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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5:42 minutes

Problem 129

Textbook Question

When equivalent amounts of methyl bromide and sodium iodide are dissolved in methanol, the concentration of iodide ion quickly decreases and then slowly returns to its original concentration. Account for this observation.

Verified step by step guidance

Step 1: Recognize that the reaction involves methyl bromide (CH₃Br) and sodium iodide (NaI) in methanol. This is a classic example of an SN2 (bimolecular nucleophilic substitution) reaction.

Step 2: In the SN2 reaction, the iodide ion (I⁻) acts as a nucleophile and attacks the carbon atom in methyl bromide, which is electrophilic due to the polar C-Br bond. This results in the displacement of the bromide ion (Br⁻) and the formation of methyl iodide (CH₃I). The reaction can be represented as: CH₃Br + I⁻ → CH₃I + Br⁻.

Step 3: The initial decrease in iodide ion concentration is due to its consumption in the SN2 reaction to form methyl iodide. This explains the rapid drop in iodide ion concentration.

Step 4: Over time, the methyl iodide (CH₃I) formed in the reaction can undergo a reverse reaction with bromide ion (Br⁻) in the methanol solvent. This reverse reaction regenerates iodide ion (I⁻) and methyl bromide (CH₃Br). The equilibrium between these species accounts for the slow return of iodide ion concentration to its original level.

Step 5: Conclude that the observed changes in iodide ion concentration are due to the dynamic equilibrium established between the forward SN2 reaction and the reverse reaction in the methanol solvent. This equilibrium allows the iodide ion concentration to eventually stabilize.

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Key Concepts

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

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. In this case, iodide ion (I-) acts as a nucleophile attacking methyl bromide (CH3Br), leading to the formation of methyl iodide (CH3I) and bromide ion (Br-). Understanding this mechanism is crucial to explain the initial decrease in iodide concentration.

Equilibrium and Reaction Dynamics

Chemical reactions often reach a state of equilibrium where the rates of the forward and reverse reactions are equal. The initial decrease in iodide concentration suggests that it is being consumed in the reaction with methyl bromide, while the slow return to its original concentration indicates that the reverse reaction or a secondary reaction is occurring, allowing iodide to regenerate over time.

Solvent Effects on Reaction Rates

The choice of solvent can significantly influence the rate and outcome of chemical reactions. Methanol, being a polar protic solvent, stabilizes ions and can affect the nucleophilicity of iodide. This stabilization may lead to a slower reaction rate after the initial consumption of iodide, contributing to the observed dynamics of iodide concentration in the solution.

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