Structural isomerism in organic compounds MCQs With Answer

Structural isomerism in organic compounds MCQs With Answer

Structural isomerism (also called constitutional isomerism) is a fundamental concept for B. Pharm students studying drug molecules, where compounds share the same molecular formula but differ in atom connectivity. Understanding types — chain (skeletal), position, functional group, metamerism, ring–chain and tautomerism — helps predict physical properties, reactivity, stability and pharmacological behavior. Structural isomers influence melting/boiling points, solubility, metabolic pathways and receptor binding, so mastery supports formulation and medicinal chemistry. This focused set of practice MCQs emphasizes examples, nomenclature, counting isomers and analytical identification techniques relevant to pharmacy studies. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the defining feature of structural (constitutional) isomers?

• They differ only in spatial arrangement around a double bond
• They have different molecular formulas
• They have the same molecular formula but different atom connectivity
• They are mirror images of each other

Correct Answer: They have the same molecular formula but different atom connectivity

Q2. Which term is synonymous with structural isomerism in organic chemistry?

• Stereoisomerism
• Conformational isomerism
• Constitutional isomerism
• Optical isomerism

Correct Answer: Constitutional isomerism

Q3. Which of the following is NOT a type of structural isomerism?

• Chain (skeletal) isomerism
• Position isomerism
• Geometrical (cis/trans) isomerism
• Functional group isomerism

Correct Answer: Geometrical (cis/trans) isomerism

Q4. Chain isomerism involves which of the following changes?

• Different position of the same functional group on the same carbon skeleton
• Different arrangement of atoms in space without bond breaking
• Different carbon skeleton or branching pattern with the same formula
• Interconversion between ring and chain forms

Correct Answer: Different carbon skeleton or branching pattern with the same formula

Q5. Which pair are chain isomers of C4H10?

• n-Butane and 2-methylpropane (isobutane)
• But-1-ene and cyclobutane
• Butan-2-ol and butan-1-ol
• Butanal and butanone

Correct Answer: n-Butane and 2-methylpropane (isobutane)

Q6. Functional group isomerism can be illustrated by which of these C2H6O pairs?

• Ethanol and dimethyl ether
• 1-Propanol and 2-propanol
• n-Butane and isobutane
• Cyclohexane and hex-1-ene

Correct Answer: Ethanol and dimethyl ether

Q7. Position isomerism refers to isomers that differ in:

• Type of functional group present
• Arrangement of carbon chain branching
• Position of a functional group or substituent on the same skeleton
• Spatial orientation around a chiral center

Correct Answer: Position of a functional group or substituent on the same skeleton

Q8. Which example correctly shows position isomerism?

• 1-Butanol and 2-Butanol
• Ethanol and dimethyl ether
• n-Butane and isobutane
• Propanone and propanal

Correct Answer: 1-Butanol and 2-Butanol

Q9. Metamerism is best described as:

• Isomerism due to different positions of double bonds
• Isomerism of compounds with different functional groups
• Isomerism in which alkyl groups separated by a functional group differ
• Stereoisomers related by rotation about single bonds

Correct Answer: Isomerism in which alkyl groups separated by a functional group differ

Q10. Which pair are metamers (same functional group, different alkyl groups) for C4H10O?

• Diethyl ether and methyl propyl ether
• 1-Butanol and 2-Butanol
• Propanone and propanal
• Benzene and toluene

Correct Answer: Diethyl ether and methyl propyl ether

Q11. Ring–chain isomerism occurs when:

• Isomers differ only by the configuration at a chiral center
• Acyclic and cyclic structures share the same molecular formula
• Functional groups are different
• Double bond positions differ along a chain

Correct Answer: Acyclic and cyclic structures share the same molecular formula

Q12. Cyclohexane and hex-1-ene are examples of which isomerism?

• Functional group isomerism
• Position isomerism
• Ring–chain isomerism
• Stereoisomerism

Correct Answer: Ring–chain isomerism

Q13. Tautomerism is a special type of isomerism characterized by:

• No exchange of atoms, only rotation around bonds
• Rapid interconversion via proton transfer and bond reorganization
• Only differences in spatial arrangement with identical connectivity
• Different isotopic composition

Correct Answer: Rapid interconversion via proton transfer and bond reorganization

Q14. Keto–enol tautomerism is an example of which broader isomerism category?

• Conformational isomerism
• Functional group isomerism / tautomerism
• Geometric isomerism
• Optical isomerism

Correct Answer: Functional group isomerism / tautomerism

Q15. Which experimental technique most directly reveals different connectivity of atoms in structural isomers?

• UV–vis spectroscopy only
• NMR spectroscopy (1H/13C, 2D NMR)
• Polarimetry only
• Simple melting point measurement without further analysis

Correct Answer: NMR spectroscopy (1H/13C, 2D NMR)

Q16. Which property is most likely to differ between structural isomers?

• Molecular weight
• Exact elemental composition
• Boiling point and solubility
• Atomic number of atoms

Correct Answer: Boiling point and solubility

Q17. How many structural isomers exist for the alkane formula C5H12?

