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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2:18 minutes

Problem 48b

Textbook Question

A solution of pure (S)-2-iodobutane ([α] = +15.90°) in acetone is allowed to react with radioactive iodide, ¹³¹I^–, until 1.0% of the iodobutane contains radioactive iodine. The specific rotation of this recovered iodobutane is found to be +15.58°.
b. What does this result suggest about the mechanism of the reaction of 2-iodobutane with iodide ion?

Verified step by step guidance

Step 1: Begin by understanding the problem. The reaction involves (S)-2-iodobutane reacting with radioactive iodide (131I-) in acetone. The specific rotation of the recovered iodobutane decreases slightly from +15.90° to +15.58°. This suggests that some change has occurred in the stereochemistry of the compound during the reaction.

Step 2: Recall the concept of specific rotation ([α]). Specific rotation is a measure of the optical activity of a chiral compound. A decrease in specific rotation indicates that the optical purity of the compound has been affected, possibly due to the formation of a racemic mixture or partial inversion of configuration.

Step 3: Consider the mechanism of the reaction. The iodide ion (I-) is a nucleophile, and the reaction likely proceeds via an SN2 or SN1 mechanism. In an SN2 reaction, the nucleophile attacks the chiral carbon, leading to inversion of configuration. In an SN1 reaction, the intermediate carbocation can lead to racemization due to attack from either side.

Step 4: Analyze the data. Since only 1.0% of the iodobutane contains radioactive iodine, the majority of the compound remains unchanged. The slight decrease in specific rotation suggests that the reaction mechanism involves partial inversion of configuration, consistent with an SN2 mechanism. This is because SN2 reactions typically result in inversion of stereochemistry at the chiral center.

Step 5: Conclude the analysis. The result suggests that the reaction of 2-iodobutane with iodide ion proceeds primarily via an SN2 mechanism, where the radioactive iodide replaces the original iodine atom with inversion of configuration. The small change in specific rotation indicates that the reaction is not complete and only a small fraction of the molecules undergo inversion.

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Key Concepts

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

Optical Activity and Specific Rotation

Optical activity refers to the ability of chiral compounds to rotate plane-polarized light. The specific rotation is a quantitative measure of this property, defined as the angle of rotation per unit concentration and path length. In this case, the specific rotation of the pure (S)-2-iodobutane is +15.90°, indicating its chiral nature. The change in specific rotation after the reaction suggests that the product may contain a mixture of chiral centers.

Nucleophilic Substitution Mechanisms

Nucleophilic substitution reactions can occur via two primary mechanisms: SN1 and SN2. The SN1 mechanism involves a two-step process where the leaving group departs first, forming a carbocation intermediate, while the SN2 mechanism is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs. The observed specific rotation change indicates that the reaction likely involves a racemization process, suggesting an SN1 mechanism where the intermediate can lead to both enantiomers.

Racemization and Reaction Kinetics

Racemization is the process by which an optically active compound is converted into a racemic mixture, containing equal amounts of both enantiomers. In the context of the reaction, the presence of radioactive iodine in only 1.0% of the product suggests that the reaction did not proceed to completion and that the mechanism allows for the formation of both enantiomers. This observation is crucial for understanding the kinetics and stereochemical outcomes of the reaction involving 2-iodobutane.

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