SN2 reaction – kinetics MCQs With Answer

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Introduction: SN2 reaction – kinetics MCQs With Answer is a focused practice resource tailored for B. Pharm students to master nucleophilic substitution mechanisms, reaction kinetics, and stereochemical outcomes. This concise guide emphasizes key keywords such as SN2 reaction, kinetics, rate law, nucleophile strength, leaving group ability, polar aprotic solvents, and transition state theory to boost exam readiness and online discoverability. Questions probe reaction order, molecularity, solvent effects, activation parameters (Arrhenius/Eyring), and factors influencing rate and selectivity—essential for pharmaceutical chemistry and drug synthesis understanding. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What best describes an SN2 reaction mechanism?

  • Bimolecular, concerted backside attack with simultaneous bond formation and bond breaking
  • Unimolecular, two-step mechanism via carbocation intermediate
  • Free radical chain mechanism with chain-propagating steps
  • Pericyclic rearrangement involving cyclic transition state

Correct Answer: Bimolecular, concerted backside attack with simultaneous bond formation and bond breaking

Q2. What is the rate law for a classic SN2 reaction?

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

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

Q3. What stereochemical outcome is typically observed in an SN2 reaction at a chiral center?

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

Correct Answer: Inversion of configuration (Walden inversion)

Q4. Which substrate type undergoes SN2 most rapidly?

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

Correct Answer: Methyl halide

Q5. How does increasing nucleophile strength affect the SN2 rate?

  • Decreases rate because of steric crowding
  • No effect on the rate
  • Increases rate because the nucleophile participates in the rate-determining step
  • Converts mechanism to SN1

Correct Answer: Increases rate because the nucleophile participates in the rate-determining step

Q6. Which leaving group among halides best promotes SN2?

  • Fluoride (F-)
  • Chloride (Cl-)
  • Bromide (Br-)
  • Iodide (I-)

Correct Answer: Iodide (I-)

Q7. Which solvent type generally accelerates SN2 reactions?

  • Polar protic solvents (e.g., water, alcohols)
  • Nonpolar solvents (e.g., hexane)
  • Polar aprotic solvents (e.g., acetone, DMF)
  • Supercritical CO2

Correct Answer: Polar aprotic solvents (e.g., acetone, DMF)

Q8. How is the SN2 transition state best described?

  • Stable carbocation intermediate
  • Concerted state with partial bonds to nucleophile and leaving group (pentacoordinate-like)
  • Radical pair with unpaired electrons
  • Cyclic aromatic intermediate

Correct Answer: Concerted state with partial bonds to nucleophile and leaving group (pentacoordinate-like)

Q9. Which energy profile characterizes an SN2 reaction?

  • Two-step profile with a stable intermediate
  • Single-step concerted profile with one transition state and no intermediate
  • Chain reaction profile with multiple initiation steps
  • Concerted pericyclic energy profile with aromatic transition state

Correct Answer: Single-step concerted profile with one transition state and no intermediate

Q10. What type of nucleophilic approach leads to SN2 displacement?

  • Front-side attack leading to retention
  • Backside attack leading to inversion
  • Side-on attack forming a radical
  • Top-side attack forming stable complex

Correct Answer: Backside attack leading to inversion

Q11. What is the molecularity of the rate-determining step in SN2?

  • Unimolecular (single species involvement)
  • Bimolecular (two species involved simultaneously)
  • Termolecular (three species involved)
  • Zero-order in all reactants

Correct Answer: Bimolecular (two species involved simultaneously)

Q12. In kinetics experiments, what are pseudo-first-order conditions for SN2?

  • When substrate is in large excess over nucleophile
  • When nucleophile concentration is held constant and in large excess
  • When both reactants are at equal low concentrations
  • When the reaction is run at very low temperature

Correct Answer: When nucleophile concentration is held constant and in large excess

Q13. Which of the following substrates is most reactive toward SN2 with iodide?

