This concise guide on Structure and uses of benzoic acid MCQs With Answer is tailored for B. Pharm students. It explains benzoic acid’s molecular structure, resonance-stabilized carboxyl group, acidity (pKa), and common derivatives such as benzoates and esters, and links these features to pharmaceutical applications—preservatives, antimicrobial agents, and synthesis intermediates. Coverage includes industrial production (oxidation of toluene), analytical identification (IR, NMR, melting point), formulation considerations, safety, and regulatory standards. Questions include mechanism, synthesis, physicochemical properties, and pharmaceutical quality control to prepare you for university exams and competitive tests. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the molecular formula of benzoic acid?
- C6H6O2
- C7H6O2
- C7H8O2
- C8H8O2
Correct Answer: C7H6O2
Q2. What is the approximate molar mass of benzoic acid?
- 122 g/mol
- 136 g/mol
- 100 g/mol
- 150 g/mol
Correct Answer: 122 g/mol
Q3. Which functional group defines benzoic acid?
- Alcohol
- Aldehyde
- Carboxylic acid
- Ketone
Correct Answer: Carboxylic acid
Q4. What is the typical pKa value of benzoic acid?
- 2.1
- 4.2
- 7.0
- 9.6
Correct Answer: 4.2
Q5. Which resonance effect most stabilizes the benzoate anion?
- Inductive donation from the ring
- Hyperconjugation of alkyl substituents
- Delocalization of negative charge into the aromatic ring and carbonyl
- Hydrogen bonding with water
Correct Answer: Delocalization of negative charge into the aromatic ring and carbonyl
Q6. How is benzoic acid industrially produced on a large scale?
- Hydrolysis of benzyl chloride
- Oxidation of toluene
- Friedel–Crafts acylation of benzene
- Nitration of benzaldehyde
Correct Answer: Oxidation of toluene
Q7. Which catalyst or condition is commonly used for oxidation of toluene to benzoic acid?
- BF3 at low temperature
- Cobalt/manganese catalysts with air or oxygen
- Nitrate salts at room temperature
- Hydrogenation over Pd/C
Correct Answer: Cobalt/manganese catalysts with air or oxygen
Q8. Which IR absorption is characteristic of the carbonyl (C=O) group in benzoic acid?
- 1600–1500 cm−1
- 1700–1725 cm−1
- 2800–3000 cm−1
- 3300–3600 cm−1
Correct Answer: 1700–1725 cm−1
Q9. What IR feature indicates the O–H stretch of a carboxylic acid like benzoic acid?
- Sharp peak at 1700 cm−1
- Broad band 2500–3300 cm−1
- Sharp peak at 2250 cm−1
- No O–H signal in IR
Correct Answer: Broad band 2500–3300 cm−1
Q10. In 1H NMR, where does the carboxylic acid proton of benzoic acid typically appear?
- 0–1 ppm
- 2–3 ppm
- 7–8 ppm
- 10–13 ppm (broad)
Correct Answer: 10–13 ppm (broad)
Q11. Which statement about the acidity of benzoic acid compared to acetic acid is correct?
- Benzoic acid is significantly less acidic than acetic acid due to resonance
- Benzoic acid is more acidic than acetic acid because the aromatic ring stabilizes the anion
- They have identical pKa values
- Benzoic acid is a base, not an acid
Correct Answer: Benzoic acid is more acidic than acetic acid because the aromatic ring stabilizes the anion
Q12. Which substitution pattern is formed predominantly when benzoic acid undergoes electrophilic aromatic nitration?
- Ortho and para directed
- Meta directed
- Only para directed
- No nitration occurs
Correct Answer: Meta directed
Q13. Why is benzoic acid a meta-director in electrophilic aromatic substitution?
- Carboxyl group is an electron-donating group by resonance
- Carboxyl group is electron-withdrawing by resonance and deactivating
- Because of steric hindrance only
- Benzoic acid has no directing influence
Correct Answer: Carboxyl group is electron-withdrawing by resonance and deactivating
Q14. Which reagent converts benzoic acid to benzoyl chloride?
- Sodium hydroxide
- Sulfuric acid
- Thionyl chloride (SOCl2)
- Hydrogen peroxide
Correct Answer: Thionyl chloride (SOCl2)
Q15. Which product forms when benzoic acid reacts with methanol in presence of acid catalyst?
- Benzyl alcohol
- Methyl benzoate (esterification)
- Toluene
- Benzoic anhydride
Correct Answer: Methyl benzoate (esterification)
Q16. Which salt of benzoic acid is commonly used as a food and pharmaceutical preservative?
- Calcium benzoate
- Potassium benzoate
- Sodium benzoate
- Magnesium benzoate
Correct Answer: Sodium benzoate
Q17. What is the primary mechanism by which benzoic acid (or benzoate) exerts antimicrobial effect in formulations?
- Alkylation of DNA
- Disruption of metabolic enzymes by the undissociated acid penetrating cell membranes
- Chelation of essential metals
- Oxidative damage to membranes
Correct Answer: Disruption of metabolic enzymes by the undissociated acid penetrating cell membranes
Q18. Under which pH condition is benzoic acid most effective as a preservative?
