Stabilities of alkenes MCQs With Answer

Stabilities of alkenes MCQs With Answer

Understanding the stabilities of alkenes is essential for B. Pharm students studying organic chemistry and medicinal chemistry. This focused introduction explains how hyperconjugation, alkyl substitution, conjugation, resonance, steric effects, and ring strain determine alkene stability. Familiarity with heat of hydrogenation, Zaitsev’s rule, E/Z stereochemistry, and electronic effects helps predict reaction outcomes, drug metabolism pathways, and synthesis strategies. These MCQs emphasize mechanistic reasoning and quantitative indicators so pharmacy students can apply concepts to drug design and analysis. Clear examples relate stability trends to real pharmaceutical problems and exam-style questions. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which factor is most important in determining the relative stability of simple acyclic alkenes?

• Number of hydrogen atoms on the double bond
• Degree of alkyl substitution on the double bond
• Presence of nearby alcohol groups
• Color of the compound

Correct Answer: Degree of alkyl substitution on the double bond

Q2. Which alkene is expected to be most stable?

• 1-Butene (monosubstituted)
• cis-2-Butene (disubstituted)
• trans-2-Butene (disubstituted)
• 2-Methyl-1-propene (trisubstituted)

Correct Answer: trans-2-Butene (disubstituted)

Q3. Hyperconjugation stabilizes alkenes by:

• Delocalizing lone pairs into the π-system
• Overlapping σ C–H orbitals with the adjacent π-system
• Forming hydrogen bonds
• Increasing ring strain

Correct Answer: Overlapping σ C–H orbitals with the adjacent π-system

Q4. Heat of hydrogenation correlates inversely with alkene stability because:

• More stable alkenes give larger exothermic hydrogenation
• Less stable alkenes release less heat on hydrogenation
• More stable alkenes have lower heat of hydrogenation
• Heat of hydrogenation is unrelated to stability

Correct Answer: More stable alkenes have lower heat of hydrogenation

Q5. Which statement about conjugated alkenes is true?

• Conjugation decreases stability relative to isolated double bonds
• Conjugation has no effect on stability
• Conjugation increases stability due to π-electron delocalization
• Conjugation always makes alkenes less reactive

Correct Answer: Conjugation increases stability due to π-electron delocalization

Q6. For substituted alkenes, the order of stability generally is:

• mono- < di- < tri- < tetra-substituted
• tetra- < tri- < di- < mono-substituted
• tri- < di- < mono- < tetra-substituted
• All substitution patterns are equally stable

Correct Answer: mono- < di- < tri- < tetra-substituted

Q7. Which alkene is more stable: cis-2-butene or trans-2-butene, and why?

• cis-2-Butene is more stable due to better hyperconjugation
• trans-2-Butene is more stable due to reduced steric strain
• They are equally stable because substitution is the same
• cis-2-Butene is more stable because of hydrogen bonding

Correct Answer: trans-2-Butene is more stable due to reduced steric strain

Q8. Which of the following decreases alkene stability?

• Increased alkyl substitution
• Conjugation with aromatic rings
• Electron-withdrawing groups directly attached to the double bond
• Hyperconjugation from adjacent C–H bonds

Correct Answer: Electron-withdrawing groups directly attached to the double bond

Q9. Allylic stabilization refers to:

• Stability of an alkene when adjacent to an sp3 carbon only
• Delocalization of electrons when a double bond is adjacent to a radical or cation
• Increased steric hindrance near the double bond
• Loss of aromaticity due to double bond placement

Correct Answer: Delocalization of electrons when a double bond is adjacent to a radical or cation

Q10. Which experimental method directly measures relative alkene stabilities?