  • tert-Butyl bromide
  • Isopropyl chloride
  • n-Propyl bromide
  • Methyl bromide

Correct Answer: Methyl bromide

Q14. For halide leaving groups, what is the general leaving group ability order?

  • F- > Cl- > Br- > I-
  • Cl- > Br- > I- > F-
  • I- > Br- > Cl- > F-
  • Br- > I- > Cl- > F-

Correct Answer: I- > Br- > Cl- > F-

Q15. In a polar aprotic solvent, which halide ion is unusually a strong nucleophile?

  • Fluoride (F-)
  • Chloride (Cl-)
  • Bromide (Br-)
  • Iodide (I-)

Correct Answer: Fluoride (F-)

Q16. How does steric hindrance near the reaction center affect SN2 rate?

  • Increases rate by stabilizing transition state
  • Decreases rate by blocking backside attack
  • No effect on rate
  • Converts mechanism to radical substitution

Correct Answer: Decreases rate by blocking backside attack

Q17. What is the typical effect of raising temperature on an SN2 reaction rate?

  • Rate decreases due to solvation changes
  • Rate increases according to the Arrhenius equation
  • Rate becomes independent of concentration
  • Reaction changes from SN2 to SN1

Correct Answer: Rate increases according to the Arrhenius equation

Q18. Which halide is the poorest leaving group in SN2 reactions?

  • Iodide (I-)
  • Bromide (Br-)
  • Chloride (Cl-)
  • Fluoride (F-)

Correct Answer: Fluoride (F-)

Q19. How can counterions influence SN2 kinetics?

  • They have no effect on reactivity
  • They can coordinate to the nucleophile and reduce its effective nucleophilicity
  • They always increase solubility and hence rate
  • They convert SN2 into SN1

Correct Answer: They can coordinate to the nucleophile and reduce its effective nucleophilicity

Q20. What is expected for a primary kinetic isotope effect (KIE) if C–H bond is not broken in the SN2 rate-determining step?

  • Large KIE (>6)
  • Moderate KIE (2–4)
  • Small or negligible primary KIE (~1)
  • Inverse KIE (<1)

Correct Answer: Small or negligible primary KIE (~1)

Q21. What does the Hammond postulate imply about the SN2 transition state?

  • Its structure resembles reactants or products closest in energy
  • It is always product-like regardless of reaction energy
  • It has no relation to reactant or product energies
  • It always resembles the reactants for substitution reactions

Correct Answer: Its structure resembles reactants or products closest in energy

Q22. Which experimental technique is commonly used to follow SN2 reaction kinetics in solution?

  • Infrared microscopy of solids
  • NMR spectroscopy monitoring concentration changes over time
  • X-ray crystallography of reaction mixture
  • Mass spectrometry of the entire solvent

Correct Answer: NMR spectroscopy monitoring concentration changes over time

Q23. What is the effect of neighboring group participation on SN2 reactions?

  • It always inhibits substitution
  • It can accelerate reaction by stabilizing transition state or forming bridged intermediates
  • It converts nucleophile into leaving group
  • It only affects radical reactions

Correct Answer: It can accelerate reaction by stabilizing transition state or forming bridged intermediates

Q24. Which condition favors E2 over SN2 for secondary substrates?

  • Use of a small, non-bulky nucleophile in polar aprotic solvent
  • Use of a bulky base at elevated temperature
  • Use of a very weak nucleophile and low temperature
  • Using dilute conditions with excess nucleophile

Correct Answer: Use of a bulky base at elevated temperature

Q25. SN2 reactions are described as stereospecific. What does this mean?

  • The reaction always gives a racemic mixture
  • The stereochemistry of the product depends on the stereochemistry of the substrate
  • Stereochemistry is lost because radicals form
  • The product configuration is random

Correct Answer: The stereochemistry of the product depends on the stereochemistry of the substrate

Q26. Why are tertiary alkyl halides poor substrates for SN2?

