Order of reactivity of alkyl halides (SN1/SN2) MCQs With Answer

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Order of reactivity of alkyl halides (SN1/SN2) MCQs With Answer — This concise guide is tailored for B.Pharm students preparing for exams and practicals. It explains how factors such as substrate structure, leaving group ability, nucleophile strength, solvent polarity, steric hindrance, resonance stabilization and carbocation stability determine the order of reactivity in SN1 and SN2 reactions. Keywords: order of reactivity, alkyl halides, SN1, SN2, nucleophilic substitution, B.Pharm, leaving group, solvent effects. The MCQs emphasize kinetics, mechanism distinctions, stereochemistry and real-world examples to sharpen problem-solving skills. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which substrate shows the highest rate in an SN2 reaction?

  • Methyl halide (CH3X)
  • Primary alkyl halide
  • Secondary alkyl halide
  • Tertiary alkyl halide

Correct Answer: Methyl halide (CH3X)

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

  • Strong nucleophile
  • Nonpolar aprotic solvent
  • Stabilization of the carbocation intermediate
  • Less steric hindrance at electrophilic carbon

Correct Answer: Stabilization of the carbocation intermediate

Q3. Order the leaving group ability for halides in protic solvents (best to worst).

  • I− > Br− > Cl− > F−
  • F− > Cl− > Br− > I−
  • Cl− > Br− > I− > F−
  • Br− > I− > Cl− > F−

Correct Answer: I− > Br− > Cl− > F−

Q4. Which solvent favors SN2 reactions most strongly?

  • Polar protic (e.g., water)
  • Polar aprotic (e.g., DMSO)
  • Nonpolar (e.g., hexane)
  • Strongly acidic solvent

Correct Answer: Polar aprotic (e.g., DMSO)

Q5. For SN1 reactions, the rate law is:

  • Rate = k[substrate][nucleophile]
  • Rate = k[substrate]
  • Rate = k[nucleophile]
  • Rate = k[substrate]^2

Correct Answer: Rate = k[substrate]

Q6. Which substrate is most reactive in SN1?

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

Correct Answer: Tertiary alkyl halide

Q7. Which statement about SN2 stereochemistry is correct?

  • Retention of configuration
  • Racemization
  • Inversion of configuration (Walden inversion)
  • No stereochemical change

Correct Answer: Inversion of configuration (Walden inversion)

Q8. Benzylic and allylic halides are unusually reactive because:

  • They are less sterically hindered
  • They form stabilized carbocations by resonance
  • They have poorer leaving groups
  • They are always inert

Correct Answer: They form stabilized carbocations by resonance

Q9. A strong nucleophile and polar aprotic solvent best promote which pathway?

  • SN1
  • SN2
  • Radical substitution
  • Electrophilic addition

Correct Answer: SN2

Q10. Which change will decrease SN2 rate most significantly?

  • Changing leaving group from Cl to I
  • Replacing a methyl with a tertiary center
  • Switching solvent to DMSO
  • Using a stronger nucleophile

Correct Answer: Replacing a methyl with a tertiary center

Q11. In SN1 reactions, which phenomenon often accompanies formation of the carbocation?

  • Nucleophile attack before ionization
  • Carbocation rearrangement
  • Concerted backside attack
  • Elimination without ionization

Correct Answer: Carbocation rearrangement

Q12. Which halide will show the fastest SN2 reaction with a strong nucleophile?

  • 1-chlorobutane
  • 1-bromobutane
  • 1-iodobutane
  • 1-fluorobutane

Correct Answer: 1-iodobutane

Q13. Which factor does NOT significantly affect the SN1 rate?

  • Substrate structure
  • Carbocation stability
  • Nucleophile strength
  • Solvent polarity

Correct Answer: Nucleophile strength

Q14. Which description fits an SN2 transition state?

  • Planar carbocation
  • Pentacoordinate, partially bonded carbon
  • Radical intermediate
  • Free anion intermediate

Correct Answer: Pentacoordinate, partially bonded carbon

Q15. Neighboring group participation accelerates which mechanism?

  • SN2 only
  • SN1 only
  • Both SN1 and SN2
  • Neither SN1 nor SN2

Correct Answer: Both SN1 and SN2

Q16. Which nucleophile will react fastest in SN2 with methyl bromide?

  • I−
  • Br−
  • OH−
  • NH3

Correct Answer: OH−

Q17. How does increased steric hindrance at the electrophilic carbon affect SN2?

  • Increases rate
  • Has no effect
  • Decreases rate
  • Makes reaction SN1 exclusively

Correct Answer: Decreases rate

Q18. In SN1, which solvent property stabilizes transition state and carbocation?

