Factors affecting SN1 and SN2 reactions MCQs With Answer

Introduction: Understanding the factors affecting SN1 and SN2 reactions is essential for B. Pharm students studying organic reaction mechanisms and medicinal chemistry. This concise guide emphasizes how substrate structure, nucleophile strength, leaving group ability, solvent type, steric hindrance, and carbocation stability govern whether a nucleophilic substitution proceeds via SN1 or SN2. Grasping kinetics, stereochemical outcomes (inversion vs racemization), and experimental conditions helps predict reaction pathways and design safer, more effective drug syntheses. These MCQs with focused explanations will reinforce core concepts and exam-ready knowledge. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which substrate favors an SN2 reaction the most?

• Primary alkyl halide with little steric hindrance
• Tertiary alkyl halide
• Benzyl halide
• Allylic tertiary halide

Correct Answer: Primary alkyl halide with little steric hindrance

Q2. Which factor most increases the rate of an SN1 reaction?

• Better leaving group
• Stronger nucleophile
• Less polar solvent
• Decreased carbocation stability

Correct Answer: Better leaving group

Q3. What is the kinetic order for an SN2 reaction?

• First order — depends only on substrate
• Second order — depends on substrate and nucleophile
• Zero order — independent of concentration
• Third order — complex dependence

Correct Answer: Second order — depends on substrate and nucleophile

Q4. In which solvent do SN2 reactions prefer to occur?

• Polar protic solvents like water and alcohols
• Polar aprotic solvents like DMSO and DMF
• Nonpolar solvents like hexane
• Strongly acidic solvents

Correct Answer: Polar aprotic solvents like DMSO and DMF

Q5. Which leaving group is the best for both SN1 and SN2?

• Fluoride
• Chloride
• Bromide
• Tosylate

Correct Answer: Tosylate

Q6. Which substrate is most likely to undergo SN1 due to resonance-stabilized carbocation?

• Benzyl chloride
• Primary alkyl chloride
• Vinyl chloride
• Allylic tertiary chloride

Correct Answer: Benzyl chloride

Q7. What stereochemical outcome is typical for SN2 at a chiral center?

• Retention of configuration
• Inversion of configuration
• Racemization
• No change in stereochemistry

Correct Answer: Inversion of configuration

Q8. Which nucleophile property most accelerates SN2?

• High steric bulk
• High nucleophilicity and low steric hindrance
• Weak base and poor nucleophile
• Large, polarizable anions in protic solvent

Correct Answer: High nucleophilicity and low steric hindrance

Q9. Which substrate will favor SN1 due to tertiary carbocation formation?

• Primary alcohol
• Secondary bromide
• Tertiary chloride
• Methyl iodide

Correct Answer: Tertiary chloride

Q10. How does a polar protic solvent affect SN1 reactions?

• Stabilizes carbocations and promotes SN1
• Destabilizes carbocations and inhibits SN1
• Preferentially accelerates SN2
• Has no effect on reaction pathway

Correct Answer: Stabilizes carbocations and promotes SN1

Q11. Which nucleophile is strongest in polar aprotic solvent for SN2?

• Fluoride ion
• Bromide ion
• Water
• Ammonia

Correct Answer: Fluoride ion

Q12. What effect does increasing temperature generally have on SN1 vs SN2?

• Favors SN1 because of entropy increase
• Always favors SN2 over SN1
• Decreases rates of both reactions
• No predictable effect

Correct Answer: Favors SN1 because of entropy increase

Q13. Which halide is the best leaving group in nucleophilic substitution?

• Fluoride
• Chloride
• Bromide
• Iodide

Correct Answer: Iodide

Q14. Which substrate will undergo SN2 fastest?

• Methyl bromide
• Primary bromide with bulky substituents
• Secondary bromide
• Tertiary bromide

Correct Answer: Methyl bromide

Q15. Neighboring group participation often leads to which effect in substitution?

• Slower reaction rate
• Formation of bridged intermediates and altered stereochemistry
• No change in mechanism
• Preventing carbocation formation

Correct Answer: Formation of bridged intermediates and altered stereochemistry

Q16. Which describes SN1 reaction kinetics?

• Second order overall
• First order, depends only on substrate concentration
• Zero order under all conditions
• Depends only on nucleophile

Correct Answer: First order, depends only on substrate concentration

Q17. In SN2, how does steric hindrance of the substrate affect rate?

• Increases rate by stabilizing transition state
• Decreases rate by blocking nucleophile approach
• No effect
• Only affects SN1

Correct Answer: Decreases rate by blocking nucleophile approach

Q18. Which phenomenon can cause racemization in an SN1 reaction?

• Concerted backside attack
• Planar carbocation intermediate attacked from both sides
• Sterically hindered nucleophile causing inversion
• Strong polar aprotic solvent

Correct Answer: Planar carbocation intermediate attacked from both sides

Q19. Which substituent increases carbocation stability most by hyperconjugation?

