Electrophilic addition reactions of alkenes MCQs With Answer

Electrophilic addition reactions of alkenes MCQs With Answer are essential for B. Pharm students studying organic reaction mechanisms, regioselectivity, and stereochemistry. This concise introduction covers electrophilic addition concepts such as carbocation formation, Markovnikov vs. anti‑Markovnikov outcomes, halonium ion intermediates, halohydrin formation, oxymercuration-demercuration, and hydroboration‑oxidation pathways. Emphasis is placed on reaction conditions, reagent roles, rearrangements, and stereochemical consequences important in drug synthesis and metabolite prediction. These MCQs will sharpen your problem-solving skills, reinforce mechanism-based reasoning, and prepare you for exam-style questions on electrophilic additions of alkenes. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which species acts as the electrophile in acid-catalyzed hydration of alkenes?

• Hydronium ion (H3O+)
• Carbocation intermediate
• Water molecule
• Alkene π bond

Correct Answer: Hydronium ion (H3O+)

Q2. According to Markovnikov’s rule, when H–X adds to an unsymmetrical alkene, the hydrogen attaches to the carbon with:

• More substituents
• Fewer substituents
• Greater steric hindrance
• Less electron density

Correct Answer: Fewer substituents

Q3. Which step is generally the rate-determining step in traditional electrophilic addition to alkenes?

• Nucleophilic attack on carbocation
• Formation of carbocation by protonation of the alkene
• Deprotonation of the intermediate
• Solvent reorganization

Correct Answer: Formation of carbocation by protonation of the alkene

Q4. In bromination of alkenes with Br2 in inert solvent, the key non-carbocation intermediate is:

• Carbanion
• Bromonium ion
• Free radical
• Carbocation

Correct Answer: Bromonium ion

Q5. Halogen addition (Br2) to an alkene typically gives which stereochemical outcome?

• Syn addition
• Anti addition
• Random stereochemistry
• Exclusive racemization

Correct Answer: Anti addition

Q6. Which reagent combination gives oxymercuration-demercuration that hydrates alkenes without rearrangement?

• BH3 followed by H2O2/NaOH
• Hg(OAc)2, H2O then NaBH4
• HBr in peroxide
• H2/Pd

Correct Answer: Hg(OAc)2, H2O then NaBH4

Q7. Hydroboration–oxidation of terminal alkenes yields alcohols with which regiochemistry?

• Markovnikov (OH on more substituted carbon)
• Anti‑Markovnikov (OH on less substituted carbon)
• Random regiochemistry
• No alcohol formation

Correct Answer: Anti‑Markovnikov (OH on less substituted carbon)

Q8. The hydroboration step of hydroboration–oxidation proceeds with what stereochemistry?

• Anti addition
• Syn addition
• Radical inversion
• Internal return

Correct Answer: Syn addition

Q9. Which reagent promotes anti‑Markovnikov addition of HBr to alkenes via a radical pathway?

• Peroxides or ROOR
• Hg(OAc)2
• Br2 in CCl4
• H2SO4

Correct Answer: Peroxides or ROOR

Q10. Why does oxymercuration avoid carbocation rearrangements?

• It forms a stable free radical
• It proceeds via a mercurinium ion which is resistant to rearrangement
• It forms a long‑lived carbocation that equilibrates
• It uses basic conditions that block rearrangement

Correct Answer: It proceeds via a mercurinium ion which is resistant to rearrangement

Q11. In halohydrin formation (Br2, H2O), the OH group attaches to which carbon of an unsymmetrical alkene?

• The less substituted carbon
• The more substituted carbon
• The carbon with fewer hydrogens always
• Either carbon equally

Correct Answer: The more substituted carbon

Q12. During halonium ion opening by nucleophile, nucleophile attacks at:

• The more substituted carbon of the halonium
• The less substituted carbon due to greater positive character
• A carbon outside the ring
• Only at terminal methyl groups

Correct Answer: The less substituted carbon due to greater positive character

Q13. Which is the correct order of carbocation stability?

• Primary > Secondary > Tertiary
• Tertiary > Secondary > Primary
• Secondary > Tertiary > Primary
• Primary > Tertiary > Secondary

Correct Answer: Tertiary > Secondary > Primary

Q14. Which electrophilic addition to alkenes is most likely to undergo hydride or methyl rearrangement?

• Oxymercuration-demercuration
• Acid-catalyzed hydration
• Hydroboration–oxidation
• Halogenation with Br2/CCl4

Correct Answer: Acid-catalyzed hydration

Q15. A resonance‑stabilized allylic carbocation will be:

• Less stable than a tertiary carbocation
• More stable than a primary carbocation
• Unreactive toward nucleophiles
• Always undergo rearrangement

Correct Answer: More stable than a primary carbocation

Q16. Which solvent is commonly used for halogenation of alkenes that avoids nucleophilic participation?

