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

SN1 Reaction

Organic Chemistry

7. Substitution Reactions

SN1 Reaction

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7:09 minutes

Problem 22

Textbook Question

Propose an S_N1 mechanism for the solvolysis of 3-bromo-2,3-dimethylpentane in ethanol.

Verified step by step guidance

Identify the substrate: The compound 3-bromo-2,3-dimethylpentane is a tertiary alkyl halide. Tertiary alkyl halides are highly favorable for SN1 reactions due to the stability of the carbocation intermediate formed during the reaction.

Step 1: Initiation of the reaction begins with the departure of the leaving group (bromide ion, Br⁻) from the substrate. This step is the rate-determining step and results in the formation of a tertiary carbocation at the 3rd carbon of the pentane chain.

Step 2: Analyze the stability of the carbocation. The tertiary carbocation formed is stabilized by hyperconjugation and inductive effects from the surrounding alkyl groups. No rearrangement is necessary as the carbocation is already in its most stable form.

Step 3: Nucleophilic attack by ethanol (CH₃CH₂OH). The lone pair of electrons on the oxygen atom of ethanol attacks the positively charged carbocation, forming an oxonium ion intermediate.

Step 4: Deprotonation of the oxonium ion. A molecule of ethanol or another base in the solution removes a proton (H⁺) from the oxonium ion, resulting in the formation of the final product, an ether (3-ethoxy-2,3-dimethylpentane).

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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 two main steps: the formation of a carbocation intermediate and the subsequent nucleophilic attack. The rate-determining step is the first step, where the leaving group departs, leading to a positively charged carbocation. This mechanism is favored in tertiary substrates due to their ability to stabilize the carbocation through hyperconjugation and inductive effects.

Carbocation Stability

Carbocation stability is crucial in determining the feasibility of an SN1 reaction. Tertiary carbocations are more stable than secondary or primary ones due to the greater number of alkyl groups that can donate electron density through hyperconjugation and inductive effects. In the case of 3-bromo-2,3-dimethylpentane, the formation of a tertiary carbocation is favored, making the SN1 pathway more likely.

Solvolysis

Solvolysis refers to a chemical reaction in which the solvent acts as a nucleophile, leading to the substitution of a leaving group. In this context, ethanol serves as the solvent and nucleophile, attacking the carbocation formed during the SN1 mechanism. The nature of the solvent can significantly influence the reaction rate and mechanism, with polar protic solvents like ethanol stabilizing the carbocation and facilitating the reaction.

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Related Practice

Textbook Question

Give the substitution products expected from solvolysis of each compound by heating in ethanol.

(a)

(b)

Textbook Question

A reluctant first-order substrate can be forced to ionize by adding some silver nitrate (one of the few soluble silver salts) to the reaction. Silver ion reacts with the halogen to form a silver halide (a highly exothermic reaction), generating the cation of the alkyl group.

Give mechanisms for the following silver-promoted rearrangements.

(a)

Textbook Question

Propose a mechanism involving a hydride shift or an alkyl shift for each solvolysis reaction. Explain how each rearrangement forms a more stable intermediate.

Hint: Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations.

(b)

Textbook Question

Give the S_N1 mechanism for the formation of 2-ethoxy-3-methylbutane, the unrearranged product in this reaction.

Textbook Question

Predict the compound in each pair that will undergo solvolysis (in aqueous ethanol) more rapidly.

(a) (CH₃CH₂)₂CH—Cl or (CH₃)₃C—Cl

(b)

Textbook Question

Choose the member of each pair that will react faster by the S_N1 mechanism.

a. 1-bromopropane or 2-bromopropane

b. 2-bromo-2-methylbutane or 2-bromo-3-methylbutane

Textbook Question

Furfuryl chloride can undergo substitution by both S_N2 and S_N1 mechanisms. Because it is a 1° alkyl halide, we expect S_N2 but not S_N1 reactions. Draw a mechanism for the S_N1 reaction shown below, paying careful attention to the structure of the intermediate. How can this primary halide undergo S_N1 reactions?

Textbook Question

Optically active butan-2-ol racemizes in dilute acid. Propose a mechanism for this racemization.

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Propose an SN1 mechanism for the solvolysis of 3-bromo-2,3-dimethylpentane in ethanol.

Key Concepts

SN1 Mechanism

Carbocation Stability

Solvolysis

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Propose an S_N1 mechanism for the solvolysis of 3-bromo-2,3-dimethylpentane in ethanol.