E1 versus E2 reactions MCQs With Answer

E1 versus E2 reactions MCQs With Answer are essential for B. Pharm students preparing for organic chemistry and medicinal chemistry exams. This focused set covers mechanistic distinctions, rate laws, stereochemical requirements like anti-periplanar geometry, carbocation stability, factors influencing regiochemistry (Zaitsev vs Hofmann), solvent and base effects, leaving group trends, and common reagents such as t-BuOK and NaOEt. Each question is designed to deepen understanding of elimination mechanisms, predict major products, and recognize competing pathways like SN1/SN2 or E1cb. These targeted MCQs will boost problem-solving skills and exam readiness for pharmaceutical coursework. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which statement best differentiates an E2 reaction from an E1 reaction?

• E2 is concerted and bimolecular; E1 proceeds via a carbocation intermediate
• E2 proceeds via a carbocation intermediate; E1 is concerted
• E2 rate depends only on substrate; E1 rate depends on substrate and base
• E2 always gives a carbocation rearrangement

Correct Answer: E2 is concerted and bimolecular; E1 proceeds via a carbocation intermediate

Q2. The rate law for a typical E2 reaction is:

• rate = k[substrate]
• rate = k[substrate][base]
• rate = k[base]
• rate = k[substrate]^2

Correct Answer: rate = k[substrate][base]

Q3. Which substrate is most likely to undergo E2 rather than E1 under strong basic conditions?

• tert-butyl bromide (3°)
• 1-bromobutane (1°)
• 2-bromopropane (2°) in weak base
• neopentyl chloride

Correct Answer: 1-bromobutane (1°)

Q4. Anti-periplanar geometry is a requirement for which elimination mechanism?

• E1 only
• E2 only
• Both E1 and E2
• Neither E1 nor E2

Correct Answer: E2 only

Q5. A tertiary alkyl halide treated with methanol (a weak base, polar protic solvent) most likely follows which pathway?

• E2
• E1
• SN2
• E1cb

Correct Answer: E1

Q6. Which factor favors E1 over E2 for the same substrate?

• Strong, bulky base
• High temperature with polar protic solvent and weak base
• Low temperature and strong base
• Small, strong nucleophile in polar aprotic solvent

Correct Answer: High temperature with polar protic solvent and weak base

Q7. Which reagent commonly leads to Hofmann (less substituted) alkene predominance in eliminations?

• Sodium ethoxide (NaOEt)
• Pyridine
• Potassium tert-butoxide (t-BuOK)
• Sodium iodide (NaI)

Correct Answer: Potassium tert-butoxide (t-BuOK)

Q8. In E1 reactions, which intermediate determines product distribution and rearrangements?

• Concerted transition state
• Carbocation intermediate
• Carbanion intermediate
• Radical intermediate

Correct Answer: Carbocation intermediate

Q9. A substrate capable of forming a resonance-stabilized carbocation will favor which mechanism?

• E2
• E1
• SN2
• E1cb

Correct Answer: E1

Q10. Which solvent is likely to favor E2 reactions by stabilizing the base less and enhancing nucleophilicity?

• Water (polar protic)
• Methanol (polar protic)
• Formic acid

Correct Answer: Dimethyl sulfoxide (DMSO) (polar aprotic)

Q11. Which elimination mechanism is concerted and usually shows second-order kinetics?

• E1
• E2
• E1cb
• SN1

Correct Answer: E2

Q12. Which of the following best describes E1cb mechanism?

• Concerted proton loss and leaving group departure
• Carbocation formation followed by deprotonation
• Carbanion intermediate formed by deprotonation before leaving group departs
• Radical chain elimination

Correct Answer: Carbanion intermediate formed by deprotonation before leaving group departs

Q13. Which substrate is most prone to undergo E1 due to carbocation stabilization?

• 1-chlorobutane
• 2-chloro-2-methylpropane (tert-butyl chloride)
• 1-bromo-2-methylpropane
• 1-chloro-2,2-dimethylpropane (neopentyl chloride)

Correct Answer: 2-chloro-2-methylpropane (tert-butyl chloride)

Q14. In a reaction where strong base and high temperature are used with a secondary halide, which product is most expected?

• SN2 substitution product
• E2 elimination product (major)
• E1 elimination product with rearrangement
• No reaction

Correct Answer: E2 elimination product (major)

Q15. Which experimental observation suggests an E1 mechanism?

• Rate doubles when base concentration doubles
• Rate is independent of base concentration
• Requires strict anti-periplanar geometry
• Instantaneous reaction with very bulky bases

Correct Answer: Rate is independent of base concentration

Q16. Predict major alkene product from 2-bromobutane with NaOEt in ethanol (heat):

• 1-butene (less substituted)
• cis-2-butene only
• trans-2-butene (Zaitsev product)
• Butane (no elimination)

Correct Answer: trans-2-butene (Zaitsev product)

Q17. Which of the following increases the rate of E2 but does not favor E1?