• 2
• 3
• 4
• 5

Correct Answer: 3

Q18. For C4H10, how many structural isomers are possible?

• 1
• 2
• 3
• 4

Correct Answer: 2

Q19. Which of the following pairs are functional group isomers of formula C3H6O?

• Propanone (acetone) and propanal (propionaldehyde)
• 1-Propanol and 2-Propanol
• Cyclopropane and propene
• Butane and isobutane

Correct Answer: Propanone (acetone) and propanal (propionaldehyde)

Q20. Which statement about isomer counts as carbon number increases is correct?

• The number of possible structural isomers decreases with chain length
• Isomer count remains constant regardless of formula
• The number of possible structural isomers increases rapidly with carbon number
• Only one isomer exists for all alkanes

Correct Answer: The number of possible structural isomers increases rapidly with carbon number

Q21. Which pair are functional group isomers among C4 compounds?

• Butan-1-ol and butan-2-ol
• Butanal and butanone
• n-Butane and isobutane
• 1-Butene and cis-2-Butene

Correct Answer: Butanal and butanone

Q22. In medicinal chemistry, why is structural isomerism important for drug molecules?

• It never affects pharmacological activity
• Different connectivity can change receptor binding, metabolism and toxicity
• It only changes isotopic labeling patterns
• All isomers always have identical ADME profiles

Correct Answer: Different connectivity can change receptor binding, metabolism and toxicity

Q23. Which analytical method is most useful to distinguish functional group isomers based on group-specific vibrations?

• Infrared (IR) spectroscopy
• Mass spectrometry without fragmentation
• Polarimetry
• Simple density measurement

Correct Answer: Infrared (IR) spectroscopy

Q24. Which of the following formulas has exactly two distinct structural isomers: propene and cyclopropane?

• C3H8
• C3H6
• C4H10
• C2H6O

Correct Answer: C3H6

Q25. Which of the following best describes valence isomerism?

• Isomers that differ by atomic masses
• Isomers that interconvert by simple rotation about single bonds
• Isomers differing by reorganization of bonding electrons while preserving atom set
• Isomers that are non-superimposable mirror images

Correct Answer: Isomers differing by reorganization of bonding electrons while preserving atom set

Q26. Which example demonstrates ring–chain isomerism for formula C6H12?

• Hexane and hexene
• Cyclohexane and hex-1-ene
• 1-Hexanol and 2-Hexanol
• Benzene and cyclohexane

Correct Answer: Cyclohexane and hex-1-ene

Q27. Which statement about metamerism is TRUE?

• Metamerism requires different functional groups
• Metamers have different numbers of carbon atoms
• Metamers have the same functional group but different alkyl groups on either side
• Metamerism is identical to geometric isomerism

Correct Answer: Metamers have the same functional group but different alkyl groups on either side

Q28. For the molecular formula C3H8O, how many common structural isomers exist (including ethers and alcohols)?

• 2
• 3
• 4
• 5

Correct Answer: 3

Q29. Which factor commonly explains lower boiling points for branched structural isomers compared to linear isomers?

• Branched isomers have higher molecular weight
• Branched isomers have lower surface area and weaker van der Waals interactions
• Branched isomers are ionic
• Branched isomers have more hydrogen bonds

Correct Answer: Branched isomers have lower surface area and weaker van der Waals interactions

Q30. Which pair best illustrates functional group isomerism affecting polarity and solubility?

• Ethanol (polar) and dimethyl ether (less polar)
• 1-Butene and 2-Butene
• n-Butane and isobutane
• Cyclohexane and cyclohexene

Correct Answer: Ethanol (polar) and dimethyl ether (less polar)

Q31. Which of the following pairs are skeletal (chain) isomers?

• n-Pentane and isopentane (methylbutane)
• Propanone and propanal
• Cyclopropane and propene
• 1-Butanol and 2-Butanol

Correct Answer: n-Pentane and isopentane (methylbutane)

Q32. Which statement accurately differentiates positional isomers from chain isomers?

• Positional isomers have different skeletons; chain isomers differ only by substituent positions
• Positional isomers differ by location of substituent/functional group on same skeleton; chain isomers differ in carbon skeleton
• They are identical concepts
• Positional isomers differ in stereochemistry only

Correct Answer: Positional isomers differ by location of substituent/functional group on same skeleton; chain isomers differ in carbon skeleton

Q33. Which of these formulas has seven structural isomers commonly listed (including alcohols and ethers)?

• C4H10O
• C3H8O
• C2H6O
• C6H6

Correct Answer: C4H10O

Q34. Which is a major reason structural isomers can have very different pharmacokinetics (ADME)?

• They always have identical 3D shape
• They have different connectivity that changes polarity, lipophilicity and metabolic sites
• They are composed of different elements
• Isomers do not interact with enzymes

Correct Answer: They have different connectivity that changes polarity, lipophilicity and metabolic sites

Q35. Which experimental observation would most strongly suggest two compounds are structural isomers rather than stereoisomers?