- High pH (alkaline)
- Neutral pH (~7)
- Low pH (acidic)
- pH has no effect
Correct Answer: Low pH (acidic)
Q19. Which is a classic qualitative test to distinguish carboxylic acids like benzoic acid from phenols?
- Sodium bicarbonate effervescence test
- Ferric chloride gives violet color
- Tollens’ test
- Lucas test
Correct Answer: Sodium bicarbonate effervescence test
Q20. Which of the following statements about solubility of benzoic acid is correct?
- Benzoic acid is highly soluble in water at room temperature
- Benzoic acid is insoluble in organic solvents like ethanol
- Benzoic acid solubility in water increases with temperature
- Benzoic acid is more soluble in water than sodium benzoate
Correct Answer: Benzoic acid solubility in water increases with temperature
Q21. Which chromatographic mobile phase is commonly used to separate benzoic acid on TLC?
- Pure hexane
- Mixture of ethyl acetate and hexane
- 100% water
- Chloroform with base
Correct Answer: Mixture of ethyl acetate and hexane
Q22. Which derivative of benzoic acid is used as an antimicrobial preservative in many pharmaceuticals (E-number E210/211)?
- Benzoic anhydride
- Sodium benzoate
- Benzoyl peroxide
- Benzyl benzoate
Correct Answer: Sodium benzoate
Q23. Which reaction condition will decarboxylate benzoic acid to give benzene?
- Heating neat at mild temperature without catalyst
- High-temperature decarboxylation with copper catalyst or solid-phase decarboxylation
- Treating with NaOH at room temperature
- Reaction with methanol and acid
Correct Answer: High-temperature decarboxylation with copper catalyst or solid-phase decarboxylation
Q24. Which of the following is NOT a typical pharmaceutical use of benzoic acid or its salts?
- Preservative in oral and topical formulations
- Synthesis intermediate for benzamide derivatives
- Primary active analgesic at therapeutic doses
- Antifungal adjunct in topical mixtures
Correct Answer: Primary active analgesic at therapeutic doses
Q25. Which chromatographic detector is most useful for detecting benzoic acid in HPLC?
- Refractive index detector
- UV detector around 230 nm
- Evaporative light scattering detector for very volatile acids
- Fluorescence detector at 500 nm
Correct Answer: UV detector around 230 nm
Q26. In pharmaceutical formulations, why are benzoic acid and sodium benzoate often used at low concentrations?
- High concentrations cause microbial resistance
- They can cause irritation or toxicity at high concentrations and regulatory limits exist
- They volatilize rapidly
- They react to form polymers at high concentration
Correct Answer: They can cause irritation or toxicity at high concentrations and regulatory limits exist
Q27. Which reagent would be used to qualitatively detect benzoic acid via titrimetric assay in pharmacopeial methods?
- 0.1 M hydrochloric acid
- Sodium chloride solution
- 0.1 M sodium hydroxide
- Silver nitrate
Correct Answer: 0.1 M sodium hydroxide
Q28. Which of the following best describes the UV-Vis absorption property of benzoic acid?
- Strong absorption above 400 nm
- Characteristic π→π* absorption around 230 nm due to the aromatic ring
- No UV absorption
- Only σ→σ* transitions visible
Correct Answer: Characteristic π→π* absorption around 230 nm due to the aromatic ring
Q29. Which reaction is disfavored on benzoic acid due to the deactivating nature of the carboxyl group?
- Nitration at meta position
- Oxidation of side chain
- Friedel–Crafts alkylation/acylation on the ring
- Conversion to benzoyl chloride
Correct Answer: Friedel–Crafts alkylation/acylation on the ring
Q30. What is the expected m/z value for the molecular ion peak (M+) of benzoic acid in mass spectrometry (approximate)?
- 106
- 122
- 138
- 150
Correct Answer: 122
Q31. Which of the following statements about benzoic acid salts is true?
- Sodium benzoate is less soluble in water than benzoic acid
- Benzoic acid salts are generated by neutralization with base
- Benzoic acid cannot form salts
- Benzoic acid salts are more volatile than the parent acid
Correct Answer: Benzoic acid salts are generated by neutralization with base
Q32. What is the effect of electron-withdrawing substituents (e.g., nitro) on the acidity of benzoic acid?
- Decrease acidity (increase pKa)
- No effect
- Increase acidity (lower pKa)
- Convert it into a base
Correct Answer: Increase acidity (lower pKa)
Q33. Which positional isomer of nitrobenzoic acid is most readily formed by direct nitration of benzoic acid?
- Ortho-nitrobenzoic acid predominantly
- Meta-nitrobenzoic acid predominantly
- Para-nitrobenzoic acid only
- No nitro substitution occurs
Correct Answer: Meta-nitrobenzoic acid predominantly
Q34. Which pharmaceutical quality parameter is commonly checked for benzoic acid raw material?
- Moisture content and assay by titration
- Optical rotation only
- Protein content
- Blood compatibility
Correct Answer: Moisture content and assay by titration
Q35. Which safety precaution is most relevant when handling benzoic acid powder in a lab?