• Better leaving group ability
• Stronger base concentration increase
• Increased carbocation stability
• Polar protic solvent

Correct Answer: Stronger base concentration increase

Q18. What stereochemical outcome is required for E2 elimination on a cyclohexane ring?

• Syn-coplanar H and leaving group
• Anti-periplanar (axial H and axial LG)
• Equatorial H and equatorial LG preferred
• No stereochemical requirement

Correct Answer: Anti-periplanar (axial H and axial LG)

Q19. Which leaving group improves both E1 and E2 reaction rates most effectively?

• Fluoride
• Hydroxide
• Bromide
• Amide

Correct Answer: Bromide

Q20. A secondary alkyl halide gives significant substitution at low temperature with a moderate base; at higher temperature elimination dominates. This change is best explained by:

• Kinetic control favoring SN2 at high temperature
• Thermodynamic control favoring elimination at higher temperature
• Change in leaving group ability with temperature
• Formation of radicals at high temperature

Correct Answer: Thermodynamic control favoring elimination at higher temperature

Q21. Which reagent is most likely to promote E2 over SN2 with a secondary substrate due to steric hindrance?

• NaI in acetone
• t-BuOK in tert-butanol
• NaCN in DMSO
• NaBr in water

Correct Answer: t-BuOK in tert-butanol

Q22. Which mechanistic feature is characteristic of E1 but not of E2?

• Concerted bond-breaking and bond-forming
• Formation of a planar carbocation intermediate
• Strict anti-periplanar transition state
• Bimolecular rate-determining step

Correct Answer: Formation of a planar carbocation intermediate

Q23. During dehydration of tertiary alcohol with concentrated H2SO4, the mechanism is typically:

• E2
• E1
• E1cb
• SN2

Correct Answer: E1

Q24. Which elimination gives a carbanion intermediate and is common for stabilized anions like β-dicarbonyl systems?

• E2
• E1
• E1cb
• SN1

Correct Answer: E1cb

Q25. Which observation supports a stepwise E1 mechanism rather than concerted E2?

• First-order dependence on substrate concentration
• Second-order dependence on substrate and base
• Strong stereospecific anti requirement
• No carbocation rearrangements observed

Correct Answer: First-order dependence on substrate concentration

Q26. A tertiary alkyl halide undergoes E1 and displays a rearranged alkene product. The rearrangement occurred because:

• SN2 intermediate caused inversion
• Carbocation intermediate rearranged to a more stable carbocation before elimination
• Concerted E2 transition state allowed bond shifts
• Base removed proton causing pericyclic rearrangement

Correct Answer: Carbocation intermediate rearranged to a more stable carbocation before elimination

Q27. Which factor disfavors E1 but has little effect on E2?

• Weak base
• Nonpolar solvent
• Good carbocation stabilization
• Primary substrate

Correct Answer: Primary substrate

Q28. When a deuterium isotope replaces a β-hydrogen in an elimination, a large kinetic isotope effect suggests:

• Leaving group departure is rate determining
• Proton (or deuteron) abstraction is involved in the rate-determining step
• Reaction proceeds by SN1
• Solvent attack is rate determining

Correct Answer: Proton (or deuteron) abstraction is involved in the rate-determining step

Q29. Which base would most strongly promote E2 over E1 for a secondary bromide?

• EtOH (ethanol)
• NEt3 (triethylamine)
• NaOEt (sodium ethoxide)
• Water

Correct Answer: NaOEt (sodium ethoxide)

Q30. For 2-chloro-2-methylbutane undergoing elimination, the major alkene formed under E1 conditions is predicted by:

• Hofmann rule (less substituted alkene)
• Zaitsev rule (more substituted alkene)
• Retention of configuration at carbon
• Formation of terminal alkyne

Correct Answer: Zaitsev rule (more substituted alkene)

Q31. Which experimental test can help distinguish E1 from E2 experimentally?

• Measure rate dependence on base concentration
• Change the temperature only
• Use a radical trap
• Replace bromide with fluoride

Correct Answer: Measure rate dependence on base concentration

Q32. A secondary alcohol is treated with thionyl chloride followed by DBU (a non-nucleophilic base). Which mechanism most likely gives the alkene?

• E1 (carbocation mediated)
• E2 (base-induced elimination)
• E1cb
• SN1 substitution

Correct Answer: E2 (base-induced elimination)

Q33. Which statement about E1 and solvent polarity is correct?