• They have identical NMR spectra
• They have distinct IR absorption bands indicating different functional groups
• They rotate plane-polarized light in opposite directions
• They interconvert rapidly by rotation about a single bond

Correct Answer: They have distinct IR absorption bands indicating different functional groups

Q36. Which pair are ring–chain isomers for C4H8?

• But-1-ene and cyclobutane
• Butane and isobutane
• 1-Butanol and butanone
• 2-Butanol and 3-Butanol

Correct Answer: But-1-ene and cyclobutane

Q37. Which reaction type can produce structural isomers through rearrangement?

• Hydrogen bonding
• Sigmatropic rearrangements and carbocation rearrangements
• Simple mixing without reaction
• Optical rotation

Correct Answer: Sigmatropic rearrangements and carbocation rearrangements

Q38. Which of these is a pair of valence isomers (same formula, different bonding arrangement)?

• Benzene and Dewar benzene
• n-Butane and isobutane
• Propan-1-ol and propan-2-ol
• Cyclohexane and cyclohexene

Correct Answer: Benzene and Dewar benzene

Q39. Which technique is most useful for distinguishing isomers that differ only by carbon skeleton connectivity?

• 1H and 13C NMR spectroscopy with DEPT and 2D experiments
• Simple refractive index measurement
• Melting point alone
• Paper chromatography without standards

Correct Answer: 1H and 13C NMR spectroscopy with DEPT and 2D experiments

Q40. Which of the following pairs are functional group isomers that will show distinct IR carbonyl bands?

• Butanol and diethyl ether
• Butanal (aldehyde) and butanone (ketone)
• Ethene and ethanol
• Cyclohexane and hexane

Correct Answer: Butanal (aldehyde) and butanone (ketone)

Q41. Which pair are metamers for formula C5H12O (example conceptually)?

• Diisopropyl ether and ethyl tert-butyl ether
• 1-Pentanol and 2-Pentanol
• Propanone and propanal
• n-Pentane and isopentane

Correct Answer: Diisopropyl ether and ethyl tert-butyl ether

Q42. Which statement about positional isomerism in substituted benzenes is true?

• Ortho, meta and para isomers are examples of chain isomerism
• They are positional isomers because substituents occupy different ring positions
• They have different molecular formulas
• They are always identical in reactivity

Correct Answer: They are positional isomers because substituents occupy different ring positions

Q43. Which compound pair demonstrates how structural isomerism can alter hydrogen bonding capability?

• Ethanol and dimethyl ether (same formula, different H-bonding)
• Butane and isobutane (both cannot H-bond)
• Propene and propane (both cannot H-bond)
• Ethylene and acetylene (both cannot H-bond)

Correct Answer: Ethanol and dimethyl ether (same formula, different H-bonding)

Q44. Which of the following is FALSE regarding structural isomers?

• They can have different chemical reactivity
• They necessarily have different molecular formulas
• They can show different physical properties such as bp and mp
• They can be separated by chromatographic techniques

Correct Answer: They necessarily have different molecular formulas

Q45. Which of the following pairs are NOT structural isomers but rather stereoisomers?

• cis-2-Butene and trans-2-Butene
• n-Butane and isobutane
• Ethanol and dimethyl ether
• Propanone and propanal

Correct Answer: cis-2-Butene and trans-2-Butene

Q46. Which property is LEAST useful to differentiate structural isomers?

• 13C NMR chemical shifts
• IR functional group bands
• Exact molecular weight by high-resolution MS only
• Chromatographic retention time with an appropriate column

Correct Answer: Exact molecular weight by high-resolution MS only

Q47. Which reaction type commonly leads to tautomerism in organic molecules?

• Hydrogen shift (proton transfer) accompanied by π-bond rearrangement
• Purely radical substitution without proton movement
• Simple physical mixing at low temperature
• Photochemical isomerization only

Correct Answer: Hydrogen shift (proton transfer) accompanied by π-bond rearrangement

Q48. Which of the following best illustrates skeletal isomerism?

• CH3CH2CH2OH and CH3OCH2CH3
• n-Butane and 2-methylpropane (isobutane)
• cis-1,2-Dichloroethene and trans-1,2-Dichloroethene
• Propanone and propanal

Correct Answer: n-Butane and 2-methylpropane (isobutane)

Q49. Which of the following changes would most likely convert one structural isomer into another in a laboratory synthesis?

• Simple heating without chemical reaction
• Chemical rearrangement reactions such as acid-catalyzed rearrangement
• Shaking the mixture vigorously
• Exposing to visible light only without catalyst

Correct Answer: Chemical rearrangement reactions such as acid-catalyzed rearrangement

Q50. Which of the following is NOT a correct example of structural isomerism?

• Ethanol and dimethyl ether (functional group isomers)
• n-Butane and isobutane (chain isomers)
• Enantiomers of lactic acid (mirror-image stereoisomers)
• Cyclohexane and hex-1-ene (ring–chain isomers)

Correct Answer: Enantiomers of lactic acid (mirror-image stereoisomers)

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com