Order of reactivity of alkyl halides (E1/E2) MCQs With Answer

Order of reactivity of alkyl halides (E1/E2) MCQs With Answer

Understanding the order of reactivity of alkyl halides in E1 and E2 eliminations is essential for B. Pharm students studying medicinal chemistry and synthetic pathways. This guide explains how substrate structure (primary, secondary, tertiary), leaving group ability, base strength, solvent polarity, steric hindrance, and carbocation stability influence whether an alkyl halide undergoes E1 or E2. Emphasis on benzylic/allylic effects, vinyl/aryl exceptions, and typical reagents (t-BuOK, NaOEt, heat, polar protic/aprotic solvents) helps predict major products and reaction rates. Gain a practical grasp of mechanism selection and product distribution to support drug synthesis problems. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which substrate shows the highest reactivity toward E1 elimination under polar protic conditions?

• Primary alkyl chloride
• Secondary alkyl bromide
• Tertiary alkyl iodide
• Vinyl chloride

Correct Answer: Tertiary alkyl iodide

Q2. For a strong, bulky base like t-BuOK, which elimination pathway is favored with a secondary alkyl bromide?

• E1 with carbocation rearrangement
• E2 anti-periplanar elimination
• SN1 substitution
• Radical halogenation

Correct Answer: E2 anti-periplanar elimination

Q3. Which factor most increases the rate of an E1 reaction?

• Stronger nucleophilic base
• Better leaving group
• Lower solvent polarity
• Sterically hindered base

Correct Answer: Better leaving group

Q4. Which solvent is most likely to favor E2 over E1 for a given alkyl halide and base?

• Polar protic solvent like ethanol
• Polar aprotic solvent like DMSO
• Nonpolar solvent like hexane
• Strongly acidic aqueous medium

Correct Answer: Polar aprotic solvent like DMSO

Q5. Which substrate is least likely to undergo E2 elimination due to required geometry?

• Cyclohexyl bromide requiring trans-diaxial H
• Primary methyl chloride
• Allylic chloride
• Benzyl chloride

Correct Answer: Cyclohexyl bromide requiring trans-diaxial H

Q6. Which leaving group improves the rate of both E1 and E2 reactions most significantly?

• Fluoride
• Chloride
• Bromide
• Hydroxide

Correct Answer: Bromide

Q7. For tertiary alkyl halides, which mechanism predominates with weak bases and polar protic solvents?

• E2 exclusively
• E1 via carbocation intermediate
• SN2 substitution
• Radical substitution

Correct Answer: E1 via carbocation intermediate

Q8. Which statement best describes the rate law of an E2 reaction?

• Rate depends only on concentration of substrate
• Rate depends on concentration of base only
• Rate depends on both substrate and base concentrations
• Rate is independent of concentrations (zero order)

Correct Answer: Rate depends on both substrate and base concentrations

Q9. Which substrate will give the fastest E2 reaction with a small, strong base like NaOEt?

• Primary alkyl iodide
• Secondary alkyl chloride
• Tertiary alkyl fluoride
• Vinyl chloride

Correct Answer: Primary alkyl iodide

Q10. Which is a common experimental indicator that an elimination occurs via E1?

• Reaction rate increases with base concentration
• Observation of rearranged products
• Strict anti-periplanar geometry requirement
• Concerted one-step mechanism

Correct Answer: Observation of rearranged products

Q11. Which alkyl halide is most reactive toward E2 when paired with a hindered base?

• Primary alkyl chloride favoring substitution
• Secondary alkyl bromide giving elimination
• Tertiary alkyl iodide giving E2 exclusively
• Vinyl bromide

Correct Answer: Secondary alkyl bromide giving elimination

Q12. What effect does increasing temperature usually have on E1 vs E2 competition?