- Benzoic acid is highly explosive so use blast shields
- Avoid inhalation and skin contact; use gloves and dust mask
- No precautions needed; it is completely inert
- Keep under nitrogen to prevent oxidation
Correct Answer: Avoid inhalation and skin contact; use gloves and dust mask
Q36. In a formulation, benzoic acid is incompatible with which type of excipient?
- Strong bases (they neutralize acid)
- Non-polar hydrocarbons
- Cellulose derivatives
- Sugars
Correct Answer: Strong bases (they neutralize acid)
Q37. Which derivative is formed when benzoic acid undergoes reaction with phosphorus pentachloride (PCl5)?
- Benzoyl fluoride
- Benzoyl chloride
- Benzyl chloride
- Benzoic aldehyde
Correct Answer: Benzoyl chloride
Q38. Which test will give a positive result for benzoic acid but not for sodium benzoate?
- Solubility in water
- Effervescence with sodium bicarbonate
- Formation of a colored complex with ferric chloride
- UV absorption at 230 nm
Correct Answer: Effervescence with sodium bicarbonate
Q39. Which pathway is used to synthesize benzoic acid derivatives in medicinal chemistry?
- Reduction of benzoic acid to benzyl alcohol followed by substitution
- Functionalization of benzoyl chloride to form amides and esters
- Complete ring hydrogenation then oxidation
- Direct enzymatic conversion from glucose
Correct Answer: Functionalization of benzoyl chloride to form amides and esters
Q40. Which property of benzoic acid is most relevant to its effectiveness as a preservative in acidic foods?
- High melting point
- Ability to exist in undissociated form at low pH and diffuse into microbes
- Bright color aiding detection
- Strong buffering capacity
Correct Answer: Ability to exist in undissociated form at low pH and diffuse into microbes
Q41. Which pharmacopoeial method is commonly used to assay benzoic acid content in raw material?
- Gravimetric precipitation only
- Titration with standard base after dissolution
- Karl Fischer titration
- Osmolarity measurement
Correct Answer: Titration with standard base after dissolution
Q42. In formulation compatibility studies, why is it important to test benzoic acid with primary amines?
- They may form insoluble benzoic salts only
- They may react to form amides or alter preservative efficacy
- There is no interaction possible
- Amines will oxidize benzoic acid
Correct Answer: They may react to form amides or alter preservative efficacy
Q43. Which of the following is a limitation of using sodium benzoate as a preservative in certain beverages?
- It forms benzene when combined with ascorbic acid under certain conditions
- It reacts with sugars to form polymers
- It is highly colored and alters appearance
- It increases the pH dramatically
Correct Answer: It forms benzene when combined with ascorbic acid under certain conditions
Q44. Which spectral signal in 13C NMR is typical for the carboxyl carbon of benzoic acid?
- 0–50 ppm
- 100–120 ppm
- 170–180 ppm
- 200–220 ppm
Correct Answer: 170–180 ppm
Q45. Which of the following is a common impurity expected in technical benzoic acid produced from toluene oxidation?
- Benzoic alcohol exclusively
- Benzaldehyde and other partially oxidized intermediates
- Only inorganic salts
- No impurities are formed in the oxidation route
Correct Answer: Benzaldehyde and other partially oxidized intermediates
Q46. Which biotransformation pathway commonly degrades benzoic acid in environmental microbial systems?
- Direct halogenation to chlorobenzoic acids only
- Conjugation to glycine forming hippuric acid in mammals and microbial ring cleavage pathways in microbes
- Polymerization into harmless solids
- Reduction to benzene extensively
Correct Answer: Conjugation to glycine forming hippuric acid in mammals and microbial ring cleavage pathways in microbes
Q47. For formulation stability, what packaging consideration is important for benzoic acid-containing products?
- Opaque, airtight containers to minimize moisture uptake and photodegradation
- Glass ampoules always because plastics react
- Open-top containers to allow venting
- No special packaging required
Correct Answer: Opaque, airtight containers to minimize moisture uptake and photodegradation
Q48. Which reagent would convert benzoic acid to benzyl alcohol?
- LiAlH4 (lithium aluminum hydride) reduction
- Oxidation with KMnO4
- Treatment with SOCl2
- Nitration followed by hydrogenation
Correct Answer: LiAlH4 (lithium aluminum hydride) reduction
Q49. In an HPLC assay for benzoic acid, what internal standard property is most desirable?
- Completely different retention time and no UV response
- Similar chemical behavior and retention time but resolvable peak
- React with benzoic acid to form adducts
- Identical compound to benzoic acid
Correct Answer: Similar chemical behavior and retention time but resolvable peak
Q50. Which regulatory consideration is most important when using benzoic acid derivatives in pharmaceutical products?
- Ensuring compliance with preservative concentration limits and labeling requirements
- Benzoic acid derivatives are always unregulated
- Only color limits matter
- Packaging must be metal only
Correct Answer: Ensuring compliance with preservative concentration limits and labeling requirements
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