Esters – structure and properties MCQs With Answer

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Esters – structure and properties MCQs With Answer is a concise, exam-focused introduction for B. Pharm students studying pharmaceutical chemistry. This topic covers ester functional groups (R–C(=O)–O–R’), IUPAC nomenclature, formation methods like Fischer esterification and Steglich coupling, and breakdown reactions such as acid/base hydrolysis and enzymatic cleavage. You will learn how ester polarity, hydrogen-bonding behavior, boiling points, solubility, and reactivity influence formulation, drug design, prodrug strategies, and stability of pharmaceutical preparations. Spectroscopic identification (IR, NMR, MS) and industrial relevance (flavoring, fragrance, lipid esters) are emphasized. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which functional group defines an ester?

  • R–C(=O)–O–R’
  • R–C(=O)–OH
  • R–O–H
  • R–C≡N

Correct Answer: R–C(=O)–O–R’

Q2. In IUPAC nomenclature, an ester of ethanol and acetic acid is named as:

  • Ethyl acetate
  • Acetic ethanol
  • Ethyl ethanoate
  • Acetyl alcohol

Correct Answer: Ethyl acetate

Q3. Which reagent combination commonly catalyzes Fischer esterification?

  • Carboxylic acid, alcohol, and strong acid catalyst
  • Carboxylic acid, alcohol, and base catalyst
  • Acyl chloride and water
  • Alcohol and hydrogen peroxide

Correct Answer: Carboxylic acid, alcohol, and strong acid catalyst

Q4. Which condition shifts the Fischer esterification equilibrium toward ester formation?

  • Removing water produced
  • Adding excess water
  • Lowering temperature drastically
  • Adding a large amount of base

Correct Answer: Removing water produced

Q5. Which reagent is commonly used for activating carboxylic acids in Steglich esterification?

  • DCC (dicyclohexylcarbodiimide)
  • LiAlH4
  • NaBH4
  • H2/Pt

Correct Answer: DCC (dicyclohexylcarbodiimide)

Q6. Which catalyst often accompanies DCC in Steglich esterification to improve rate and yield?

  • DMAP (4-dimethylaminopyridine)
  • NaOH
  • Pd/C
  • H2SO4

Correct Answer: DMAP (4-dimethylaminopyridine)

Q7. Ester hydrolysis under basic conditions is called:

  • Saponification
  • Amidation
  • Transesterification
  • Hydrogenation

Correct Answer: Saponification

Q8. What are the products of base-catalyzed hydrolysis (saponification) of an ester?

  • Carboxylate salt and alcohol
  • Carboxylic acid and ester
  • Ketone and water
  • Amide and alcohol

Correct Answer: Carboxylate salt and alcohol

Q9. Acid-catalyzed hydrolysis of an ester gives:

  • Carboxylic acid and alcohol
  • Carboxylate salt and alcohol
  • Ketone and amine
  • Alcohol and ether

Correct Answer: Carboxylic acid and alcohol

Q10. Which spectroscopic absorption is characteristic of esters in IR spectroscopy?

  • C=O stretch around 1735–1750 cm⁻¹
  • O–H broad band at 3200–3600 cm⁻¹
  • N–H stretch at 3300 cm⁻¹
  • C≡N stretch at 2200–2250 cm⁻¹

Correct Answer: C=O stretch around 1735–1750 cm⁻¹

Q11. In 1H NMR, ester alpha-protons (adjacent to carbonyl) typically appear:

  • Downfield relative to alkyl protons (deshielded)
  • Far upfield at negative ppm
  • As a very sharp singlet only
  • Indistinguishable from aromatic protons

Correct Answer: Downfield relative to alkyl protons (deshielded)

Q12. Which of the following increases ester hydrolysis rate?

  • Electron-withdrawing groups on the acyl carbon
  • Bulky alkyl groups on the alcohol side only
  • Presence of strong base inhibitors
  • Decreased temperature

Correct Answer: Electron-withdrawing groups on the acyl carbon

Q13. Transesterification is the exchange of:

  • The alkoxy group of an ester with another alcohol
  • Carbonyl oxygen for sulfur
  • An ester into a ketone directly
  • Amide to ester without reagents

Correct Answer: The alkoxy group of an ester with another alcohol

Q14. Which catalyst is commonly used for acid-catalyzed transesterification?

  • Sulfuric acid
  • Lithium aluminum hydride
  • Palladium catalyst
  • Silver nitrate

Correct Answer: Sulfuric acid

Q15. Which ester property explains why many esters have pleasant fruity odors?

  • Volatility and specific molecular shapes interacting with olfactory receptors
  • High polarity causing no smell
  • Strong ionic character
  • High melting points

Correct Answer: Volatility and specific molecular shapes interacting with olfactory receptors

Q16. Which ester is commonly used as an analgesic prodrug example (acetylated phenol)?

  • Aspirin (acetylsalicylic acid)
  • Ibuprofen
  • Paracetamol free base
  • Penicillin G

Correct Answer: Aspirin (acetylsalicylic acid)

Q17. Which of these reagents converts a carboxylic acid to an acyl chloride, facilitating ester formation?

