Stability of conjugated dienes MCQs With Answer

Stability of conjugated dienes MCQs With Answer

The stability of conjugated dienes is a fundamental topic for B. Pharm students studying organic chemistry and medicinal chemistry. This introduction explains how conjugation, resonance energy, hyperconjugation, and substitution patterns influence thermodynamic and kinetic stability of dienes, affecting reactivity in reactions like Diels–Alder and electrophilic addition. Key concepts include s-cis vs s-trans conformations, delocalization, allylic stabilization, heat of hydrogenation comparisons, and UV-visible absorption shifts. Understanding these principles helps predict reaction outcomes, drug metabolite formation, and stability of unsaturated compounds in formulations. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which factor most directly increases the thermodynamic stability of a conjugated diene?

• Increased ring strain
• Greater conjugation and resonance delocalization
• More steric hindrance near double bonds
• Higher degree of saturation

Correct Answer: Greater conjugation and resonance delocalization

Q2. How does heat of hydrogenation compare for a conjugated diene versus two isolated double bonds?

• Conjugated diene has a more negative heat of hydrogenation
• Conjugated diene has a less negative (lower magnitude) heat of hydrogenation
• They are always equal
• Conjugated diene gives no heat of hydrogenation

Correct Answer: Conjugated diene has a less negative (lower magnitude) heat of hydrogenation

Q3. What is the main reason 1,3-butadiene is more stable than 1,4-pentadiene?

• Ring formation in 1,3-butadiene
• Greater conjugation across adjacent double bonds in 1,3-butadiene
• 1,4-Pentadiene is aromatic
• 1,3-Butadiene has stronger sigma bonds

Correct Answer: Greater conjugation across adjacent double bonds in 1,3-butadiene

Q4. Which conformation of a 1,3-diene is required for an intramolecular Diels–Alder reaction?

• s-trans conformation
• s-cis conformation
• Both s-cis and s-trans equally
• Anti-periplanar conformation

Correct Answer: s-cis conformation

Q5. Allylic stabilization in conjugated dienes primarily involves:

• Sigma bond rotation
• Delocalization of pi electrons over three atoms
• Hydrogen bonding
• Formation of carbocations only at sp3 centers

Correct Answer: Delocalization of pi electrons over three atoms

Q6. Which experimental measurement is commonly used to estimate conjugation energy?

• Boiling point determination
• Heat of hydrogenation
• Melting point analysis
• Optical rotation

Correct Answer: Heat of hydrogenation

Q7. In UV-visible spectroscopy, conjugation in dienes causes:

• Hypsochromic (blue) shift
• No change in absorption
• Bathochromic (red) shift to longer wavelengths
• Disappearance of peaks

Correct Answer: Bathochromic (red) shift to longer wavelengths

Q8. Which substituent increases conjugated diene stability most by resonance donation?

• Nitro (-NO2)
• Alkyl (-CH3)
• Methoxy (-OCH3)
• Trifluoromethyl (-CF3)

Correct Answer: Methoxy (-OCH3)

Q9. Hyperconjugation contributes to diene stability by:

• Delocalizing sigma electrons into the pi system
• Forming hydrogen bonds
• Increasing ring strain
• Creating ionic bonds

Correct Answer: Delocalizing sigma electrons into the pi system

Q10. Which statement about s-trans and s-cis conformers is true regarding their relative populations at room temperature for an unsubstituted 1,3-butadiene?

• s-cis is heavily favored
• s-trans is slightly more stable and more populated
• They are present in equal amounts
• Neither conformation exists at room temperature

Correct Answer: s-trans is slightly more stable and more populated

Q11. In electrophilic addition to conjugated dienes, the initial carbocation intermediate is stabilized by:

• Resonance delocalization across the conjugated system
• Only inductive effects
• Loss of conjugation
• Formation of a radical

Correct Answer: Resonance delocalization across the conjugated system

Q12. Which of the following increases conjugation energy most effectively?