• Water
• Acetone
• CCl4 (carbon tetrachloride)
• Methanol

Correct Answer: CCl4 (carbon tetrachloride)

Q17. During electrophilic addition, the alkene acts as:

• An electrophile donating a proton
• A nucleophile via its π bond
• A radical initiator
• A Lewis acid

Correct Answer: A nucleophile via its π bond

Q18. In bromination followed by nucleophilic attack, the product stereochemistry relative to initial alkene substituents is typically:

• Retention at both centers
• Inversion at both centers (trans relative positions)
• Random racemic mix only
• Syn addition product

Correct Answer: Inversion at both centers (trans relative positions)

Q19. Which factor most strongly influences regiochemistry in electrophilic addition?

• Temperature only
• Stability of the intermediate carbocation or transition state
• Color of reactants
• Pressure only

Correct Answer: Stability of the intermediate carbocation or transition state

Q20. Hydrohalogenation of an alkene in presence of peroxides produces anti‑Markovnikov product for which hydrogen halide?

• HCl
• HBr
• HI
• HF

Correct Answer: HBr

Q21. Which reagent set converts an alkene to an alcohol without rearrangement and with Markovnikov regiochemistry?

• BH3, then H2O2/NaOH
• HBr then H2O
• Hg(OAc)2, H2O then NaBH4
• Br2 in H2O

Correct Answer: Hg(OAc)2, H2O then NaBH4

Q22. In electrophilic addition, what role does a Lewis acid catalyst (e.g., AlCl3) commonly play?

• Generates a stronger electrophile by coordinating to nucleophile
• Generates a stronger electrophile by coordinating to reagent
• Acts as nucleophile
• Removes solvent

Correct Answer: Generates a stronger electrophile by coordinating to reagent

Q23. Which addition pathway involves a concerted transition state rather than discrete carbocation intermediates?

• Acid-catalyzed hydration
• Hydroboration of alkenes
• Carbocation rearrangement
• Bromination via bromonium ion

Correct Answer: Hydroboration of alkenes

Q24. Which product results from addition of Br2 to cyclohexene in CCl4?

• 1,2‑dibromocyclohexane (trans)
• Bromocyclohexane plus HBr
• Bicyclic bromonium polymer
• Cyclohexanol

Correct Answer: 1,2‑dibromocyclohexane (trans)

Q25. Which statement about halohydrin formation is true?

• Water attacks the more substituted carbon of the bromonium ion
• Water attacks the less substituted carbon of the bromonium ion
• It always gives syn addition products
• It proceeds through a free radical mechanism

Correct Answer: Water attacks the more substituted carbon of the bromonium ion

Q26. In nucleophilic attack on a bromonium ion, ring opening is favored at the carbon that:

• Is more substituted because it bears more positive character
• Is less substituted because of steric hindrance
• Is less substituted because it is more positively charged in the three-membered ion
• Has an adjacent aromatic ring

Correct Answer: Is less substituted because it is more positively charged in the three-membered ion

Q27. Which reagent is best to convert an alkene to an alcohol via syn addition and anti‑Markovnikov regiochemistry?

• Hg(OAc)2 followed by NaBH4
• BH3 followed by H2O2/NaOH
• HBr in peroxides
• Br2 in CCl4

Correct Answer: BH3 followed by H2O2/NaOH

Q28. Why does hydroboration give anti‑Markovnikov addition of water after oxidation?

• Because boron attaches to the less hindered carbon and is replaced by OH
• Because boron stabilizes tertiary carbocation
• Because reaction proceeds via free radicals
• Because acid catalysis flips regiochemistry

Correct Answer: Because boron attaches to the less hindered carbon and is replaced by OH

Q29. Which of the following additions is not an electrophilic addition?

• Halogenation with Br2
• Acid-catalyzed hydration
• Free radical addition of HBr under peroxides
• Oxymercuration

Correct Answer: Free radical addition of HBr under peroxides

Q30. During HBr addition in the absence of peroxides, the regioselectivity follows:

• Anti‑Markovnikov rule
• Markovnikov rule
• No regiochemistry
• Always gives symmetric products

Correct Answer: Markovnikov rule

Q31. Which intermediate explains anti addition and trans stereochemistry in Br2 addition?

• Carbocation stabilized by solvent
• Bromonium ion bridging the two carbons
• Free radical bromide
• Organometallic complex

Correct Answer: Bromonium ion bridging the two carbons

Q32. In electrophilic additions, polar solvents typically:

• Decrease reaction rate by stabilizing reactants
• Stabilize ionic intermediates and often increase rate
• Promote radical pathways
• Always prevent rearrangements

Correct Answer: Stabilize ionic intermediates and often increase rate

Q33. Which condition favors formation of a more stable carbocation intermediate?

• Electron‑withdrawing substituents on alkene
• Electron‑donating substituents on alkene
• Strongly basic medium
• High concentration of radicals

Correct Answer: Electron‑donating substituents on alkene

Q34. Which reagent pair gives halohydrin formation from an alkene?

• Br2 in CCl4
• Br2 in H2O
• BH3 then H2O2
• HBr with peroxide

Correct Answer: Br2 in H2O

Q35. Which of the following correctly describes the regiochemistry of acid-catalyzed hydration of alkenes?