  • SOCl2 (thionyl chloride)
  • NaBH4
  • H2/Pd
  • PCC

Correct Answer: SOCl2 (thionyl chloride)

Q18. Esters are generally less polar than corresponding carboxylic acids because:

  • They cannot donate hydrogen bonds as acids do
  • They are ionic
  • They contain nitrogen
  • They have higher molecular weights only

Correct Answer: They cannot donate hydrogen bonds as acids do

Q19. Which statement about boiling points is true for esters compared to alcohols of similar molecular weight?

  • Esters usually have lower boiling points than alcohols
  • Esters always boil at higher temperatures
  • Esters and alcohols have identical boiling points
  • Esters are solids while alcohols are gases

Correct Answer: Esters usually have lower boiling points than alcohols

Q20. Which enzyme class catalyzes ester hydrolysis in biological systems?

  • Esterases (or lipases)
  • Oxidases
  • Kinases
  • Isomerases

Correct Answer: Esterases (or lipases)

Q21. Which factor stabilizes esters against hydrolysis in drug formulations?

  • Steric hindrance around the ester bond
  • Presence of strong acids
  • High water content
  • High temperature during storage

Correct Answer: Steric hindrance around the ester bond

Q22. Which reaction converts an ester into an alcohol and an aldehyde or ketone under reducing conditions?

  • Reduction with DIBAL-H at low temperature to aldehyde
  • Oxidation with KMnO4
  • Saponification
  • Acid-catalyzed dehydration

Correct Answer: Reduction with DIBAL-H at low temperature to aldehyde

Q23. In mass spectrometry, esters commonly show which fragmentation?

  • Alpha cleavage producing acylium ions
  • Stable molecular ion only
  • Complete absence of fragmentation
  • Formation of nitrilium ions

Correct Answer: Alpha cleavage producing acylium ions

Q24. Which statement about conjugated esters is correct?

  • Conjugation with a double bond lowers the C=O stretching frequency
  • They are more reactive than non-conjugated esters toward nucleophiles always
  • They lack any UV absorbance
  • They cannot undergo transesterification

Correct Answer: Conjugation with a double bond lowers the C=O stretching frequency

Q25. Which laboratory test would best distinguish an ester from a carboxylic acid?

  • Treat with NaHCO3: carboxylic acids effervesce, esters do not
  • Tollens’ test: esters give silver mirror
  • Iodoform test: esters always give yellow precipitate
  • Bromine water decolorization: esters always decolorize

Correct Answer: Treat with NaHCO3: carboxylic acids effervesce, esters do not

Q26. Which of the following is a common industrial application of esters?

  • Flavorings and fragrances
  • Primary metal extraction
  • Explosive fillers only
  • Electrical insulation exclusively

Correct Answer: Flavorings and fragrances

Q27. Which structural feature makes vinyl esters like vinyl acetate polymerizable?

  • Presence of a vinyl group (C=C) adjacent to the ester
  • High ionic character
  • Strong acidity
  • Unusually stable carbonyl

Correct Answer: Presence of a vinyl group (C=C) adjacent to the ester

Q28. Which ester derivative is most reactive toward nucleophilic acyl substitution?

  • Acyl chloride
  • Carboxylic acid
  • Ester
  • Amide

Correct Answer: Acyl chloride

Q29. Which process is commonly used to produce biodiesel from vegetable oils (triacylglycerols)?

  • Transesterification with methanol
  • Direct hydrogenation to alkanes
  • Saponification followed by combustion
  • Fischer esterification with phenol

Correct Answer: Transesterification with methanol

Q30. Which statement is true about cyclic esters (lactones)?

  • Lactones are intramolecular esters formed from hydroxy acids
  • Lactones are always unstable and cannot be isolated
  • Lactones are amide analogs with nitrogen
  • Lactones are ionic salts

Correct Answer: Lactones are intramolecular esters formed from hydroxy acids

Q31. Which reagent converts an alcohol and carboxylic acid to an ester using DCC-mediated coupling in peptide chemistry?

  • DCC and DMAP
  • NaOH and H2O
  • LiAlH4
  • PCC

Correct Answer: DCC and DMAP

Q32. Which property of esters affects their use as solvents in pharmaceuticals?

  • Moderate polarity and ability to dissolve nonpolar and slightly polar compounds
  • Strong ionic dissociation in water
  • Extreme acidity
  • High melting point solids only

Correct Answer: Moderate polarity and ability to dissolve nonpolar and slightly polar compounds

Q33. Which product arises from reaction of an ester with excess ammonia (NH3)?

  • Amide and alcohol
  • Ketone and water
  • Carboxylic acid only
  • Ether and alkene

Correct Answer: Amide and alcohol

Q34. Which of the following decreases ester solubility in water?

  • Increasing alkyl chain length on either side
  • Adding polar functional groups
  • Lowering molecular weight
  • Introducing ionic groups

Correct Answer: Increasing alkyl chain length on either side

Q35. Which reagent selectively cleaves esters in the presence of amides under basic conditions?

