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

8. Elimination Reactions

Cumulative Substitution/Elimination

Organic Chemistry

8. Elimination Reactions

Cumulative Substitution/Elimination

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Problem 33

Textbook Question

The deuterium kinetic isotope effect (k_H2O/k_D2O) for the hydrolysis of aspirin is 2.2. What does this tell you about the kind of catalysis exerted by the ortho-carboxyl substituent? (Hint: It is easier to break an O–H bond than an O–D bond.)

Verified step by step guidance

Understand the concept of the kinetic isotope effect (KIE): The KIE is the ratio of reaction rates for isotopically substituted molecules. Here, kH2O/kD2O = 2.2 indicates that the reaction involving H2O is faster than the one involving D2O. This suggests that breaking the O-H bond is easier than breaking the O-D bond due to the lower bond dissociation energy of O-H compared to O-D.

Recognize the role of the ortho-carboxyl substituent: The ortho-carboxyl group in aspirin can act as a catalyst by participating in proton transfer or hydrogen bonding, which facilitates the hydrolysis reaction. The observed KIE suggests that the breaking of an O-H bond is a rate-determining step in the reaction.

Interpret the significance of the KIE value: A KIE of 2.2 is consistent with a primary kinetic isotope effect, which occurs when the bond to the isotopically substituted atom (H or D) is directly involved in the rate-determining step. This implies that the ortho-carboxyl group is likely involved in a proton transfer step during the hydrolysis of aspirin.

Relate the KIE to the mechanism of catalysis: The ortho-carboxyl substituent may stabilize the transition state by donating or accepting a proton, which involves breaking an O-H bond. The slower reaction in D2O (due to the stronger O-D bond) supports this mechanism.

Conclude the type of catalysis: Based on the KIE and the role of the ortho-carboxyl group, the catalysis exerted by the ortho-carboxyl substituent is likely general acid-base catalysis, where proton transfer plays a critical role in facilitating the hydrolysis reaction.

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Key Concepts

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

Kinetic Isotope Effect (KIE)

The kinetic isotope effect refers to the change in reaction rate when one atom in a molecule is replaced by one of its isotopes. In this context, the ratio of reaction rates (kH2O/kD2O) indicates how the presence of deuterium affects the hydrolysis of aspirin. A KIE greater than 1 suggests that the bond involving the lighter isotope (H) is broken more easily than that involving the heavier isotope (D), providing insights into the reaction mechanism.

Catalysis

Catalysis is the process by which the rate of a chemical reaction is increased by the presence of a catalyst, which is not consumed in the reaction. In the case of the hydrolysis of aspirin, the ortho-carboxyl substituent may stabilize the transition state or lower the activation energy, thus facilitating the breaking of the O-H bond. Understanding the type of catalysis (acid-base, covalent, etc.) is crucial for interpreting the KIE results.

O-H vs. O-D Bond Strength

The strength of O-H bonds is generally weaker than that of O-D bonds due to the difference in mass and bond length. This difference is significant in reactions involving proton transfer or bond cleavage. The hint in the question emphasizes that it is easier to break an O-H bond than an O-D bond, which is critical for understanding why the KIE of 2.2 suggests a specific catalytic mechanism influenced by the ortho-carboxyl group.

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