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

13. Alcohols and Carbonyl Compounds

Protecting Alcohols from Organometallics

Organic Chemistry

13. Alcohols and Carbonyl Compounds

Protecting Alcohols from Organometallics

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Problem 43

Textbook Question

A chemist failed to generate the diol using the reaction shown here.
(a) Suggest a reason why this reaction did not work as written.
(b) How could the reaction conditions be modified to allow formation of the diol? [It may require more than one step.]

Verified step by step guidance

Analyze the reaction: The starting material is a β-hydroxy ketone, and the reaction involves a Grignard reagent (BrMgCH=CH2) followed by an acidic quench. The desired product is a diol, but the reaction fails to produce it.

Identify the issue: Grignard reagents are highly reactive nucleophiles and bases. The hydroxyl group (-OH) in the starting material is acidic and can react with the Grignard reagent, leading to protonation of the Grignard reagent and deactivation of its nucleophilicity. This prevents the Grignard reagent from attacking the carbonyl group.

Propose a solution: To prevent the Grignard reagent from reacting with the hydroxyl group, the hydroxyl group must be protected. A common protecting group for alcohols is a silyl ether, such as a trimethylsilyl (TMS) group. The hydroxyl group can be converted to a TMS ether using a reagent like TMSCl and a base (e.g., Et3N).

Modify the reaction: After protecting the hydroxyl group, the Grignard reagent can be added to react with the carbonyl group, forming the desired intermediate. After the reaction is complete, the protecting group can be removed using an acidic or fluoride-based deprotection step (e.g., TBAF) to regenerate the hydroxyl group.

Summarize the steps: (1) Protect the hydroxyl group using a silyl ether. (2) Perform the Grignard reaction with the protected compound. (3) Quench the reaction with H3O+ to form the alcohol. (4) Deprotect the silyl ether to regenerate the hydroxyl group, yielding the desired diol.

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Key Concepts

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

Reaction Mechanisms

Understanding reaction mechanisms is crucial in organic chemistry as it explains the step-by-step process by which reactants transform into products. This includes identifying intermediates, transition states, and the role of catalysts. A failure to generate the diol may stem from an incomplete or incorrect mechanism that does not favor the formation of the desired product.

Functional Group Transformations

Functional group transformations involve the conversion of one functional group into another through chemical reactions. In the context of diol formation, recognizing the starting materials and the necessary transformations to introduce hydroxyl groups is essential. Modifying reaction conditions may involve using reagents that facilitate these transformations effectively.

Reaction Conditions

Reaction conditions, including temperature, pressure, solvent, and the presence of catalysts, significantly influence the outcome of chemical reactions. Adjusting these conditions can enhance the yield of the desired product, such as a diol. For instance, using a different solvent or temperature may stabilize intermediates or transition states, leading to successful diol formation.

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