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

Good Leaving Groups

Organic Chemistry

7. Substitution Reactions

Good Leaving Groups

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6:30 minutes

Problem 56a,b,c

Textbook Question

Which reaction in each of the following pairs takes place more rapidly? (EtOH is ethyl alcohol; Et₂O is diethyl ether.)
a.
b.
c.

Verified step by step guidance

Step 1: Analyze the reaction mechanism for each pair of reactions. For part (a), both reactions involve the nucleophilic substitution of CH3Br. The first reaction uses HO⁻ (a strong nucleophile), while the second uses H2O (a weaker nucleophile). Stronger nucleophiles generally lead to faster reactions in nucleophilic substitution.

Step 2: For part (b), compare the leaving groups in the two reactions. The leaving group ability increases as the halide ion size increases (I⁻ > Br⁻ > Cl⁻ > F⁻). CH3I has a better leaving group (I⁻) compared to CH3Cl (Cl⁻), so the reaction with CH3I is expected to proceed more rapidly.

Step 3: For part (c), compare the nucleophiles and reaction conditions. In the first reaction, NH3 acts as a nucleophile, while in the second reaction, H2O is the nucleophile. NH3 is a stronger nucleophile than H2O, so the reaction with NH3 is expected to proceed more rapidly.

Step 4: Consider the solvent effects in each reaction. Polar protic solvents (like H2O and EtOH) can stabilize the transition state and the leaving group, which can influence the reaction rate. However, the strength of the nucleophile and the leaving group ability are the dominant factors in these cases.

Step 5: Summarize the findings for each pair of reactions. For part (a), the reaction with HO⁻ is faster. For part (b), the reaction with CH3I is faster. For part (c), the reaction with NH3 is faster. These conclusions are based on the nucleophile strength and leaving group ability.

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Key Concepts

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

Nucleophilicity

Nucleophilicity refers to the ability of a nucleophile to donate an electron pair to an electrophile during a chemical reaction. Stronger nucleophiles are more reactive and can displace leaving groups more effectively. In the given reactions, the strength of the nucleophile (e.g., HO− vs. H2O) significantly influences the reaction rate, as stronger nucleophiles will lead to faster reactions.

Leaving Group Ability

The leaving group ability is a measure of how easily a group can depart from the parent molecule during a reaction. Good leaving groups, such as bromide (Br−) and iodide (I−), stabilize the negative charge after leaving, facilitating the reaction. In the comparisons provided, the nature of the leaving group affects the overall reaction rate, with better leaving groups leading to faster reactions.

Solvent Effects

The solvent can significantly impact the rate of a reaction by stabilizing reactants, intermediates, or products. Polar protic solvents, like water and alcohols, can stabilize ions through solvation, while polar aprotic solvents can enhance nucleophilicity. In the reactions presented, the choice of solvent (EtOH vs. Et2O) can influence the speed of the reaction by affecting the nucleophile's reactivity and the leaving group's stability.

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