Electrophilic addition in conjugated dienes MCQs With Answer

Electrophilic addition in conjugated dienes MCQs With Answer

Understanding electrophilic addition in conjugated dienes is crucial for B.Pharm students studying organic reaction mechanisms and drug synthesis. This introduction covers key concepts like resonance-stabilized allylic carbocations, the formation of 1,2- and 1,4-addition products, kinetic versus thermodynamic control, and common reagents (HBr, Br2, HCl, H2O). Emphasis on regioselectivity, stereochemistry, and reaction conditions will help you predict major products and mechanism pathways useful in pharmaceutical intermediate design. These MCQs are tailored to deepen your mechanistic insight and problem-solving skills for exams and lab applications. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary intermediate formed during electrophilic addition of HBr to a conjugated diene?

• Carbanion
• Allylic carbocation
• Free radical
• Bromonium ion

Correct Answer: Allylic carbocation

Q2. Which two major products result from electrophilic addition to a simple conjugated diene like 1,3-butadiene?

• 1,2- and 1,3-addition products
• 1,2- and 1,4-addition products
• 1,1- and 1,3-addition products
• 1,3- and 1,4-addition products

Correct Answer: 1,2- and 1,4-addition products

Q3. Which factor favors the kinetic product (1,2-addition) in electrophilic addition to conjugated dienes?

• High temperature
• Long reaction time
• Low temperature and short reaction time
• Strong base present

Correct Answer: Low temperature and short reaction time

Q4. Which product is generally favored under thermodynamic control in electrophilic addition to conjugated dienes?

• Less substituted alkene (1,2-product)
• More substituted alkene (1,4-product)
• Conjugated, more stable product (usually 1,4-product)
• Non-conjugated product

Correct Answer: Conjugated, more stable product (usually 1,4-product)

Q5. What explains the formation of both 1,2- and 1,4-products from a single allylic carbocation intermediate?

• Resonance delocalization of the positive charge
• Nucleophilic substitution at a saturated carbon
• Radical chain propagation
• Pericyclic rearrangement

Correct Answer: Resonance delocalization of the positive charge

Q6. Which reagent typically gives an anti addition across an isolated double bond but leads to multiple products with conjugated dienes?

• H2 with Pd catalyst
• Br2
• Ozone (O3)
• Potassium permanganate

Correct Answer: Br2

Q7. In the electrophilic addition of HBr to 1,3-butadiene at low temperature, which product predominates?

• 1-bromo-2-butene (1,2-product)
• 3-bromo-1-butene (1,4-product)
• 1,4-dibromobutane
• Butane

Correct Answer: 1-bromo-2-butene (1,2-product)

Q8. Which concept best describes why 1,4-addition product is often more stable than the 1,2-product?

• Hyperconjugation and conjugation stabilization
• Inductive effect from bromine
• Steric hindrance reduces reactivity
• Loss of aromaticity

Correct Answer: Hyperconjugation and conjugation stabilization

Q9. What is the stereochemical outcome typically observed for halogen addition to an isolated double bond?

• Syn addition
• Anti addition
• Radical inversion
• No stereochemical preference

Correct Answer: Anti addition

Q10. Which intermediate is proposed when halogen (Br2) reacts with a conjugated diene?

• Bromonium ion followed by resonance opening to allylic cation
• Free bromide radical
• Bromide anion substitution intermediate
• Peroxide intermediate

Correct Answer: Bromonium ion followed by resonance opening to allylic cation

Q11. How does solvent polarity affect the distribution of 1,2- vs 1,4-products?

• Polar solvents always favor 1,2-products
• Nonpolar solvents always favor 1,4-products
• Polar solvents stabilize carbocations and can increase 1,4-product formation under equilibrium
• Solvent has no effect

Correct Answer: Polar solvents stabilize carbocations and can increase 1,4-product formation under equilibrium

Q12. Which of the following reagents leads to electrophilic hydration (Markovnikov) of conjugated dienes producing allylic alcohols?

• Br2 in CCl4
• HBr in absence of peroxide
• H2O in presence of H+ (acid-catalyzed)
• O3 followed by Zn

Correct Answer: H2O in presence of H+ (acid-catalyzed)

Q13. In the mechanism of electrophilic addition to conjugated dienes, resonance contributors place positive charge on which carbon positions?