  • Because they form very stable carbocations
  • Because steric hindrance prevents effective backside attack by the nucleophile
  • Because they are too soluble in polar aprotic solvents
  • Because they have very good leaving groups

Correct Answer: Because steric hindrance prevents effective backside attack by the nucleophile

Q27. In SN2 kinetics, which step is rate-determining?

  • The formation of a carbocation intermediate
  • The single concerted bond-making and bond-breaking step (the only step)
  • Product solvation after substitution
  • A pre-equilibrium ion-pair formation

Correct Answer: The single concerted bond-making and bond-breaking step (the only step)

Q28. What is the order of reaction with respect to nucleophile in SN2?

  • Zero order
  • First order
  • Second order
  • Third order

Correct Answer: First order

Q29. How do polar protic solvents influence SN2 reactions?

  • They increase nucleophilicity by stabilizing cations
  • They decrease nucleophilicity by hydrogen-bonding to nucleophiles and slow SN2
  • They have no effect on SN2 rates
  • They always favor SN2 over E2

Correct Answer: They decrease nucleophilicity by hydrogen-bonding to nucleophiles and slow SN2

Q30. In the SN2 transition state, what is the distribution of partial charges?

  • Partial negative charge on nucleophile and leaving group, partial positive on carbon
  • Partial positive on nucleophile and leaving group, partial negative on carbon
  • No partial charges develop in the transition state
  • Only the leaving group carries partial positive charge

Correct Answer: Partial negative charge on nucleophile and leaving group, partial positive on carbon

Q31. If an SN2 reaction occurs at a stereogenic carbon, what is the product configuration relative to the starting material?

  • Racemate regardless of starting configuration
  • Retention of configuration
  • Inversion of configuration
  • No chiral product formed

Correct Answer: Inversion of configuration

Q32. How do electron-withdrawing substituents on the substrate affect SN2 rate at the reaction center?

  • They typically decrease SN2 rate by destabilizing the transition state
  • They typically increase SN2 rate by making the carbon more electrophilic
  • They always convert SN2 into radical substitutions
  • They have no electronic effect on SN2

Correct Answer: They typically increase SN2 rate by making the carbon more electrophilic

Q33. Which analysis is used to obtain activation parameters ΔH‡ and ΔS‡ for SN2 reactions?

  • Hammett plot
  • Eyring (transition state) analysis
  • UV-Vis absorbance calibration
  • Isotopic labeling only

Correct Answer: Eyring (transition state) analysis

Q34. Why does hydrogen bonding in protic solvents reduce nucleophilicity of small anions?

  • Because hydrogen bonding increases their polarizability
  • Because hydrogen bonding stabilizes and solvates anions, making them less available to attack
  • Because hydrogen bonding makes anions heavier
  • Because protic solvents form covalent bonds with anions

Correct Answer: Because hydrogen bonding stabilizes and solvates anions, making them less available to attack

Q35. Which statement is true about intermediates in a concerted SN2 mechanism?

  • A stable carbocation intermediate is formed
  • No discrete intermediate; reaction proceeds through a single transition state
  • A persistent carbanion intermediate forms
  • A radical intermediate is always observed

Correct Answer: No discrete intermediate; reaction proceeds through a single transition state

Q36. What is the Walden inversion phenomenon in substitution chemistry?

  • Conversion of an sp2 carbon to sp3
  • Inversion of configuration at a stereocenter during nucleophilic substitution
  • Retention of configuration due to double inversion
  • Racemization caused by free radicals

Correct Answer: Inversion of configuration at a stereocenter during nucleophilic substitution

Q37. Which substitution mechanism typically gives a racemic product from a chiral substrate?

  • SN2
  • SN1 involving a planar carbocation intermediate
  • Concerted pericyclic substitution
  • Radical chain substitution exclusively

Correct Answer: SN1 involving a planar carbocation intermediate

Q38. Under pseudo-first-order conditions for SN2 where nucleophile is in large excess, what is the observed rate law?