• NMR coupling constants only
• Infrared spectroscopy of C=O groups
• Heat of hydrogenation measurements
• Melting point determination

Correct Answer: Heat of hydrogenation measurements

Q11. In a substituted alkene, electron-donating groups (EDGs) attached to the double bond generally:

• Destabilize the alkene by withdrawing electron density
• Stabilize the alkene by donating electron density into the π-system
• Have no effect on stability
• Convert the alkene into an alkyne

Correct Answer: Stabilize the alkene by donating electron density into the π-system

Q12. Which alkene is likely least stable due to angle strain?

• Cyclohexene
• Cyclopentene
• Cyclobutene
• 1-Butene

Correct Answer: Cyclobutene

Q13. In conjugated dienes, resonance stabilization leads to:

• Higher heat of hydrogenation than isolated dienes
• Lower heat of hydrogenation than isolated dienes
• No change in heat of hydrogenation
• Complete loss of double bond reactivity

Correct Answer: Lower heat of hydrogenation than isolated dienes

Q14. Which is a cause of decreased stability in internal alkenes?

• Increased hyperconjugation
• Steric interaction between substituents
• Conjugation with adjacent π-systems
• Electron donation from substituents

Correct Answer: Steric interaction between substituents

Q15. The Zaitsev rule predicts the formation of which alkene in elimination reactions?

• The least substituted alkene
• The most substituted (more stable) alkene
• The alkene with trans stereochemistry only
• A conjugated alkene always

Correct Answer: The most substituted (more stable) alkene

Q16. Which effect explains stabilization by an adjacent oxygen lone pair via conjugation?

• Hyperconjugation
• Inductive effect
• Resonance (mesomeric) effect
• Van der Waals forces

Correct Answer: Resonance (mesomeric) effect

Q17. Allenes (cumulated dienes) are generally less stable than conjugated dienes because:

• They have more hyperconjugation
• They lack effective π-conjugation between the double bonds
• They are aromatic
• They are planar and highly stabilized

Correct Answer: They lack effective π-conjugation between the double bonds

Q18. Which substituent directly attached to an alkene would most stabilize a positive charge in an adjacent carbocation intermediate?

• Nitro group (–NO2)
• Methyl group (–CH3)
• Fluoro group (–F)
• Trifluoromethyl (–CF3)

Correct Answer: Methyl group (–CH3)

Q19. Which is true about aromatic conjugation vs simple alkene conjugation?

• Aromatic conjugation always destabilizes adjacent alkenes
• Aromatic systems can provide extra stabilization through resonance
• Aromatic conjugation reduces hyperconjugation effects
• Aromaticity is unrelated to alkene stability

Correct Answer: Aromatic systems can provide extra stabilization through resonance

Q20. Which stereochemical configuration is determined by CIP rules for alkenes?

• R/S configuration for alkenes
• E/Z configuration for alkenes
• cis/trans only for cyclic systems
• Optical rotation sign

Correct Answer: E/Z configuration for alkenes

Q21. In an alkene bearing an electron-withdrawing group (EWG), the stability is often:

• Increased due to electron donation
• Decreased because EWGs destabilize the π-electron density
• Unchanged because EWGs only affect sigma bonds
• Converted into an aromatic system

Correct Answer: Decreased because EWGs destabilize the π-electron density

Q22. Which measurement helps compare stabilities of isomeric alkenes experimentally?

• UV-Vis absorption maximum only
• Heat of combustion
• Heat of formation or hydrogenation
• Refractive index

Correct Answer: Heat of formation or hydrogenation

Q23. Steric hindrance between substituents on the same side of an alkene tends to:

• Increase stability
• Decrease stability
• Have no effect
• Always cause polymerization

Correct Answer: Decrease stability

Q24. Which alkene is stabilized by resonance with a carbonyl group?

• An alkene directly conjugated to the carbonyl (enone)
• An isolated terminal alkene far from the carbonyl
• An alkene with no π-overlap
• All alkenes are destabilized by carbonyls

Correct Answer: An alkene directly conjugated to the carbonyl (enone)

Q25. For pharmaceutically relevant alkenes, why is understanding stability important?