• Isolated double bonds
• Alternating single and double bonds over a longer chain
• Replacing double bonds with triple bonds
• Introducing saturated centers between double bonds

Correct Answer: Alternating single and double bonds over a longer chain

Q13. For conjugated dienes, stabilization of the radical intermediate is often due to:

• Allylic radical delocalization
• Steric crowding
• Hydrogen bonding
• Increased acidity

Correct Answer: Allylic radical delocalization

Q14. Which of the following is a resonance form of the allylic carbocation?

• Carbocation localized only at one carbon with no delocalization
• Two equivalent resonance structures with positive charge at terminal carbons
• A structure with negative charge on a terminal carbon
• Radical centered at the middle carbon

Correct Answer: Two equivalent resonance structures with positive charge at terminal carbons

Q15. Which diene is expected to be most stable: 1,3-butadiene, 1,4-pentadiene, or trans,trans-1,3-pentadiene?

• 1,4-Pentadiene
• 1,3-Butadiene due to conjugation
• trans,trans-1,3-Pentadiene because of trans stabilizing effect only
• They are all equally stable

Correct Answer: 1,3-Butadiene due to conjugation

Q16. Which computational property correlates well with conjugation and diene stability?

• Higher HOMO–LUMO gap
• Lower HOMO energy only
• Lower overall molecular energy and delocalized frontier orbitals
• Only atomic charges on hydrogens

Correct Answer: Lower overall molecular energy and delocalized frontier orbitals

Q17. Substituted conjugated dienes where substituents donate electrons by resonance typically:

• Decrease diene stability
• Increase diene stability and raise HOMO energy
• Always make dienes aromatic
• Remove conjugation

Correct Answer: Increase diene stability and raise HOMO energy

Q18. Which heat of hydrogenation result indicates greater conjugation energy?

• Measured heat is equal to sum of isolated double bond hydrogenations
• Measured heat is less exothermic than the sum for isolated double bonds
• Measured heat is more exothermic than expected
• No heat is released

Correct Answer: Measured heat is less exothermic than the sum for isolated double bonds

Q19. The term “conjugation energy” refers to:

• Energy required to break sigma bonds
• Stabilization energy gained by delocalization compared to isolated double bonds
• Ionization energy of the diene
• Heat needed to isomerize the diene

Correct Answer: Stabilization energy gained by delocalization compared to isolated double bonds

Q20. Which mechanism outcome is influenced by the relative stability of s-cis versus s-trans conformers?

• SN2 nucleophilic substitution
• Diels–Alder cycloaddition reactivity and stereochemistry
• Free radical halogenation only
• Formation of ionic salts

Correct Answer: Diels–Alder cycloaddition reactivity and stereochemistry

Q21. Which of the following increases the stability of a conjugated diene via inductive effect?

• Electron-withdrawing group attached by conjugation
• Alkyl substituents providing electron donation by +I effect
• Introducing a sp-hybridized carbon between double bonds
• Adding a nitro group conjugated to the diene

Correct Answer: Alkyl substituents providing electron donation by +I effect

Q22. In an electrophilic addition to a conjugated diene, the product distribution between 1,2- and 1,4-addition depends mainly on:

• Only temperature
• Reaction conditions and stability of intermediates (kinetic vs thermodynamic control)
• The solvent viscosity
• Color of the reagents

Correct Answer: Reaction conditions and stability of intermediates (kinetic vs thermodynamic control)

Q23. Conjugated dienes typically have lower heat of hydrogenation because:

• They are less reactive
• Delocalization lowers the overall energy of the pi system
• They form stronger C–H bonds upon hydrogenation
• They always form cyclic products

Correct Answer: Delocalization lowers the overall energy of the pi system

Q24. Which of these is a resonance contributor that stabilizes the 1,3-butadiene pi system?

• Structure with two separate isolated double bonds only
• Structures where positive and negative charges are separated by three atoms (allylic forms)
• Only ionic structures with full charges on carbon
• No valid resonance contributors exist

Correct Answer: Structures where positive and negative charges are separated by three atoms (allylic forms)

Q25. Aromatic stabilization is NOT applicable to which diene?

• Cyclobutadiene (4 π electrons)
• Benzene-derived conjugated diene fragments (6 π electrons in ring)
• Conjugated open-chain dienes like 1,3-butadiene
• Annulenes with 10 π electrons

Correct Answer: Conjugated open-chain dienes like 1,3-butadiene

Q26. Which structural feature leads to stronger conjugation in a polyene?