• Favors substitution over elimination
• Promotes elimination pathways (both E1 and E2)
• Has no effect on mechanism distribution
• Always converts E2 to E1

Correct Answer: Promotes elimination pathways (both E1 and E2)

Q13. Which substrate is most stabilized as a carbocation and thus favors E1?

• Primary alkyl halide
• Secondary alkyl halide
• Tertiary alkyl halide
• Vinyl halide

Correct Answer: Tertiary alkyl halide

Q14. In E2 elimination, what stereochemical requirement often controls product formation?

• Syn-periplanar hydrogen only
• Anti-periplanar hydrogen for concerted bond breaking
• No stereochemical requirement
• Random hydrogen abstraction

Correct Answer: Anti-periplanar hydrogen for concerted bond breaking

Q15. Which alkyl halide generally does not undergo E1 due to inability to form a stable carbocation?

• Tertiary allylic chloride
• Benzylic bromide
• Vinyl chloride
• Secondary tertiary mixed halide

Correct Answer: Vinyl chloride

Q16. Which reagent is most likely to push a secondary alkyl bromide toward E2 rather than E1?

• Weak nucleophile in polar protic solvent
• Strong, non-nucleophilic base like KOt-Bu
• Neutral solvent at low temperature
• Silver nitrate to remove halide

Correct Answer: Strong, non-nucleophilic base like KOt-Bu

Q17. Which concept explains why benzylic and allylic halides react faster in E1/E2?

• Inductive electron withdrawal only
• Resonance stabilization of carbocation or transition state
• Steric hindrance blocking elimination
• Poor leaving group tendency

Correct Answer: Resonance stabilization of carbocation or transition state

Q18. For SN1/E1 competitions, which condition increases the proportion of E1 products?

• Lower temperature
• Very strong nucleophile
• High temperature and weak nucleophile/base
• Use of a bulky nucleophile that favors SN2

Correct Answer: High temperature and weak nucleophile/base

Q19. Which halide shows negligible reactivity in both E1 and E2 due to strong C–X bond strength?

• Alkyl iodide
• Alkyl bromide
• Alkyl chloride
• Alkyl fluoride

Correct Answer: Alkyl fluoride

Q20. Which outcome suggests a concerted E2 mechanism during analysis of kinetics?

• First-order kinetics dependent only on substrate
• Second-order kinetics dependent on substrate and base
• Formation of carbocation intermediate observed
• Major product arises after rearrangement

Correct Answer: Second-order kinetics dependent on substrate and base

Q21. In the order of reactivity for E2 with a small strong base, which substrate is most reactive?

• Methyl halide
• Primary halide
• Secondary halide
• Tertiary halide

Correct Answer: Methyl halide

Q22. Why are vinyl and aryl halides poor substrates for E2?

• They form very stable carbocations easily
• The C–X bond is stronger and geometry prevents anti-periplanar H
• They have excellent leaving groups
• They always undergo SN2 instead

Correct Answer: The C–X bond is stronger and geometry prevents anti-periplanar H

Q23. Which is the major product when tert-butyl bromide is heated in ethanol (polar protic)?

• SN2 substitution product predominates
• E1 elimination forming an alkene predominates
• No reaction occurs
• Radical substitution occurs

Correct Answer: E1 elimination forming an alkene predominates

Q24. When a leaving group is improved (e.g., Cl → I), what is the general effect on E1 and E2 rates?

• Both E1 and E2 rates decrease
• E1 rate increases much more than E2
• Both E1 and E2 rates increase
• Only SN2 is affected

Correct Answer: Both E1 and E2 rates increase

Q25. Which mechanism is most sensitive to solvent polarity (polar protic solvents)?

• E2 because base strength increases in protic solvents
• E1 because carbocation stabilization by solvent is important
• Neither mechanism is affected by solvent
• SN2 is only affected

Correct Answer: E1 because carbocation stabilization by solvent is important

Q26. What product orientation does Zaitsev’s rule predict for E2 eliminations with small bases?