• It determines taste of the drug
• It influences metabolic oxidation and bioavailability
• It only affects color
• It is irrelevant to drug design

Correct Answer: It influences metabolic oxidation and bioavailability

Q26. Which best describes the inductive effect on alkene stability?

• Electron donation or withdrawal through σ-bonds affecting π-electron density
• Resonance delocalization through π-systems
• Steric interactions only
• Hydrogen bonding to the double bond

Correct Answer: Electron donation or withdrawal through σ-bonds affecting π-electron density

Q27. Which alkene would show the lowest heat of hydrogenation (most stable)?

• 1-hexene (terminal)
• 2-hexene cis
• 2-hexene trans
• 1,3-hexadiene isolated

Correct Answer: 2-hexene trans

Q28. In a ring system, the most stable alkene usually has the double bond located where?

• At a position causing maximum angle strain
• Where it minimizes torsional and steric strain (often equatorial-like)
• Only at bridgehead carbons
• Always between the largest substituents

Correct Answer: Where it minimizes torsional and steric strain (often equatorial-like)

Q29. Which alkene is stabilized by conjugation with a benzene ring?

• Styrene (vinylbenzene)
• 1-Octene
• Trans-2-butene
• Cyclopropene

Correct Answer: Styrene (vinylbenzene)

Q30. Which process differentiates kinetic vs thermodynamic alkene products?

• Kinetic product is more stable; thermodynamic is formed faster
• Kinetic product forms faster but may be less stable; thermodynamic product is more stable
• They are always the same product
• Kinetic control depends only on temperature

Correct Answer: Kinetic product forms faster but may be less stable; thermodynamic product is more stable

Q31. Which substituent stabilizes an alkene via resonance donation?

• –NO2
• –OMe (methoxy)
• –CF3
• –Cl

Correct Answer: –OMe (methoxy)

Q32. Which alkene geometry minimizes steric crowding between substituents?

• cis configuration for large groups
• trans configuration for large groups
• Both are equally noncrowded
• It depends only on electronic effects

Correct Answer: trans configuration for large groups

Q33. Why are tetrasubstituted alkenes generally rare in drug molecules?

• They are too reactive and unstable
• They have increased steric hindrance that can affect synthesis and binding
• They are colorless
• They always form salts

Correct Answer: They have increased steric hindrance that can affect synthesis and binding

Q34. The presence of a neighboring π-system (conjugation) typically changes alkene reactivity by:

• Making the alkene more basic
• Delocalizing charge, altering electrophilic addition pathways
• Preventing any reactions
• Converting the alkene to an alkyne

Correct Answer: Delocalizing charge, altering electrophilic addition pathways

Q35. Which computational parameter is useful to assess alkene stability theoretically?

• HOMO-LUMO gap and calculated heat of formation
• Only empirical boiling point
• Optical rotation
• Melting behavior under UV light

Correct Answer: HOMO-LUMO gap and calculated heat of formation

Q36. An allylic carbocation is stabilized primarily by:

• Inductive withdrawal of electrons
• Resonance delocalization over the allylic system
• Loss of hyperconjugation
• Steric compression

Correct Answer: Resonance delocalization over the allylic system

Q37. How does hydrogen bonding near an alkene affect its stability?

• Hydrogen bonding always destabilizes the alkene
• It can stabilize or destabilize depending on geometry and electronic effects
• It has no possible effect
• It converts the alkene into a ketone

Correct Answer: It can stabilize or destabilize depending on geometry and electronic effects

Q38. Which alkene derivative would most likely resist hydrogenation (remain stable)?

• A conjugated alkene adjacent to an aromatic ring
• A terminal isolated alkene with no conjugation
• A highly strained cyclopropene
• A simple monosubstituted alkene

Correct Answer: A conjugated alkene adjacent to an aromatic ring

Q39. Which is true about vinyl carbocations formed from alkenes?