  • Low dielectric constant
  • High dielectric constant (polar)
  • Strongly basic
  • Nonpolar

Correct Answer: High dielectric constant (polar)

Q19. Which halogen substituent most favors SN1 over SN2 on the same tertiary carbon?

  • Fluoride
  • Chloride
  • Bromide
  • Iodide

Correct Answer: Iodide

Q20. Which experimental observation indicates an SN1 mechanism?

  • Rate depends on nucleophile concentration
  • Inversion of configuration exclusively
  • Racemization of a chiral center
  • No effect of solvent on rate

Correct Answer: Racemization of a chiral center

Q21. For a secondary alkyl halide, which condition favors SN2 over SN1?

  • Polar protic solvent and weak nucleophile
  • Polar aprotic solvent and strong nucleophile
  • Highly stabilizing solvent for carbocation
  • Elevated temperature favoring elimination

Correct Answer: Polar aprotic solvent and strong nucleophile

Q22. Which statement about leaving groups is true?

  • Better leaving groups are strong bases
  • Poor leaving groups leave faster
  • Weaker bases make better leaving groups
  • Leaving group ability is irrelevant

Correct Answer: Weaker bases make better leaving groups

Q23. The Hammond postulate helps predict:

  • Which nucleophile is strongest
  • Relative structure of transition states
  • Exact reaction rates numerically
  • Product solubility

Correct Answer: Relative structure of transition states

Q24. In competition experiments, SN2 is favored when:

  • Weak nucleophile and polar protic solvent
  • Sterically hindered nucleophile
  • Strong nucleophile in polar aprotic solvent
  • Substrate forms a very stable carbocation

Correct Answer: Strong nucleophile in polar aprotic solvent

Q25. Which substrate will undergo the fastest SN1 reaction: neopentyl, tert-butyl, benzylic, or methyl?

  • Neopentyl
  • Tert-butyl
  • Benzylic
  • Methyl

Correct Answer: Benzylic

Q26. Which is a common consequence of SN1 that reduces stereochemical purity?

  • Complete retention
  • Optical purity increase
  • Partial or full racemization
  • New chiral centers created

Correct Answer: Partial or full racemization

Q27. Which factor makes a tertiary halide unreactive in SN2 but reactive in SN1?

  • Good leaving group
  • High nucleophile concentration
  • Severe steric hindrance blocking backside attack
  • Polar aprotic solvent

Correct Answer: Severe steric hindrance blocking backside attack

Q28. Allylic halides often react faster because:

  • They are less polar
  • They form resonance-stabilized cations and transition states
  • They never undergo SN1
  • They have worse leaving groups

Correct Answer: They form resonance-stabilized cations and transition states

Q29. Which experimental technique helps distinguish SN1 from SN2?

  • Measuring reaction order (kinetics)
  • Measuring melting point
  • Checking color only
  • Thin layer chromatography in all cases

Correct Answer: Measuring reaction order (kinetics)

Q30. How does an increase in temperature generally affect competition between substitution and elimination?

  • Favors substitution over elimination
  • Has no effect
  • Favors elimination due to higher activation energy of elimination
  • Always stops both reactions

Correct Answer: Favors elimination due to higher activation energy of elimination

Q31. Which substituent on a benzyl halide increases SN1 reactivity most?

  • Electron-withdrawing nitro group at benzylic position
  • Electron-donating methoxy group on aromatic ring
  • Bulky tert-butyl substituent on ring
  • Fluorine on benzylic carbon

Correct Answer: Electron-donating methoxy group on aromatic ring

Q32. In an SN2 reaction, nucleophile strength correlates most closely with:

  • Pka of conjugate acid (higher pKa = stronger nucleophile)
  • Molecular weight
  • Color
  • Boiling point

Correct Answer: Pka of conjugate acid (higher pKa = stronger nucleophile)

Q33. Which substrate is most likely to undergo intramolecular SN2 faster than intermolecular SN2?

  • Long-chain primary halide
  • Vicinal dihalide that forms 5-membered ring
  • Methyl halide
  • Tertiary halide

Correct Answer: Vicinal dihalide that forms 5-membered ring

Q34. Which halide is least likely to undergo SN1 in aqueous solution?

  • tert-Butyl chloride
  • Benzyl bromide
  • Methyl chloride
  • Isopropyl bromide

Correct Answer: Methyl chloride

Q35. Which reagent pairing favors E2 over SN2 for a secondary halide?

  • Weak nucleophile, polar protic solvent
  • Strong bulky base like t-BuOK in tert-butanol
  • Weak base, strong nucleophile in DMSO
  • Mild nucleophile at low temperature

Correct Answer: Strong bulky base like t-BuOK in tert-butanol

Q36. What role does resonance play in determining SN1 reactivity?