  • rate = k [nucleophile][substrate]^2
  • rate = k’ [substrate] where k’ = k [nucleophile]initial
  • rate = k [nucleophile]
  • rate = constant independent of concentrations

Correct Answer: rate = k’ [substrate] where k’ = k [nucleophile]initial

Q39. The Finkelstein reaction (R–Cl + NaI in acetone → R–I + NaCl) proceeds by which mechanism?

  • SN1
  • SN2
  • E2
  • Free radical substitution

Correct Answer: SN2

Q40. Which solvent would you choose to maximize SN2 rate for alkyl halide substitution?

  • Water (polar protic)
  • Methanol (polar protic)
  • Acetone (polar aprotic)
  • Hexane (nonpolar)

Correct Answer: Acetone (polar aprotic)

Q41. According to the Arrhenius equation, how does activation energy (Ea) relate to the rate constant k?

  • Higher Ea gives larger k
  • Higher Ea gives smaller k
  • Ea has no effect on k
  • k is directly proportional to Ea

Correct Answer: Higher Ea gives smaller k

Q42. What is the ideal angle between nucleophile and leaving group in a perfect SN2 backside attack?

  • 90°
  • 120°
  • 180° (colinear, directly opposite)
  • 60°

Correct Answer: 180° (colinear, directly opposite)

Q43. Intramolecular SN2 reactions that form rings are most favorable for which ring size due to geometric constraints?

  • Three-membered rings exclusively
  • Four-membered rings only
  • Five-membered rings (often favored)
  • Seven-membered rings preferentially

Correct Answer: Five-membered rings (often favored)

Q44. How do phase-transfer catalysts affect SN2 rates in biphasic systems?

  • They slow the reaction by stabilizing the leaving group
  • They transfer nucleophile into organic phase, increasing its effective concentration and rate
  • They sequester nucleophile in the aqueous phase and decrease rate
  • They convert SN2 to SN1 mechanism

Correct Answer: They transfer nucleophile into organic phase, increasing its effective concentration and rate

Q45. What is the effect of adding crown ethers to an SN2 reaction involving an alkali metal halide nucleophile?

  • Crown ethers bind the nucleophile and reduce rate
  • Crown ethers sequester the metal cation, increasing free nucleophile reactivity and rate
  • Crown ethers act as leaving groups
  • Crown ethers promote radical pathways

Correct Answer: Crown ethers sequester the metal cation, increasing free nucleophile reactivity and rate

Q46. How does increased polarizability of a nucleophile influence SN2 reactivity in protic solvents?

  • Less polarizable nucleophiles are more reactive
  • More polarizable nucleophiles (e.g., I-) are more nucleophilic in protic solvents
  • Polarizability has no impact in protic solvents
  • Polarizability only matters in gas phase

Correct Answer: More polarizable nucleophiles (e.g., I-) are more nucleophilic in protic solvents

Q47. What does the term “molecularity” refer to in the context of an elementary SN2 step?

  • The number of atoms in the substrate
  • The number of species that simultaneously collide in the elementary step
  • The molecular weight of the nucleophile
  • The stoichiometric coefficient in the overall equation

Correct Answer: The number of species that simultaneously collide in the elementary step

Q48. What information does the slope of a Hammett plot provide for a series of substituted benzyl halide SN2 reactions?

  • The slope is meaningless for SN2 reactions
  • The slope indicates sensitivity of reaction rate to electronic substituent effects
  • The slope gives the activation energy directly
  • The slope predicts solvent polarity

Correct Answer: The slope indicates sensitivity of reaction rate to electronic substituent effects

Q49. When a bond to a labeled atom is broken in the rate-determining step, what is expected for the kinetic isotope effect (KIE)?

  • KIE is negligible (~1)
  • KIE is significant (>1) for a normal primary isotope effect
  • KIE is always exactly 0.5
  • KIE is only observed for oxygen isotopes

Correct Answer: KIE is significant (>1) for a normal primary isotope effect

Q50. In the SN2 reaction: CH3Cl + OH- → CH3OH + Cl-, which species is the nucleophile?

  • CH3Cl
  • Cl-
  • OH-
  • CH3OH

Correct Answer: OH-

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