• Only on the terminal carbon
• On both allylic carbons and the internal carbon
• Only on the central carbon of the diene
• Only on non-conjugated carbons

Correct Answer: On both allylic carbons and the internal carbon

Q14. Which experimental observation indicates kinetic control in a diene addition?

• Product ratio changes with increased temperature
• Product ratio is same at all times
• Short reaction time gives one major product, long time leads to different major product
• Products are racemic

Correct Answer: Short reaction time gives one major product, long time leads to different major product

Q15. Electrophilic addition to conjugated dienes is important in pharmaceutical chemistry because:

• It always yields single enantiomeric products
• It provides methods to build substituted alkenes and allylic centers used in drug scaffolds
• It never produces side products
• It only involves free radical steps

Correct Answer: It provides methods to build substituted alkenes and allylic centers used in drug scaffolds

Q16. Which product results when HBr adds to 2-methyl-1,3-butadiene under thermodynamic control?

• 1-bromo-2-methyl-1-butene (1,2-product)
• 3-bromo-2-methyl-1-butene (1,4-product)
• 2,3-dibromo-2-methylbutane
• Unreacted diene

Correct Answer: 3-bromo-2-methyl-1-butene (1,4-product)

Q17. Which statement about the allylic carbocation intermediate is true?

• It is less stable than a primary carbocation
• It is stabilized by resonance across the conjugated system
• It cannot form in conjugated dienes
• It is always converted to a radical

Correct Answer: It is stabilized by resonance across the conjugated system

Q18. Which technique can be used to determine the ratio of 1,2- to 1,4-products after reaction?

• Infrared spectroscopy only
• Gas chromatography or NMR spectroscopy
• Melting point determination
• Paper chromatography only

Correct Answer: Gas chromatography or NMR spectroscopy

Q19. Which is a distinguishing feature of conjugated dienes compared to isolated dienes in electrophilic addition?

• Conjugated dienes never react with electrophiles
• Conjugated dienes give only one product
• Conjugated dienes can delocalize positive charge leading to multiple regioisomers
• Conjugated dienes immediately polymerize under any condition

Correct Answer: Conjugated dienes can delocalize positive charge leading to multiple regioisomers

Q20. In the addition of HBr to a conjugated diene, which factor would increase the proportion of 1,2-product?

• Higher temperature
• Presence of peroxide
• Very short reaction time and low temperature
• Equilibration over long time

Correct Answer: Very short reaction time and low temperature

Q21. Which term best describes the 1,2-addition product obtained rapidly at low temperature?

• Thermodynamic product
• Polymerization product
• Kinetic product
• Revived product

Correct Answer: Kinetic product

Q22. During electrophilic addition to conjugated dienes, what role does the nucleophile play?

• It generates the electrophile
• It attacks the resonance-stabilized carbocation to form the final product
• It forms a radical chain
• It removes a proton to form an alkene

Correct Answer: It attacks the resonance-stabilized carbocation to form the final product

Q23. Which of the following reagents would most likely lead to allylic bromination instead of simple electrophilic addition when reacting with conjugated dienes under radical conditions?

• HBr with peroxides
• HBr at low temp without light
• Br2 in CCl4 in absence of light
• Hydrogen gas with Pd/C

Correct Answer: HBr with peroxides

Q24. Why does 1,4-addition often give the more substituted double bond in conjugated dienes?

• Because nucleophile attacks the less substituted carbon always
• Because resonance leads to formation of the more substituted, conjugated alkene which is thermodynamically favored
• Because the electrophile cannot add at terminal carbon
• Because the solvent forces this outcome

Correct Answer: Because resonance leads to formation of the more substituted, conjugated alkene which is thermodynamically favored

Q25. Which experimental observation supports the resonance-stabilized allylic carbocation mechanism?

• Only a single product is isolated
• Nucleophile adds at two different carbon positions yielding regioisomers
• Reaction produces only radicals
• All products are identical in NMR

Correct Answer: Nucleophile adds at two different carbon positions yielding regioisomers

Q26. What is the expected product when Br2 adds to 1,3-butadiene at low temperature?

• Only 1,2-dibromo product
• Only 1,4-dibromo product
• A mixture of 1,2- and 1,4-dibromo products with 1,2 predominating
• No reaction

Correct Answer: A mixture of 1,2- and 1,4-dibromo products with 1,2 predominating

Q27. In academic problems, which rule often predicts where the electrophile will add to an unsymmetrical conjugated diene?

