

• TLC and IR spectroscopy
• Flame test
• Refractive index only
• Polarimetry only

Correct Answer: TLC and IR spectroscopy

Q33. What is the effect of strong electron-withdrawing groups on orientation for sulfonation?

• They direct the incoming electrophile to the meta position
• They direct to ortho and para positions
• They make the ring inert to any substitution
• They cause ring cleavage

Correct Answer: They direct the incoming electrophile to the meta position

Q34. Which reagent pair can generate the electrophile for sulfonation in situ?

• Concentrated H2SO4 + SO3
• HCl + NaNO2
• NaOH + H2O2
• AlCl3 + Br2

Correct Answer: Concentrated H2SO4 + SO3

Q35. Why is sulfonation useful in medicinal chemistry?

• It increases water solubility and introduces a handle for further derivatization
• It always makes compounds inactive biologically
• It prevents any further chemical modifications
• It solely increases volatility

Correct Answer: It increases water solubility and introduces a handle for further derivatization

Q36. Which of the following best describes monosulfonation selectivity on substituted benzenes?

• Dependent on substituent electronic effects, reagent concentration, and temperature
• Completely random regardless of conditions
• Independent of substituents but dependent on pressure only
• Only steric effects determine selectivity

Correct Answer: Dependent on substituent electronic effects, reagent concentration, and temperature

Q37. What is the major challenge when sulfonating strongly activated aromatic rings?

• Over-sulfonation (multiple sulfonic groups) and resinification
• Complete inactivity to sulfonation
• Formation of azides
• Conversion to carbonyl compounds

Correct Answer: Over-sulfonation (multiple sulfonic groups) and resinification

Q38. Which of the following correctly ranks reactivity toward sulfonation (most to least)?

• Anisole > Toluene > Benzene > Nitrobenzene
• Nitrobenzene > Benzene > Toluene > Anisole
• Benzene > Anisole > Toluene > Nitrobenzene
• Toluene > Nitrobenzene > Anisole > Benzene

Correct Answer: Anisole > Toluene > Benzene > Nitrobenzene

Q39. Which product would form if phenol is sulfonated and then the sulfonic acid group is replaced by bromine via Sandmeyer-like chemistry?

• Such a direct replacement is not standard; sulfonation can be used as a blocking group before bromination at other sites
• p-Bromophenol from direct substitution
• Bromobenzene directly
• No reaction is possible under any conditions

Correct Answer: Such a direct replacement is not standard; sulfonation can be used as a blocking group before bromination at other sites

Q40. What happens to the aromaticity of benzene during formation of the sigma complex in sulfonation?

• Aromaticity is temporarily lost and restored after deprotonation
• Aromaticity is permanently destroyed
• Aromaticity is unaffected throughout
• Aromaticity is converted to antiaromaticity permanently

Correct Answer: Aromaticity is temporarily lost and restored after deprotonation

Q41. Which of the following is a typical workup step after sulfonation to isolate the product?

• Pouring reaction mixture into ice/water and neutralization or extraction
• Direct distillation without quenching
• Evaporation under high vacuum at high temperature only
• Adding concentrated base and heating to reflux forever

Correct Answer: Pouring reaction mixture into ice/water and neutralization or extraction

Q42. In which situation is sulfonation used as a protective (blocking) group in synthesis?

• When one needs to block a position on the ring to direct future substitutions elsewhere
• When permanent deactivation of the molecule is required
• When a leaving group for SN1 is needed
• When radical polymerization is desired

Correct Answer: When one needs to block a position on the ring to direct future substitutions elsewhere

Q43. How does the presence of water influence sulfonation reactions?

• Water can hydrolyze SO3 and favor desulfonation; moisture control is important
• Water accelerates formation of sulfonic acids always
• Water converts sulfonic acids to sulfones directly
• Water has no effect on sulfonation chemistry

Correct Answer: Water can hydrolyze SO3 and favor desulfonation; moisture control is important

Q44. Which statement about electrophile strength in sulfonation compared to nitration is correct?

• SO3 is a strong electrophile but the overall sulfonation electrophile activity depends on medium; both are powerful EAS electrophiles
• SO3 is always weaker than NO2+ in any medium
• NO2+ is always weaker than SO3 in any medium
• Neither SO3 nor NO2+ acts as electrophiles in EAS

Correct Answer: SO3 is a strong electrophile but the overall sulfonation electrophile activity depends on medium; both are powerful EAS electrophiles

Q45. Which laboratory precaution is important when adding SO3 to concentrated sulfuric acid?

• Add SO3 slowly with cooling to control temperature and limit exotherm
• Add SO3 all at once at room temperature
• Mix in open air without ventilation
• Use no protective equipment because it is safe

Correct Answer: Add SO3 slowly with cooling to control temperature and limit exotherm

Q46. Which of the following is a common derivative prepared from benzenesulfonic acid in drug synthesis?

• Sulfonamides via reaction with amines
• Esters via Fischer esterification only
• Peroxides by oxidation
• Alkyl halides by direct substitution

Correct Answer: Sulfonamides via reaction with amines

Q47. What happens to the activation energy when an electron-withdrawing group is present on benzene for sulfonation?

• The activation energy increases, slowing the reaction
• The activation energy decreases, speeding the reaction
• There is no change in activation energy
• Activation energy becomes zero

Correct Answer: The activation energy increases, slowing the reaction

Q48. Which technique can most directly confirm the position (ortho/meta/para) of sulfonation on a monosubstituted benzene?

• 1H and 13C NMR combined with comparison of splitting patterns and chemical shifts
• Thin layer chromatography alone
• Simple melting point measurement only
• Visual color change observation

Correct Answer: 1H and 13C NMR combined with comparison of splitting patterns and chemical shifts

Q49. Which property of benzenesulfonic acid makes it useful as an intermediate for salt formation in pharmaceuticals?

• Its strong acidity allows formation of stable water-soluble salts
• It is nonpolar and insoluble, useful for extraction
• It forms unstable salts only
• It is highly volatile

Correct Answer: Its strong acidity allows formation of stable water-soluble salts

Q50. Which change in procedure would help avoid polysulfonation when sulfonating an activated aromatic compound?

• Use lower temperature, dilute sulfonating agent, and short reaction time
• Increase temperature and use excess SO3
• Use radical initiators and long reaction times
• Perform the reaction in strongly basic aqueous solution

Correct Answer: Use lower temperature, dilute sulfonating agent, and short reaction time

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