Organic chemistryLesson 2 of 3 in the course Nucleophilic substitution and elimination

SN2 Mechanism, Rate Factors, and Competing Pathways

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

This lesson introduces the defining features of the bimolecular nucleophilic substitution reaction, or SN2. Understanding its concerted mechanism explains how bonds change, why the nucleophile approaches from a particular direction, and how a chiral center changes configuration.

An SN2 reaction is a nucleophilic substitution at an sp3sp^3-hybridised carbon. A nucleophile, written as Nu, forms a bond to the carbon while a leaving group, often a halide written as X, departs.

All flashcards

Why is SN2 called a bimolecular reaction?
The substrate and nucleophile both participate in the rate-determining step.
What is the role of the SN2 transition state?
It represents the high-energy arrangement during concerted bond formation and bond breaking; it is not a stable intermediate.
Does inversion at a chiral center necessarily change its R/S label?
No. Replacing a substituent can change priority assignments, so geometric inversion does not necessarily change the R/S label.
How can tert-butoxide affect substitution at a crowded carbon?
It is a strong, bulky base, so it may be less effective at nucleophilic attack and may favor elimination.
What happens in the reported SN2 roundabout observation?
In a gas-phase collision, methyl iodide rotates before chloride completes the SN2 displacement.

Quiz questions

  1. Which rate law describes a typical SN2 reaction?Answer: r=k[RX][Nu−]r=k[RX][Nu^-]
  2. Which order gives the usual SN2 reactivity trend for ordinary alkyl substrates?Answer: Methyl > primary > secondary >> tertiary
  3. What type of reaction is the Finkelstein reaction?Answer: An SN2 halide exchange in which one halide replaces another.
  4. Why does racemization alone not prove that a reaction followed an SN1 pathway?Answer: Repeated SN2 attacks by a nucleophilic leaving group can also cause inversions that produce a racemic mixture.
  5. In a polar protic solvent, what nucleophilicity trend is commonly observed among halide ions?Answer: I−>Br−>Cl−>F−I^- > Br^- > Cl^- > F^-

Based on the Wikipedia article SN2 reaction, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.

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