• Saturated methylene interruptions
• Alternating double and single bonds without interruption
• Isolated double bonds separated by two single bonds
• Presence of sp3 hybridized carbons between double bonds

Correct Answer: Alternating double and single bonds without interruption

Q27. Which diene is most likely to undergo rapid electrophilic addition at the terminal carbon due to resonance stabilization of the carbocation?

• 1,4-Pentadiene with no conjugation
• 1,3-Butadiene
• Isolated alkene like 1-hexene
• Alkyne

Correct Answer: 1,3-Butadiene

Q28. In a substituted conjugated diene, which arrangement of substituents stabilizes the conjugated system via conjugative overlap?

• Substituents orthogonal to the pi system
• Substituents coplanar and able to donate or withdraw by resonance
• Bulky substituents forcing out-of-plane geometry only
• Saturated substituents that break conjugation

Correct Answer: Substituents coplanar and able to donate or withdraw by resonance

Q29. Which observation supports the presence of conjugation in an organic molecule?

• Higher than expected heat of hydrogenation
• Lower UV absorption wavelength than isolated double bonds
• Lower than expected heat of hydrogenation and bathochromic UV shift
• Absence of any IR absorption

Correct Answer: Lower than expected heat of hydrogenation and bathochromic UV shift

Q30. Which is true about the reactivity of conjugated dienes compared to isolated alkenes?

• Conjugated dienes are always less reactive to electrophiles
• Conjugated dienes can form stabilized allylic intermediates leading to distinct product distributions
• Isolated alkenes form allylic carbocations more easily
• Conjugation prevents any addition reactions

Correct Answer: Conjugated dienes can form stabilized allylic intermediates leading to distinct product distributions

Q31. In terms of molecular orbital theory, conjugation in dienes causes:

• Formation of localized orbitals only
• Formation of delocalized pi molecular orbitals that lower the system energy
• Increase in energy of all occupied orbitals uniformly
• Complete separation of HOMO and LUMO

Correct Answer: Formation of delocalized pi molecular orbitals that lower the system energy

Q32. Which product is thermodynamically favored in electrophilic addition to 1,3-butadiene at high temperature?

• 1,2-Addition product only (kinetic product)
• 1,4-Addition product (thermodynamic product)
• Polymerization only
• No reaction occurs at high temperature

Correct Answer: 1,4-Addition product (thermodynamic product)

Q33. Which factor makes trans,trans-1,3-butadiene more stable than cis,trans isomers?

• Increased ring strain in trans isomers
• Less steric hindrance and better conjugation in trans arrangements
• Trans isomers cannot conjugate
• Trans isomers are aromatic

Correct Answer: Less steric hindrance and better conjugation in trans arrangements

Q34. Allylic carbocations are stabilized because the positive charge is:

• Localized on a single sp3 carbon
• Delocalized over multiple carbons via resonance
• Neutralized by inductive withdrawal only
• Converted immediately to radicals

Correct Answer: Delocalized over multiple carbons via resonance

Q35. Which measurement would directly show delocalization in a conjugated diene?

• Single-point pKa
• Equalization of C–C bond lengths in X-ray or computational geometry
• Boiling point only
• Specific rotation measurement

Correct Answer: Equalization of C–C bond lengths in X-ray or computational geometry

Q36. When an electron-donating group is conjugated to a diene, the HOMO energy typically:

• Decreases significantly
• Increases, making the diene more nucleophilic
• Remains unchanged
• Becomes unoccupied

Correct Answer: Increases, making the diene more nucleophilic

Q37. Which reaction pathway is enhanced by conjugation in dienes?

• SN1 at sp3 carbon unrelated to the pi system
• Diels–Alder cycloaddition due to stabilized frontier orbitals
• Direct substitution at saturated carbons only
• Hydrolysis of esters

Correct Answer: Diels–Alder cycloaddition due to stabilized frontier orbitals

Q38. In the context of drug stability, why is conjugation in unsaturated systems important?

