Elimination reactions: E1 and E2, Hoffman vs Saytzeff rules MCQs With Answer

Introduction

This blog presents a focused set of multiple-choice questions on elimination reactions — E1 and E2 mechanisms — and the competition between Hofmann and Saytzeff (Zaitsev) product formation, tailored for M.Pharm students studying MPC 102T Advanced Organic Chemistry I. Questions emphasize mechanistic details, rate laws, stereochemical requirements (anti-periplanar geometry), influence of bases, solvents and leaving groups, and special cases such as E1cB and rearrangements. Each MCQ tests applied understanding relevant to pharmaceutical chemistry (reaction conditions, product prediction, and mechanistic rationale). Answers are provided to help students evaluate their grasp of factors governing regioselectivity and stereoselectivity in elimination reactions.

Q1. Which rate law correctly describes a bimolecular E2 elimination reaction?

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

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

Q2. Which statement describes the rate-determining step in a classical E1 elimination?

• Concerted proton abstraction and leaving group departure
• Formation of a carbocation by loss of the leaving group
• Deprotonation at the β-carbon followed by leaving group departure
• Nucleophilic attack on the β-carbon

Correct Answer: Formation of a carbocation by loss of the leaving group

Q3. For an E2 elimination to occur efficiently, what geometric relationship between the β-hydrogen and leaving group is generally required?

• Synclinal orientation
• Anti-periplanar orientation
• Gauche conformation
• Perpendicular arrangement

Correct Answer: Anti-periplanar orientation

Q4. Which reaction condition most favors Hofmann (less substituted) alkene formation over Saytzeff (more substituted) product?

• Use of a small, non-bulky base
• Use of a bulky base such as t-BuOK
• Carbocation-stabilizing polar protic solvent
• High temperature with no base

Correct Answer: Use of a bulky base such as t-BuOK

Q5. Saytzeff (Zaitsev) rule predicts the major alkene product will be:

• The least substituted alkene
• The alkene with the most internal double bond (most substituted)
• An equal mixture of all possible alkenes
• The alkene formed by elimination from the α-carbon only

Correct Answer: The alkene with the most internal double bond (most substituted)

Q6. Which intermediate is characteristic of the E1 elimination mechanism?

• Carbanion intermediate
• Radical cation intermediate
• Carbocation intermediate
• Concerted transition state only, no intermediate

Correct Answer: Carbocation intermediate

Q7. Which substrate is most likely to undergo E2 elimination when treated with a strong base?

• Primary alkyl halide with a small nucleophile
• Primary alkyl halide with a bulky base
• Tertiary alkyl halide
• Methyl halide

Correct Answer: Tertiary alkyl halide

Q8. How does improving leaving-group ability (e.g., Br- vs F-) generally affect the rate of E2 and E1 reactions?

• Decreases both E1 and E2 rates
• Increases E1 rate but decreases E2 rate
• Increases both E1 and E2 rates
• No significant effect on either mechanism

Correct Answer: Increases both E1 and E2 rates

Q9. Which solvent type tends to favor E1 eliminations by stabilizing carbocations?

• Nonpolar aprotic solvents
• Polar aprotic solvents (e.g., DMSO)
• Polar protic solvents (e.g., ethanol, water)
• Supercritical CO2

Correct Answer: Polar protic solvents (e.g., ethanol, water)

Q10. What structural requirement must be met on the β-carbon for a β-elimination to give an alkene?

• Presence of at least one α-hydrogen
• Presence of at least one β-hydrogen
• Aromatic stabilization at the β-carbon
• Conjugation with an oxygen atom only

Correct Answer: Presence of at least one β-hydrogen

Q11. In cyclohexane substrates undergoing E2, which conformational arrangement of the leaving group and β-hydrogen is required for efficient elimination?

• Both equatorial positions
• Both axial positions (axial-axial anti-periplanar)
• One axial and one equatorial in a syn relationship
• No specific conformation is required

Correct Answer: Both axial positions (axial-axial anti-periplanar)

Q12. Which phenomenon commonly accompanies E1 eliminations but not E2?

• Concerted bond-making and bond-breaking
• Formation of a single stereospecific product
• Carbocation rearrangements such as hydride or alkyl shifts
• Requirement for anti-periplanar geometry

Correct Answer: Carbocation rearrangements such as hydride or alkyl shifts

Q13. The E1cB elimination mechanism is most likely when which combination is present?

• Good leaving group and non-acidic β-hydrogen
• Poor leaving group and an acidic β-hydrogen stabilized by electron-withdrawing groups
• Primary carbocation intermediate stabilized by resonance
• Bulky base and tertiary substrate

Correct Answer: Poor leaving group and an acidic β-hydrogen stabilized by electron-withdrawing groups

Q14. Treatment of 2-bromobutane with tert-butoxide (a bulky base) predominantly gives which alkene product?

• Only Zaitsev alkene
• Only Hofmann alkene
• Predominantly Hofmann (less substituted) alkene
• Predominantly Zaitsev (more substituted) alkene

Correct Answer: Predominantly Hofmann (less substituted) alkene

Q15. For disubstituted alkenes formed by elimination, which stereoisomer is usually thermodynamically favored?

• Z (cis) isomer
• E (trans) isomer
• Neither; equal amounts of E and Z form
• Cyclic alkenes are always favored over acyclic

Correct Answer: E (trans) isomer

Q16. Observation of a large primary kinetic isotope effect (kH/kD >>1) for an elimination reaction indicates:

• Leaving-group departure is rate-determining
• C–H bond cleavage is involved in the rate-determining step
• Reaction proceeds via a carbocation intermediate exclusively
• The reaction is diffusion controlled

Correct Answer: C–H bond cleavage is involved in the rate-determining step

Q17. Which practical transformation is commonly used to convert an alcohol into a good leaving group prior to elimination?

• Oxidation to a ketone
• Tosylation (formation of a tosylate)
• Hydrogenation of the alcohol
• Formation of an acetal

Correct Answer: Tosylation (formation of a tosylate)

Q18. Which statement correctly characterizes the E2 mechanism?

• It proceeds stepwise with a stable carbocation intermediate
• It is a concerted, single-step process involving simultaneous β-H deprotonation and leaving-group departure
• It requires initial formation of a carbanion followed by leaving-group loss
• It only occurs in polar protic solvents

Correct Answer: It is a concerted, single-step process involving simultaneous β-H deprotonation and leaving-group departure

Q19. Dehydrohalogenation of 2-bromo-2-methylbutane with ethanol as a weak base at elevated temperature most likely follows which pathway and gives which product preference?

• E2 pathway giving Hofmann product
• E1 pathway giving predominantly Saytzeff (Zaitsev) product
• SN2 substitution exclusively
• E1cB pathway giving a conjugated alkene only

Correct Answer: E1 pathway giving predominantly Saytzeff (Zaitsev) product

Q20. When 1-bromobutane is treated with sodium methoxide in methanol, which reaction pathway predominates under standard conditions?

• E1 elimination to give butene
• E2 elimination to give butene
• SN2 substitution to give methoxybutane
• Radical substitution

Correct Answer: SN2 substitution to give methoxybutane

Download