• E1 is accelerated by polar protic solvents due to carbocation stabilization
• E1 is accelerated by nonpolar solvents only
• E1 is unaffected by solvent polarity
• E1 is inhibited by polar protic solvents

Correct Answer: E1 is accelerated by polar protic solvents due to carbocation stabilization

Q34. When a bulky base is used on a hindered secondary substrate, the major product is often:

• Most substituted alkene (Zaitsev)
• Less substituted alkene (Hofmann)
• SN2 substitution product
• Alcohol via hydrolysis

Correct Answer: Less substituted alkene (Hofmann)

Q35. Which reagent pair would favor elimination by E2 in an aprotic solvent for a secondary bromide?

• NaI in acetone
• NaOEt in ethanol
• KOt-Bu in DMSO
• HCl in water

Correct Answer: KOt-Bu in DMSO

Q36. For a benzylic halide, which pathway is particularly facile and often competes with substitution?

• E2 only
• E1 via stabilized carbocation
• E1cb only
• Radical chain elimination exclusively

Correct Answer: E1 via stabilized carbocation

Q37. In an E2 reaction the transition state has partial bond formation and bond breaking. According to Hammond’s postulate, for a late transition state the reaction is likely:

• Highly exothermic
• Endothermic with transition state resembling products
• Unrelated to thermochemistry
• Always barrierless

Correct Answer: Endothermic with transition state resembling products

Q38. A base extracts a proton from the β-carbon while the leaving group departs simultaneously. This describes:

• E1
• E2
• E1cb
• SN1

Correct Answer: E2

Q39. Which substrate is least likely to undergo E2 due to inability to adopt anti-periplanar conformation?

• trans-1-bromo-2-methylcyclohexane when only syn is available
• 1-bromobutane
• 2-bromobutane open-chain
• tert-butyl bromide

Correct Answer: trans-1-bromo-2-methylcyclohexane when only syn is available

Q40. Which elimination mechanism can show significant carbocation rearrangement leading to unexpected products?

• E2
• E1
• E1cb
• Concerted pericyclic eliminations

Correct Answer: E1

Q41. Sodium hydride (NaH) often promotes which elimination type when used with an alkyl halide?

• E1
• E2
• SN1
• E1cb exclusively

Correct Answer: E2

Q42. Which of the following is true about E1cb compared to E1 and E2?

• E1cb never involves a carbanion
• E1cb proceeds via a carbanion before leaving group departs
• E1cb is concerted like E2
• E1cb requires strong acid

Correct Answer: E1cb proceeds via a carbanion before leaving group departs

Q43. An observed racemization at the chiral center adjacent to the leaving group suggests which intermediate?

• Concerted E2 with inversion
• Planar carbocation as in E1
• Carbanion intermediate only
• No reaction occurred

Correct Answer: Planar carbocation as in E1

Q44. Which of the following is a sign that SN2 competes with E2?

• Product exclusively alkenes
• Both substitution and elimination products formed, depending on nucleophile strength and sterics
• Only rearranged products observed
• Reaction rate independent of nucleophile or base

Correct Answer: Both substitution and elimination products formed, depending on nucleophile strength and sterics

Q45. When a poor leaving group like -OH must be eliminated, converting it to a better leaving group (e.g., tosylate) often shifts mechanism toward:

• E2 if strong base present; E1 if acid and heat used
• E1cb only
• SN2 exclusively
• No change in mechanism

Correct Answer: E2 if strong base present; E1 if acid and heat used

Q46. Which spectroscopic evidence can indicate formation of a carbocation intermediate during reaction?

• IR peak at 1700 cm^-1
• Observation of rearranged products typical of carbocation intermediates
• Sharp NMR singlet only
• No change in mass spectrum

Correct Answer: Observation of rearranged products typical of carbocation intermediates

Q47. For elimination leading to conjugated alkene systems (e.g., formation of styrene), which mechanism is often operative due to resonance stabilization?

• E2 only
• E1 via resonance-stabilized carbocation
• E1cb exclusively
• SN2 followed by isomerization

Correct Answer: E1 via resonance-stabilized carbocation

Q48. In a competition experiment, increasing the concentration of a weak base does not change the rate. This supports which mechanism?

• E2
• E1
• SN2
• E1cb

Correct Answer: E1

Q49. Which product outcome would you predict when 2-chloro-3-methylbutane reacts with bulky base at high temperature?

• Highly substituted internal alkene exclusively
• Less substituted terminal alkene (Hofmann major)
• SN2 substitution predominates
• Retention of chloride with no reaction

Correct Answer: Less substituted terminal alkene (Hofmann major)

Q50. Which rule helps predict the major alkene from elimination when multiple β-hydrogens are available under thermodynamic control?

• Markovnikov’s rule
• Zaitsev’s rule (more substituted alkene favored)
• Baldwin’s rule
• Hammond’s postulate exclusively

Correct Answer: Zaitsev’s rule (more substituted alkene favored)

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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