• Methyl group
• Hydrogen
• Fluorine
• Alcohol group

Correct Answer: Methyl group

Q20. How does a good nucleophile but weak base (e.g., I-) influence substitution?

• Favors E2 elimination
• Favors SN2 substitution
• Favors SN1 exclusively
• Inhibits all reactions

Correct Answer: Favors SN2 substitution

Q21. Which structural feature of allylic substrates affects substitution?

• Lack of resonance stabilization
• Resonance-stabilized carbocation favoring SN1 and SN2 reactivity
• Impossible to substitute
• Always undergo elimination

Correct Answer: Resonance-stabilized carbocation favoring SN1 and SN2 reactivity

Q22. What is the role of counterions (e.g., Na+, K+) in SN2 reactions in aprotic solvent?

• They always block nucleophiles
• Smaller cations can tightly solvate nucleophiles and reduce reactivity
• No role at all
• They convert SN2 to SN1

Correct Answer: Smaller cations can tightly solvate nucleophiles and reduce reactivity

Q23. Which factor disfavors SN2 but may favor SN1?

• Strong unhindered nucleophile
• Tertiary substrate with good leaving group
• Polar aprotic solvent
• Methyl substrate

Correct Answer: Tertiary substrate with good leaving group

Q24. How does resonance delocalization adjacent to the leaving group affect SN1?

• Destabilizes carbocation and slows SN1
• Stabilizes carbocation and accelerates SN1
• Makes SN2 faster only
• No effect on either mechanism

Correct Answer: Stabilizes carbocation and accelerates SN1

Q25. Which experimental observation indicates an SN1 mechanism?

• Rate depends on nucleophile concentration
• Rate depends only on substrate concentration and formation of intermediate
• Complete stereospecific inversion of configuration
• No intermediates detected

Correct Answer: Rate depends only on substrate concentration and formation of intermediate

Q26. In SN2, what is the geometry of the transition state?

• Planar carbocation
• Pentacoordinate, partially bonded trigonal bipyramidal-like arrangement
• Free radical intermediate
• Tetrahedral carbocation

Correct Answer: Pentacoordinate, partially bonded trigonal bipyramidal-like arrangement

Q27. How does increasing the concentration of a strong nucleophile affect SN2 rate?

• Rate decreases
• Rate increases proportionally
• No change in rate
• Converts reaction to SN1

Correct Answer: Rate increases proportionally

Q28. Which halogenated substrate is least reactive toward SN2?

• Methyl iodide
• Primary chloride
• Secondary bromide with bulky substituents
• Tertiary chloride

Correct Answer: Tertiary chloride

Q29. Which is a hallmark of SN1 in terms of energy profile?

• Single-step concerted transition state
• Two-step process with high-energy carbocation intermediate
• No energy barrier
• Barrierless radical chain

Correct Answer: Two-step process with high-energy carbocation intermediate

Q30. How does the presence of a neighboring resonance-stabilizing group (e.g., phenyl) affect SN2?

• Always accelerates SN2
• Can slow SN2 due to steric or electronic effects, but may stabilize carbocation favoring SN1
• No effect
• Makes substrate inert

Correct Answer: Can slow SN2 due to steric or electronic effects, but may stabilize carbocation favoring SN1

Q31. Which reagent-solvent combination is ideal for promoting SN2 reaction of a secondary halide?

• Bulky base in polar protic solvent
• Small, strong nucleophile in polar aprotic solvent
• Weak nucleophile in aqueous solution
• Heat without nucleophile

Correct Answer: Small, strong nucleophile in polar aprotic solvent

Q32. What does the term “backside attack” describe in SN2?

• Nucleophile attacking the same side as leaving group
• Nucleophile attacking opposite side causing inversion
• Nucleophile abstracting a proton
• Formation of a carbocation

Correct Answer: Nucleophile attacking opposite side causing inversion

Q33. Which of the following promotes SN1 over SN2 for a given substrate?

• Using a strong, charged nucleophile in aprotic solvent
• Using a polar protic solvent and weak nucleophile
• Lowering temperature drastically
• Using a very hindered nucleophile in aprotic solvent

Correct Answer: Using a polar protic solvent and weak nucleophile

Q34. How does polarizability of a nucleophile affect SN2 reactions?

• Less polarizable nucleophiles are always better
• More polarizable (softer) nucleophiles are often more reactive in protic solvents
• Polarizability has no effect
• Only affects SN1

Correct Answer: More polarizable (softer) nucleophiles are often more reactive in protic solvents

Q35. Which evidence supports an SN2 mechanism experimentally?

• Observation of carbocation by spectroscopy
• Rate increases with nucleophile concentration and inversion of configuration
• Racemization of chiral center
• Reaction proceeds in absence of nucleophile

Correct Answer: Rate increases with nucleophile concentration and inversion of configuration

Q36. Why is fluoride a poor nucleophile in protic solvents compared to aprotic solvents?