• Less substituted alkene predominates
• More substituted (thermodynamically stable) alkene predominates
• Only terminal alkene forms
• No elimination products form

Correct Answer: More substituted (thermodynamically stable) alkene predominates

Q27. How does a bulky base influence the Hofmann vs Zaitsev outcome?

• Bulky base favors Zaitsev product
• Bulky base favors Hofmann (less substituted) product
• Bulky base converts reaction to SN1
• Bulky base prevents any elimination

Correct Answer: Bulky base favors Hofmann (less substituted) product

Q28. Which experimental observation supports a stepwise E1 mechanism?

• Rate increases with base concentration
• Rate shows first-order dependence on substrate only
• Strict stereospecific anti elimination with base present
• Immediate concerted proton abstraction

Correct Answer: Rate shows first-order dependence on substrate only

Q29. What role does neighboring group participation (anchimeric assistance) play in eliminations?

• It never affects elimination rates
• It can accelerate reactions by stabilizing intermediates or transition states
• It always directs reactions to radical pathways
• It prevents carbocation formation entirely

Correct Answer: It can accelerate reactions by stabilizing intermediates or transition states

Q30. For E2 on a substituted cyclohexane, what conformation often must be adopted for elimination?

• Equatorial–equatorial H alignment
• Any conformation works equally
• Axial hydrogen and axial leaving group (trans-diaxial)
• Shoe conformation with syn hydrogen only

Correct Answer: Axial hydrogen and axial leaving group (trans-diaxial)

Q31. Which reagent pair is likely to favor E2 over SN2 on a secondary bromide?

• Sodium azide in DMF
• Potassium tert-butoxide in tert-butanol
• Sodium iodide in acetone
• Sodium methoxide in methanol at low temperature

Correct Answer: Potassium tert-butoxide in tert-butanol

Q32. Which statement about kinetics of E1 and E2 is correct?

• E1 is bimolecular, E2 is unimolecular
• E1 is unimolecular, E2 is bimolecular
• Both are termolecular
• Both have zero-order kinetics

Correct Answer: E1 is unimolecular, E2 is bimolecular

Q33. Which halide among the following will most rapidly undergo E1 due to resonance stabilization?

• Allyl chloride
• Primary alkyl chloride
• Vinyl chloride
• Alkyl fluoride

Correct Answer: Allyl chloride

Q34. During E2, which hydrogen is usually abstracted to give the more stable alkene?

• Hydrogen leading to less substituted alkene
• Hydrogen leading to more substituted, conjugated alkene
• Only hydrogens on methyl groups
• Any hydrogen gives equal products

Correct Answer: Hydrogen leading to more substituted, conjugated alkene

Q35. Which experimental change will slow down an E2 reaction?

• Using a stronger base
• Switching to a poorer leaving group
• Raising temperature
• Using polar aprotic solvent

Correct Answer: Switching to a poorer leaving group

Q36. In predicting mechanism for 2-bromo-2-methylbutane with NaOEt in ethanol, which is expected?

• SN2 due to primary substrate
• E1 predominating because NaOEt is weak
• Mixture with significant E2 product because base is strong and substrate is tertiary
• No reaction occurs

Correct Answer: Mixture with significant E2 product because base is strong and substrate is tertiary

Q37. Which design principle helps medicinal chemists avoid unwanted eliminations during synthesis?

• Use very bulky leaving groups to speed elimination
• Choose reaction conditions with strong bases and high temperature
• Protect β-hydrogens or use milder nucleophiles to minimize elimination
• Always use polar protic solvents

Correct Answer: Protect β-hydrogens or use milder nucleophiles to minimize elimination

Q38. How does conjugation affect the stability of alkenes formed by elimination?

• Conjugation decreases alkene stability
• Conjugation increases alkene stability and favors formation
• Conjugation has no effect on stability
• Conjugation always favors the less substituted alkene

Correct Answer: Conjugation increases alkene stability and favors formation

Q39. Which reagent combination is most likely to give an E1 elimination product from a secondary bromide?