• It always makes the drug inert
• Conjugation affects metabolic sites, UV stability, and reactivity toward electrophiles
• It prevents any enzymatic transformation
• Conjugation reduces solubility only

Correct Answer: Conjugation affects metabolic sites, UV stability, and reactivity toward electrophiles

Q39. Which type of isomerization can change the extent of conjugation in a diene system?

• Rotation around single bonds converting s-trans to s-cis
• Bond homolysis only
• Oxidation of terminal methyl groups only
• Conversion of double bonds to triple bonds

Correct Answer: Rotation around single bonds converting s-trans to s-cis

Q40. Which conjugated diene is best stabilized by resonance with an adjacent carbonyl group?

• Diene with carbonyl conjugated directly to the π-system (enone)
• Diene separated by a saturated carbon from carbonyl
• Nonconjugated diene
• Diene with carbonyl at remote position not in conjugation

Correct Answer: Diene with carbonyl conjugated directly to the π-system (enone)

Q41. A lower heat of hydrogenation for a conjugated diene indicates:

• Less stability than isolated alkenes
• Greater stabilization due to conjugation compared to isolated double bonds
• That the compound is not unsaturated
• That the diene is aromatic

Correct Answer: Greater stabilization due to conjugation compared to isolated double bonds

Q42. Which of the following will destabilize conjugation in a diene?

• Planarization of the π system
• Introduction of steric bulk causing out-of-plane twist
• Electron donating resonance groups coplanar with the system
• Extension of conjugation length

Correct Answer: Introduction of steric bulk causing out-of-plane twist

Q43. Which type of spectroscopy is most informative about conjugation length in polyenes?

• 1H NMR only
• UV-Visible spectroscopy
• Mass spectrometry only
• Elemental analysis

Correct Answer: UV-Visible spectroscopy

Q44. The presence of conjugation in a drug molecule can influence its photostability by:

• Making it completely immune to light
• Shifting absorption into UV-visible range and promoting photo-induced reactions
• Only affecting only boiling point
• Preventing any oxidation

Correct Answer: Shifting absorption into UV-visible range and promoting photo-induced reactions

Q45. Which statement correctly compares conjugated and isolated dienes toward radical formation?

• Conjugated dienes form less stable radicals than isolated dienes
• Conjugated dienes form stabilized allylic radicals that are more stable
• Isolated dienes always form resonance-stabilized radicals
• Neither can form radicals

Correct Answer: Conjugated dienes form stabilized allylic radicals that are more stable

Q46. In kinetic vs thermodynamic control for addition to conjugated dienes, the kinetic product forms faster because:

• It is always more stable
• It proceeds via the lower energy transition state despite being less stable overall
• Temperature has no role in selectivity
• Solvent completely controls product

Correct Answer: It proceeds via the lower energy transition state despite being less stable overall

Q47. Which of the following best describes the effect of conjugation on bond lengths in a diene?

• All C–C bonds become equal length regardless of bond order
• Double bonds shorten and single bonds lengthen more than usual
• Partial bond equalization: double bonds slightly longer and single bonds slightly shorter than in isolated analogs
• No change from isolated alkenes

Correct Answer: Partial bond equalization: double bonds slightly longer and single bonds slightly shorter than in isolated analogs

Q48. Which of the following diene behaviors is explained by conjugation?

• Unexpectedly low reactivity in all reactions
• Formation of both 1,2- and 1,4-addition products upon electrophilic attack
• Complete resistance to polymerization
• Loss of all UV absorption

Correct Answer: Formation of both 1,2- and 1,4-addition products upon electrophilic attack

Q49. Effective conjugation requires what geometric feature of the molecule?

• Orthogonality between adjacent p orbitals
• Coplanarity or good p-orbital overlap across the conjugated path
• Presence of sp3 centers in conjugation path
• Any random orientation of substituents

Correct Answer: Coplanarity or good p-orbital overlap across the conjugated path

Q50. Which of the following best summarizes why conjugated dienes are important in pharmaceutical chemistry?

• They are never reactive and thus unimportant
• Conjugation affects chemical stability, metabolic pathways, photostability, and reactivity useful in synthesis
• Conjugated dienes make all drugs water-insoluble
• They guarantee drug safety

Correct Answer: Conjugation affects chemical stability, metabolic pathways, photostability, and reactivity useful in synthesis

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