• Hückel rule
• Markovnikov-type reasoning with resonance-stabilized carbocations
• Le Chatelier’s principle
• Avogadro’s law

Correct Answer: Markovnikov-type reasoning with resonance-stabilized carbocations

Q28. How does substitution on the diene affect regioselectivity of electrophilic addition?

• More substituted carbons stabilize positive charge and direct electrophile addition
• Substitution never affects regioselectivity
• Only electron-withdrawing groups increase reactivity
• Steric bulk always prevents addition

Correct Answer: More substituted carbons stabilize positive charge and direct electrophile addition

Q29. Which mechanistic feature differentiates addition to conjugated dienes from addition to isolated alkenes?

• Formation of a single, localized carbocation
• Formation of resonance-stabilized allylic carbocations leading to multiple products
• Inability to form halonium ions
• Reactions proceed only by radical pathways

Correct Answer: Formation of resonance-stabilized allylic carbocations leading to multiple products

Q30. What product ratio change would you expect if a reaction of HBr with a conjugated diene is warmed after initial low-temperature addition?

• Ratio remains unchanged
• Kinetic 1,2-product may convert to thermodynamic 1,4-product increasing 1,4 proportion
• 1,2-product increases further
• Both products convert to unreacted starting material

Correct Answer: Kinetic 1,2-product may convert to thermodynamic 1,4-product increasing 1,4 proportion

Q31. Which spectroscopic evidence indicates formation of a conjugated system after addition?

• Disappearance of all IR bands
• UV-Vis absorption shift to longer wavelength (bathochromic shift)
• Mass increase only
• Only NMR solvent peaks change

Correct Answer: UV-Vis absorption shift to longer wavelength (bathochromic shift)

Q32. In electrophilic addition to conjugated dienes, what is the role of temperature in controlling product distribution?

• Temperature has no role
• Lower temperatures favor kinetic 1,2-products; higher temperatures favor thermodynamic 1,4-products
• Higher temperatures always stop the reaction
• Lower temperatures favor polymerization

Correct Answer: Lower temperatures favor kinetic 1,2-products; higher temperatures favor thermodynamic 1,4-products

Q33. Which of the following best explains why bromination of conjugated dienes can give anti stereochemistry in certain products?

• Because bromine adds via a bridged bromonium that forces anti attack
• Because bromine adds by syn addition only
• Because radical intermediates enforce syn addition
• Because there is no intermediate

Correct Answer: Because bromine adds via a bridged bromonium that forces anti attack

Q34. When water acts as the nucleophile in electrophilic addition to conjugated dienes, the final product is typically:

• An ether
• An alcohol (allylic or vinylic depending on position)
• A ketone directly
• A carboxylic acid

Correct Answer: An alcohol (allylic or vinylic depending on position)

Q35. Which statement about pericyclic reactions vs electrophilic addition is correct for conjugated dienes?

• Electrophilic addition proceeds via discrete ionic intermediates while pericyclic (e.g., Diels–Alder) is concerted
• Both always proceed via radicals
• Pericyclic reactions require ionic intermediates
• They are the same mechanism

Correct Answer: Electrophilic addition proceeds via discrete ionic intermediates while pericyclic (e.g., Diels–Alder) is concerted

Q36. Which condition would favor formation of a more substituted alkene during electrophilic addition?

• Extremely low temperature and no equilibration
• Allowing equilibration at higher temperature to reach thermodynamic product
• Using radical initiators only
• Strictly nonpolar solvent at -78°C

Correct Answer: Allowing equilibration at higher temperature to reach thermodynamic product

Q37. Which product is expected from addition of HCl to 1,3-butadiene at equilibrium?

• Only 1,2-chloro product
• Only 1,4-chloro product
• A mixture with significant 1,4-chloro (thermodynamic) product
• Polymeric chlorinated product exclusively

Correct Answer: A mixture with significant 1,4-chloro (thermodynamic) product

Q38. Which laboratory technique helps avoid formation of thermodynamic 1,4-products and isolate kinetic 1,2-products?

• Heating the reaction mixture to reflux
• Performing the reaction at low temperature and quenching rapidly
• Prolonged stirring at room temperature
• Adding base to the reaction

Correct Answer: Performing the reaction at low temperature and quenching rapidly

Q39. How does conjugation affect the activation energy for electrophilic attack compared to an isolated alkene?