• Anti‑Markovnikov always
• Markovnikov unless rearrangement occurs
• No regiochemistry because both carbons are equivalent
• Always gives alcohol on terminal carbon

Correct Answer: Markovnikov unless rearrangement occurs

Q36. Which technique would you use to avoid carbocation rearrangement when converting an alkene to alcohol?

• Use concentrated H2SO4
• Perform hydroboration–oxidation
• Use HBr in peroxides
• Perform acid-catalyzed hydration at high temperature

Correct Answer: Perform hydroboration–oxidation

Q37. In electrophilic addition to conjugated dienes, initial attack giving the more substituted carbocation leads to:

• Only 1,2‑addition products
• Possible 1,2‑ and 1,4‑addition products depending on conditions
• Only 1,4‑addition products always
• No reaction occurs

Correct Answer: Possible 1,2‑ and 1,4‑addition products depending on conditions

Q38. Which mechanism best explains product distribution when Br2 adds to an alkene in presence of nucleophiles?

• Free radical chain mechanism
• Formation of halonium ion followed by nucleophilic attack
• Concerted pericyclic addition
• Single electron transfer to form carbanions

Correct Answer: Formation of halonium ion followed by nucleophilic attack

Q39. When a tertiary carbocation can form during electrophilic addition, what is a likely outcome?

• Rapid rearrangement to less stable forms
• Increased tendency for rearrangement compared to primary
• Lower reactivity than primary carbocations
• Formation of stable carbanions instead

Correct Answer: Increased tendency for rearrangement compared to primary

Q40. Which reagent is best to add a bromine to an alkene without forming an alcohol?

• Br2 in H2O
• Br2 in CCl4
• Br2 in methanol
• HBr in peroxides

Correct Answer: Br2 in CCl4

Q41. The reagent NBS in presence of light or peroxides is typically used for:

• Electrophilic addition to alkenes
• Allylic bromination via radical mechanism
• Hydroboration of alkenes
• Oxymercuration-demercuration

Correct Answer: Allylic bromination via radical mechanism

Q42. Which statement best distinguishes hydroboration from acid-catalyzed hydration?

• Both proceed via carbocation intermediates
• Hydroboration is concerted and syn, avoiding rearrangement
• Hydroboration gives Markovnikov products
• Acid hydration proceeds through a concerted mechanism

Correct Answer: Hydroboration is concerted and syn, avoiding rearrangement

Q43. In electrophilic addition, a substituent that donates electron density by resonance to the double bond will:

• Destabilize any carbocation formed
• Stabilize carbocation intermediate and increase reactivity
• Always prevent addition reactions
• Convert mechanism to radical pathway

Correct Answer: Stabilize carbocation intermediate and increase reactivity

Q44. Which product results from addition of Br2 to propene in water (halohydrin formation)?

• 1,2‑dibromopropane
• 2‑bromopropan‑1‑ol (OH on carbon-2)
• 1‑bromopropan‑2‑ol
• Isopropyl bromide only

Correct Answer: 2‑bromopropan‑1‑ol (OH on carbon-2)

Q45. In a bromonium ion formed from an unsymmetrical alkene, which carbon is more positively charged?

• The more substituted carbon
• The less substituted carbon
• Both carbons equally
• Neither, charge is on bromine only

Correct Answer: The more substituted carbon

Q46. Which experimental observation indicates anti addition has occurred?

• Formation of enantiomeric pairs from a cis‑alkene
• Formation of meso compound from a cis‑alkene leading to identical substituents trans to each other
• No change in stereochemistry
• Only constitutional isomers are formed

Correct Answer: Formation of meso compound from a cis‑alkene leading to identical substituents trans to each other

Q47. What is the effect of electron-withdrawing groups on the alkene toward electrophilic addition?

• They increase the rate by stabilizing carbocations
• They decrease reactivity by reducing nucleophilicity of the π bond
• They promote anti‑Markovnikov addition exclusively
• They have no effect

Correct Answer: They decrease reactivity by reducing nucleophilicity of the π bond

Q48. Which of the following pathways gives syn addition across a double bond?

• Addition of Br2 in CCl4
• Hydroboration–oxidation followed by oxidation
• Acid-catalyzed hydration via carbocation
• Formation and opening of bromonium ion

Correct Answer: Hydroboration–oxidation followed by oxidation

Q49. For electrophilic addition reactions relevant to pharmaceutical synthesis, why is understanding regiochemistry important?

• It determines melting point only
• It affects the position of functional groups which influence biological activity and metabolism
• It only affects color of compound
• Regiochemistry is irrelevant in drug design

Correct Answer: It affects the position of functional groups which influence biological activity and metabolism

Q50. Which mechanistic feature helps predict whether rearrangement will occur during electrophilic addition?

• Whether the reaction is photochemical
• Whether a discrete, long‑lived carbocation intermediate is formed
• The pH of water in the lab only
• The boiling point of solvent exclusively

Correct Answer: Whether a discrete, long‑lived carbocation intermediate is formed

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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