  • Strong base like NaOH (saponification)
  • Pd/C hydrogenation
  • Neutral alumina only
  • Weak acids like acetic acid at cold temperature

Correct Answer: Strong base like NaOH (saponification)

Q36. In pharmaceutical prodrug design, esters are often used to:

  • Improve lipophilicity and membrane permeability then release active drug by hydrolysis
  • Make the drug permanently inactive
  • Convert drug into a metal complex
  • Prevent any metabolic transformation

Correct Answer: Improve lipophilicity and membrane permeability then release active drug by hydrolysis

Q37. Which ester formation method avoids strong acid and is suitable for acid-sensitive substrates?

  • Steglich esterification (DCC/DMAP)
  • Fischer esterification with HCl
  • Direct heating with H2SO4
  • Using HBr as catalyst

Correct Answer: Steglich esterification (DCC/DMAP)

Q38. Which of the following best describes the mechanism of base-catalyzed ester hydrolysis?

  • Nucleophilic attack by hydroxide followed by collapse to carboxylate
  • Electrophilic aromatic substitution
  • Radical chain mechanism
  • Pericyclic concerted rearrangement

Correct Answer: Nucleophilic attack by hydroxide followed by collapse to carboxylate

Q39. Which ester-containing natural biomolecules are major components of fats and oils?

  • Triglycerides (triacylglycerols)
  • Polysaccharides
  • Proteins
  • Nucleic acids

Correct Answer: Triglycerides (triacylglycerols)

Q40. Which position in an ester is most susceptible to nucleophilic attack in acyl substitution?

  • Carbonyl carbon
  • Alpha carbon to the alkoxy only
  • Terminal methyl far from carbonyl
  • Oxygen of the alkoxy only

Correct Answer: Carbonyl carbon

Q41. Which solvent choice often accelerates esterification reactions by removing water azeotropically?

  • Toluene (with Dean–Stark apparatus)
  • Water only
  • Methanol without drying agents
  • Liquid ammonia

Correct Answer: Toluene (with Dean–Stark apparatus)

Q42. Which side reaction can occur during base-catalyzed transesterification of ethyl esters with methanol?

  • Saponification yielding carboxylate salt
  • Formation of nitriles directly
  • Complete polymerization to polyethylene
  • Conversion to acyl chlorides

Correct Answer: Saponification yielding carboxylate salt

Q43. The reactivity order of carboxylic acid derivatives toward nucleophiles is:

  • Acyl chloride > anhydride > ester > amide
  • Amide > ester > anhydride > acyl chloride
  • Esters are the most reactive of all
  • All have identical reactivity

Correct Answer: Acyl chloride > anhydride > ester > amide

Q44. Which analytical method would best confirm ester formation by disappearance of acid OH?

  • IR spectroscopy (loss of broad O–H band and C=O of ester)
  • Flame test
  • Conductivity measurement only
  • Thin-layer chromatography without standards

Correct Answer: IR spectroscopy (loss of broad O–H band and C=O of ester)

Q45. Which reagent is commonly used to methylate carboxylic acids to form methyl esters for GC analysis?

  • Diazomethane
  • Sodium borohydride
  • PCl5
  • NBS (N-bromosuccinimide)

Correct Answer: Diazomethane

Q46. Which structural change generally increases the hydrolysis half-life of an ester in plasma?

  • Introducing bulky groups adjacent to the ester bond
  • Replacing alkyl with hydrogen
  • Adding strong electron-donating groups on acyl carbonyl
  • Making the ester more polar and less sterically hindered

Correct Answer: Introducing bulky groups adjacent to the ester bond

Q47. Which industrial hazard is commonly associated with large-scale use of esters like ethyl acetate?

  • Flammability and volatile organic compound (VOC) emissions
  • High radioactivity
  • Strong oxidizing properties
  • Permanently toxic residues in soil

Correct Answer: Flammability and volatile organic compound (VOC) emissions

Q48. Which of the following best explains why esters are often used as solvents for paints and coatings?

  • Good solvency for resins and moderate evaporation rates
  • They polymerize rapidly at room temperature
  • They are fully miscible with water
  • They are non-volatile solids

Correct Answer: Good solvency for resins and moderate evaporation rates

Q49. In acid-catalyzed ester hydrolysis mechanism, what is the role of acid?

  • Protonation of the carbonyl oxygen to make the carbonyl more electrophilic
  • Generating nucleophilic hydroxide ion
  • Acting as radical initiator
  • Converting ester to acyl chloride directly

Correct Answer: Protonation of the carbonyl oxygen to make the carbonyl more electrophilic

Q50. Which phenomenon explains formation of esters from carboxylic acids and phenols being difficult without strong activation?

  • Phenols are less nucleophilic than aliphatic alcohols, requiring stronger activation
  • Phenols readily form esters without activation
  • Phenols decompose violently in presence of acids
  • Phenols are too volatile to react

Correct Answer: Phenols are less nucleophilic than aliphatic alcohols, requiring stronger activation

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