• Weak base in polar protic solvent at elevated temperature
• Strong base in polar aprotic solvent at low temperature
• Bulky base at very low temperature
• Strong nucleophile in polar aprotic solvent

Correct Answer: Weak base in polar protic solvent at elevated temperature

Q40. Which observation distinguishes E2 elimination from E1 by product stereochemistry?

• E2 often shows stereospecificity requiring anti-periplanar alignment
• E2 products always rearrange
• E2 yields racemic carbocations
• E2 is not influenced by substrate stereochemistry

Correct Answer: E2 often shows stereospecificity requiring anti-periplanar alignment

Q41. Which alkyl halide would you predict to follow E1 most readily: neopentyl bromide, benzyl chloride, or tert-butyl chloride?

• Neopentyl bromide
• Benzyl chloride
• Tert-butyl chloride
• None of them

Correct Answer: Tert-butyl chloride

Q42. What is the expected major product when 2-chlorobutane reacts with strong base NaOH in ethanol at room temperature?

• SN2 substitution giving 2-ethoxybutane
• E2 elimination giving 2-butene (mixture of E/Z, more trans)
• Formation of tertiary alcohol
• No reaction

Correct Answer: E2 elimination giving 2-butene (mixture of E/Z, more trans)

Q43. For which substrate does hyperconjugation most stabilize the carbocation intermediate in E1?

• Methyl carbocation
• Primary carbocation
• Secondary carbocation
• Tertiary carbocation

Correct Answer: Tertiary carbocation

Q44. Which of the following is true about the regioselectivity of E2 with an anti-periplanar requirement?

• Any β-hydrogen can be abstracted equally
• Only hydrogens that are anti-periplanar to leaving group are abstracted leading to specific alkene isomers
• Regioselectivity is independent of conformation
• Anti-periplanar requirement applies only to E1

Correct Answer: Only hydrogens that are anti-periplanar to leaving group are abstracted leading to specific alkene isomers

Q45. Which experimental variable would favor E2 formation of a less substituted alkene (Hofmann product)?

• Use of small, unhindered base like methoxide
• Use of bulky base like tert-butoxide
• Lowering the temperature dramatically
• Using a highly polar protic solvent

Correct Answer: Use of bulky base like tert-butoxide

Q46. Why might a benzylic halide undergo both E1 and E2 readily?

• Because benzylic carbocations are destabilized
• Because benzylic position lacks β-hydrogens
• Because resonance stabilizes carbocations and transition states, facilitating both pathways
• Because benzylic halides are unreactive

Correct Answer: Because resonance stabilizes carbocations and transition states, facilitating both pathways

Q47. Which analytical observation supports formation of a carbocation intermediate during E1?

• Stereospecific inversion at reaction center
• Formation of racemic mixture from a chiral center
• Strict anti elimination geometry
• Second-order rate law

Correct Answer: Formation of racemic mixture from a chiral center

Q48. When comparing reactivity trends, which order is correct for E1 reactivity of alkyl halides?

• Methyl > primary > secondary > tertiary
• Tertiary > secondary > primary > methyl
• Primary > tertiary > secondary > methyl
• All equal

Correct Answer: Tertiary > secondary > primary > methyl

Q49. Which structural feature most disfavors both E1 and E2 eliminations?

• Presence of adjacent π-system (allylic)
• Absence of β-hydrogens
• Good leaving group
• Polar protic solvent

Correct Answer: Absence of β-hydrogens

Q50. For a pharmaceutical synthesis requiring selective formation of the more substituted alkene, which conditions are preferred?

• Bulky base, low temperature
• Small strong base in polar aprotic solvent, thermodynamic control (higher temperature if needed)
• Very weak base in nonpolar solvent
• No base and radical initiator

Correct Answer: Small strong base in polar aprotic solvent, thermodynamic control (higher temperature if needed)

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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