• Because fluoride is too large
• Because strong solvation by protic solvents lowers its nucleophilicity
• It forms a stable carbocation
• It is always the best nucleophile regardless of solvent

Correct Answer: Because strong solvation by protic solvents lowers its nucleophilicity

Q37. Which of the following substituents on carbon adjacent to leaving group increases SN1 rate?

• Electron-donating alkyl groups
• Strong electron-withdrawing groups
• Very bulky electron-withdrawing group that destabilizes carbocation
• Nonpolar inert substituents only

Correct Answer: Electron-donating alkyl groups

Q38. What role can a solvent’s dielectric constant play in SN1?

• Low dielectric constant stabilizes ions and favors SN1
• High dielectric constant stabilizes separated charges and favors SN1
• Dielectric constant only affects SN2
• No role in reaction mechanisms

Correct Answer: High dielectric constant stabilizes separated charges and favors SN1

Q39. Which of the following is true about neighboring group participation (anchimeric assistance)?

• It always prevents substitution
• It can accelerate reaction by forming a cyclic intermediate
• It only occurs in SN2 and never in SN1
• It is irrelevant for drug synthesis

Correct Answer: It can accelerate reaction by forming a cyclic intermediate

Q40. How does a bulky base like tert-butoxide influence substitution on a secondary substrate?

• Favors SN2 substitution exclusively
• Favors E2 elimination due to steric hindrance
• Promotes SN1 by stabilizing carbocation
• No reaction occurs

Correct Answer: Favors E2 elimination due to steric hindrance

Q41. Which observation suggests an SN1 intermediate rearrangement has occurred?

• Product retains original carbon skeleton strictly
• Product shows rearranged carbon skeleton like hydride or alkyl shift
• Only substitution at the same carbon without migration
• Complete stereochemical inversion only

Correct Answer: Product shows rearranged carbon skeleton like hydride or alkyl shift

Q42. For drug synthesis, why is understanding SN1 vs SN2 important?

• It determines toxicology only
• It helps predict stereochemistry, reaction conditions, and impurity formation
• Mechanisms are irrelevant in real synthesis
• Only applies to gas-phase reactions

Correct Answer: It helps predict stereochemistry, reaction conditions, and impurity formation

Q43. Which condition increases likelihood of SN2 in biological systems?

• High water concentration and protic media
• Active site that orients nucleophile for backside attack and lowers steric hindrance
• Formation of stable carbocation intermediate
• Random collisions without orientation

Correct Answer: Active site that orients nucleophile for backside attack and lowers steric hindrance

Q44. Which electronic effect on the substrate will slow down SN1?

• Electron-donating groups adjacent to leaving group
• Electron-withdrawing groups that destabilize carbocation
• Resonance stabilization of carbocation
• Hyperconjugation from alkyl groups

Correct Answer: Electron-withdrawing groups that destabilize carbocation

Q45. What is “solvolysis” in the context of SN1 reactions?

• Substitution where the solvent acts as the nucleophile
• Substitution only with sulfide nucleophiles
• Elimination reaction in nonpolar solvents
• SN2 with added salt

Correct Answer: Substitution where the solvent acts as the nucleophile

Q46. Which method can increase nucleophilicity of an anion in aprotic solvents?

• Strong solvation with hydrogen-bond donors
• Adding crown ethers to complex cations and free the anion
• Lowering temperature to freeze solvation shells
• Using very polar protic solvents

Correct Answer: Adding crown ethers to complex cations and free the anion

Q47. Which structural feature makes an alkyl halide unreactive in SN2 but reactive in SN1?

• Methyl carbon
• Tertiary carbon bearing leaving group
• Primary carbon with no hindrance
• Terminal vinyl halide

Correct Answer: Tertiary carbon bearing leaving group

Q48. Why are vinyl and aryl halides generally unreactive toward SN1 and SN2?

• They form very stable carbocations easily
• The C–X bond has partial double bond character and cannot form carbocations or undergo backside attack easily
• They rapidly undergo SN2 under all conditions
• They are highly polarized and reactive

Correct Answer: The C–X bond has partial double bond character and cannot form carbocations or undergo backside attack easily

Q49. Which experimental technique can help distinguish SN1 from SN2?

• Measuring rate dependence on nucleophile concentration
• Infrared only
• Thin layer chromatography only
• Measuring color change only

Correct Answer: Measuring rate dependence on nucleophile concentration

Q50. Which best summarizes the main difference between SN1 and SN2?

• SN1 is concerted, SN2 is stepwise
• SN1 proceeds via carbocation intermediate (first order); SN2 is concerted bimolecular (second order)
• SN1 always gives inversion, SN2 gives racemization
• SN2 depends only on solvent and not nucleophile

Correct Answer: SN1 proceeds via carbocation intermediate (first order); SN2 is concerted bimolecular (second order)

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