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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3:26 minutes

Problem 29a

Textbook Question

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

Verified step by step guidance

Step 1: Understand the difference between Sₙ1 and Sₙ2 mechanisms. Sₙ1 (unimolecular nucleophilic substitution) involves a two-step process where the leaving group departs first, forming a carbocation intermediate, followed by nucleophilic attack. Sₙ2 (bimolecular nucleophilic substitution) is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs.

Step 2: Analyze the substrate structure. Sₙ1 is favored by substrates that can stabilize a carbocation intermediate, such as tertiary carbons or benzylic and allylic carbons. Sₙ2 is favored by substrates with less steric hindrance, such as primary or methyl carbons.

Step 3: Consider the strength of the nucleophile. Sₙ2 is favored by strong nucleophiles because the nucleophile directly participates in the rate-determining step. Sₙ1, on the other hand, is less dependent on nucleophile strength since the rate-determining step is the formation of the carbocation.

Step 4: Evaluate the solvent. Polar protic solvents (e.g., water, alcohols) stabilize carbocations and favor Sₙ1. Polar aprotic solvents (e.g., acetone, DMSO) do not stabilize carbocations as effectively and favor Sₙ2 by enhancing the nucleophilicity of the nucleophile.

Step 5: Assess the leaving group. A good leaving group (e.g., halides like I⁻, Br⁻, or tosylate) is important for both Sₙ1 and Sₙ2, but it is especially critical for Sₙ1 since the leaving group departure is the rate-determining step.

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Key Concepts

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

Nucleophilic Substitution Mechanisms

Nucleophilic substitution reactions can occur via two primary mechanisms: Sₙ1 and Sₙ2. Sₙ1 is a two-step process involving the formation of a carbocation intermediate, while Sₙ2 is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs. Understanding these mechanisms is crucial for predicting the outcome of reactions based on substrate structure and reaction conditions.

Carbocation Stability

The stability of carbocations plays a significant role in determining whether an Sₙ1 or Sₙ2 mechanism will be favored. More stable carbocations, such as tertiary carbocations, are more likely to form in Sₙ1 reactions, while less stable substrates favor Sₙ2 mechanisms. Factors influencing carbocation stability include the degree of alkyl substitution and resonance effects.

Nucleophile Strength and Solvent Effects

The strength of the nucleophile and the solvent used can significantly influence the preference for Sₙ1 or Sₙ2 mechanisms. Strong nucleophiles and polar aprotic solvents typically favor Sₙ2 reactions, while weak nucleophiles and polar protic solvents can stabilize carbocations, thus favoring Sₙ1. Recognizing these factors helps in predicting the reaction pathway under specific conditions.

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