  • Resonance destabilizes carbocations
  • Resonance stabilizes carbocations, increasing SN1 rate
  • Resonance only affects nucleophiles
  • Resonance makes leaving groups worse

Correct Answer: Resonance stabilizes carbocations, increasing SN1 rate

Q37. Which describes concerted SN2 mechanism?

  • Two-step with cation intermediate
  • Simultaneous bond making and breaking in single step
  • Radical chain process
  • Involves carbocation rearrangement

Correct Answer: Simultaneous bond making and breaking in single step

Q38. For predicting reactivity, which is more important for SN1: leaving group ability or nucleophile strength?

  • Nucleophile strength is more important
  • Leaving group ability and stabilization of cation are most important
  • Neither matters
  • Only solvent matters

Correct Answer: Leaving group ability and stabilization of cation are most important

Q39. Which substrate would undergo fastest SN2 despite having a good leaving group?

  • tert-Butyl bromide
  • Isopropyl bromide
  • Ethyl bromide
  • Benzyl bromide with severe steric hindrance

Correct Answer: Ethyl bromide

Q40. What is a key kinetic difference between SN1 and SN2?

  • SN1 is bimolecular in rate-determining step
  • SN2 is unimolecular in rate-determining step
  • SN1 rate depends only on substrate concentration, SN2 depends on both substrate and nucleophile
  • Both have identical rate laws always

Correct Answer: SN1 rate depends only on substrate concentration, SN2 depends on both substrate and nucleophile

Q41. Which observation supports an SN2 pathway experimentally?

  • First-order kinetics in substrate only
  • Second-order kinetics with rate dependent on nucleophile concentration
  • Complete racemization
  • Carbocation rearrangement observed

Correct Answer: Second-order kinetics with rate dependent on nucleophile concentration

Q42. How does increasing nucleophile concentration affect SN1 reactions?

  • Increases rate linearly
  • Has little to no effect on rate-determining step
  • Decreases rate
  • Converts SN1 to SN2 automatically

Correct Answer: Has little to no effect on rate-determining step

Q43. Which is true about fluoride as a leaving group in SN2?

  • Fluoride is an excellent leaving group in protic solvents
  • Fluoride is a poor leaving group due to strong C–F bond and high basicity
  • Fluoride always speeds up SN2
  • Fluoride is the best leaving group among halides

Correct Answer: Fluoride is a poor leaving group due to strong C–F bond and high basicity

Q44. Which situation can lead to neighboring group participation accelerating substitution?

  • Presence of a nearby lone pair that forms a bridged intermediate
  • Absence of any neighboring heteroatoms
  • Weak leaving group only
  • Extremely low temperature preventing reaction

Correct Answer: Presence of a nearby lone pair that forms a bridged intermediate

Q45. Which explains why tertiary alkyl halides rarely undergo SN2?

  • They form very stable carbocations
  • Strong leaving groups prevent SN2
  • Steric hindrance blocks backside attack by nucleophiles
  • They are too polar

Correct Answer: Steric hindrance blocks backside attack by nucleophiles

Q46. Which experimental modification would increase SN2 rate of a primary bromide?

  • Use of polar protic solvent like ethanol
  • Switching to a weaker nucleophile
  • Use of polar aprotic solvent like acetone with a strong nucleophile
  • Increasing steric bulk around electrophilic carbon

Correct Answer: Use of polar aprotic solvent like acetone with a strong nucleophile

Q47. Which is a correct ranking of substrates for SN1 reactivity (most to least)?

  • Methyl > primary > secondary > tertiary
  • Tertiary > secondary > primary > methyl
  • Primary > tertiary > secondary > methyl
  • Secondary > methyl > primary > tertiary

Correct Answer: Tertiary > secondary > primary > methyl

Q48. Which type of halide is unique in being reactive in both SN1 and SN2 depending on conditions?

  • Methyl halide
  • Benzylic halide
  • Tertiary halide
  • Neopentyl halide

Correct Answer: Benzylic halide

Q49. Which is the main reason bromide is a better leaving group than chloride in nucleophilic substitution?

  • Bromide is less polarizable
  • Bromide is a stronger base
  • Bromide is more stable as an anion (weaker base) and more polarizable
  • Bromide forms stronger bonds to carbon

Correct Answer: Bromide is more stable as an anion (weaker base) and more polarizable

Q50. When designing a B.Pharm experiment to compare SN1/SN2 rates, which variable is most informative to change systematically?

  • Volume of reaction vessel only
  • Substrate structure (primary, secondary, tertiary) and solvent/nucleophile conditions
  • The color of substrate
  • Brand of glassware

Correct Answer: Substrate structure (primary, secondary, tertiary) and solvent/nucleophile conditions

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