• They are stabilized by hyperconjugation extensively
• They are generally less stable than allylic carbocations
• They are as stable as tertiary alkyl carbocations
• They readily form without high energy

Correct Answer: They are generally less stable than allylic carbocations

Q40. A benzyl-substituted alkene gains stability because the benzyl group:

• Is strongly electron-withdrawing by induction
• Provides resonance delocalization when conjugated
• Always increases steric strain
• Reduces all π-contributions

Correct Answer: Provides resonance delocalization when conjugated

Q41. Which is true about trans-cycloalkenes?

• Small rings (e.g., cyclohexene) can readily accommodate trans double bonds
• Trans double bonds in small rings are highly strained and often unstable
• All cyclic trans alkenes are more stable than cis
• Trans-cycloalkenes are aromatic

Correct Answer: Trans double bonds in small rings are highly strained and often unstable

Q42. Which reaction outcome reflects thermodynamic control favoring more stable alkenes?

• Formation of the least substituted alkene at low temperature
• Reversible conditions and higher temperature giving the most substituted alkene
• Irreversible, fast elimination giving the least stable alkene
• Only radical additions occur

Correct Answer: Reversible conditions and higher temperature giving the most substituted alkene

Q43. How does conjugation with heteroatoms (e.g., O, N) affect alkene stability?

• Always destabilizes due to lone pair repulsion
• Can stabilize by resonance donation of lone pairs into the π-system
• Has no electronic effect
• Converts alkenes into alkanes

Correct Answer: Can stabilize by resonance donation of lone pairs into the π-system

Q44. Which is the correct stability order for isomeric butenes?

• 1-Butene > cis-2-butene > trans-2-butene
• trans-2-butene > cis-2-butene > 1-butene
• cis-2-butene > trans-2-butene > 1-butene
• All are equally stable

Correct Answer: trans-2-butene > cis-2-butene > 1-butene

Q45. In medicinal chemistry, alkene stability influences which metabolic pathway most directly?

• Glucuronidation only
• Hydrogenation by gut flora exclusively
• Oxidative metabolism (e.g., epoxidation or hydroxylation)
• Direct excretion unchanged

Correct Answer: Oxidative metabolism (e.g., epoxidation or hydroxylation)

Q46. What is the effect of a fluorine substituent on an adjacent alkene?

• Fluorine strongly donates by resonance and stabilizes the alkene
• Fluorine withdraws by induction and can destabilize or polarize the double bond
• Fluorine has no electronic effect
• Fluorine always causes immediate polymerization

Correct Answer: Fluorine withdraws by induction and can destabilize or polarize the double bond

Q47. Which alkene functional group is most susceptible to electrophilic addition?

• Highly substituted, conjugated aromatic double bonds only
• Electron-rich alkenes with donating substituents
• Electron-poor alkenes with strong EWGs
• All alkenes are equally susceptible regardless of substitution

Correct Answer: Electron-rich alkenes with donating substituents

Q48. Which phenomenon explains why tert-butyl-substituted alkenes are often relatively stable?

• Strong electron withdrawal by tert-butyl
• Hyperconjugation from multiple adjacent C–H bonds and inductive donation
• Resonance donation from tert-butyl
• Formation of internal hydrogen bonds

Correct Answer: Hyperconjugation from multiple adjacent C–H bonds and inductive donation

Q49. For predicting alkene stability, which textbook trend should B. Pharm students memorize?

• cis > trans > terminal alkene
• terminal > cis > trans
• trans > cis > terminal substitution
• No general trend exists

Correct Answer: trans > cis > terminal substitution

Q50. Which description best helps predict alkene stability in drug molecules?

• Only count the number of carbons in the molecule
• Assess substitution pattern, conjugation/resonance, steric strain, and electronic effects
• Ignore stereochemistry and electronic effects
• Assume all alkenes are equally reactive in vivo

Correct Answer: Assess substitution pattern, conjugation/resonance, steric strain, and electronic effects

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