• Conjugation always increases activation energy
• Conjugation can lower activation energy by stabilizing the transition state via resonance
• Conjugation prevents any electrophilic attack
• Activation energy is unaffected

Correct Answer: Conjugation can lower activation energy by stabilizing the transition state via resonance

Q40. In the addition of HBr to 1,3-butadiene, which resonance form leads to attack giving the 1,4-product?

• Resonance placing positive charge at C2 only
• Resonance placing positive charge at C4 (terminal carbon) enabling nucleophilic attack at that position
• Resonance forming a radical at C1
• No resonance forms are involved

Correct Answer: Resonance placing positive charge at C4 (terminal carbon) enabling nucleophilic attack at that position

Q41. Which of the following is true for conjugated diene addition in the presence of strong nucleophiles?

• Strong nucleophiles always prevent carbocation formation
• Strong nucleophiles may trap the carbocation rapidly, increasing kinetic product proportion
• Strong nucleophiles convert reaction to elimination only
• Strong nucleophiles have no effect

Correct Answer: Strong nucleophiles may trap the carbocation rapidly, increasing kinetic product proportion

Q42. What is the role of a bridged halonium ion in halogen addition to conjugated dienes?

• It is irrelevant to the mechanism
• It provides a three-membered intermediate that directs anti attack and can open to allylic cation pathways
• It immediately forms radicals
• It converts diene to aromatic compound

Correct Answer: It provides a three-membered intermediate that directs anti attack and can open to allylic cation pathways

Q43. Why might a pharmaceutical chemist prefer 1,4-addition over 1,2-addition when synthesizing an intermediate?

• 1,4-addition products are always achiral
• 1,4-addition often yields more conjugated and thermodynamically stable intermediates useful in further functionalization
• 1,2-addition products are always toxic
• 1,4-addition avoids any need for purification

Correct Answer: 1,4-addition often yields more conjugated and thermodynamically stable intermediates useful in further functionalization

Q44. Which experimental variable can be used to shift product distribution from 1,2 to 1,4 in lab synthesis?

• Using radical initiators
• Changing solvents and increasing temperature to promote equilibration
• Adding an excess of peroxide
• Removing the electrophile

Correct Answer: Changing solvents and increasing temperature to promote equilibration

Q45. In an NMR spectrum, how might the 1,4-addition product differ from the 1,2-product?

• No differences are ever observed
• Different chemical shifts for vinylic protons due to different substitution and conjugation
• Only multiplicity changes but chemical shifts remain identical
• Only solvent peaks change

Correct Answer: Different chemical shifts for vinylic protons due to different substitution and conjugation

Q46. Which mechanistic step is rate-determining in many electrophilic additions to conjugated dienes?

• Nucleophilic attack step
• Formation of the resonance-stabilized carbocation (electrophile addition to diene)
• Product isolation
• Rotation about single bonds after product formation

Correct Answer: Formation of the resonance-stabilized carbocation (electrophile addition to diene)

Q47. What product results when Br2 adds to a conjugated diene in the presence of a nucleophile that rapidly traps bromonium intermediates?

• Unreacted starting material
• Vicinal dibromide only
• Mixture including products from nucleophilic trapping and dibromides, with anti stereochemistry from bromonium opening
• Only polymeric bromide

Correct Answer: Mixture including products from nucleophilic trapping and dibromides, with anti stereochemistry from bromonium opening

Q48. Which pathway explains formation of 1,4-addition product in conjugated dienes?

• Direct concerted addition across both double bonds simultaneously
• Formation of allylic cation followed by nucleophilic attack at the terminal resonant carbon
• Pericyclic sigmatropic rearrangement exclusively
• Radical polymerization

Correct Answer: Formation of allylic cation followed by nucleophilic attack at the terminal resonant carbon

Q49. For educational practice, which model helps visualize resonance-stabilized intermediates in conjugated dienes?

• Molecular orbital diagrams showing delocalized pi system
• Simple ball-and-stick without electrons
• Only IR vibration models
• Chromatography models

Correct Answer: Molecular orbital diagrams showing delocalized pi system

Q50. When designing a synthetic route in medicinal chemistry, why is mechanistic understanding of electrophilic addition to conjugated dienes essential?

• It allows prediction of regio- and stereochemical outcomes and control of kinetic vs thermodynamic products important for target molecule synthesis
• Mechanism is irrelevant for planning
• Only yields matter, not mechanism
• It prevents any further reactions from occurring

Correct Answer: It allows prediction of regio- and stereochemical outcomes and control of kinetic vs thermodynamic products important for target molecule 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.

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