Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

7. Substitution Reactions

Substitution Comparison

Organic Chemistry

7. Substitution Reactions

Substitution Comparison

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Problem 29b

Textbook Question

Would you expect the following conditions to favor S_N1 or S_N2?
(b)

Verified step by step guidance

Analyze the substrate: The substrate contains a bromine atom attached to a secondary carbon, which is adjacent to a pyridine ring. The pyridine ring is an electron-withdrawing group, which can stabilize a carbocation intermediate, favoring SN1 reactions.

Consider the nucleophile: Sodium acetate (NaOAc) is a weak nucleophile. Weak nucleophiles typically favor SN1 mechanisms because they do not strongly attack the substrate, allowing the carbocation intermediate to form.

Evaluate the solvent: The solvent appears to be dimethyl sulfoxide (DMSO), which is a polar aprotic solvent. Polar aprotic solvents generally favor SN2 mechanisms because they stabilize the nucleophile without stabilizing the carbocation. However, the substrate and nucleophile characteristics may override this solvent preference.

Assess steric hindrance: The secondary carbon attached to bromine is moderately hindered, which can make SN2 reactions less favorable due to steric hindrance. This steric hindrance supports the likelihood of an SN1 mechanism.

Combine all factors: The electron-withdrawing pyridine ring stabilizes the carbocation, the weak nucleophile favors SN1, and the steric hindrance disfavors SN2. While the polar aprotic solvent typically favors SN2, the other factors suggest that SN1 is the more likely mechanism under these conditions.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Sₙ1 Mechanism

The Sₙ1 mechanism, or unimolecular nucleophilic substitution, involves a two-step process where the leaving group departs first, forming a carbocation intermediate. This mechanism is favored in polar protic solvents and with tertiary substrates due to the stability of the carbocation. The rate of reaction depends only on the concentration of the substrate, making it a first-order reaction.

Sₙ2 Mechanism

The Sₙ2 mechanism, or bimolecular nucleophilic substitution, occurs in a single concerted step where the nucleophile attacks the substrate as the leaving group departs. This mechanism is favored by strong nucleophiles and occurs in polar aprotic solvents. The reaction rate depends on both the substrate and nucleophile concentrations, making it a second-order reaction.

Factors Influencing Sₙ1 vs Sₙ2

Several factors influence whether a reaction will proceed via Sₙ1 or Sₙ2 mechanisms, including substrate structure (primary, secondary, or tertiary), the strength of the nucleophile, solvent type, and steric hindrance. Tertiary substrates and polar protic solvents favor Sₙ1, while primary substrates and strong nucleophiles in polar aprotic solvents favor Sₙ2. Understanding these factors is crucial for predicting reaction pathways.

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