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

Grignard Reaction

Organic Chemistry

13. Alcohols and Carbonyl Compounds

Grignard Reaction

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Problem 45a

Textbook Question

We discuss the reaction of Grignard reagents (organomagnesium compounds) to ketones in Chapter 17. Mechanistically, the reaction proceeds by the nucleophilic addition of a methyl carbanion to the electrophilic carbon of the carbonyl, breaking the C―Oπ bond, resulting in an alkoxide intermediate that is subsequently protonated to produce the 3° alcohol.
(a) Why does this reaction produce a racemic mixture of 3° alcohols?

Verified step by step guidance

Understand the concept of a Grignard reagent: Grignard reagents are organomagnesium compounds, typically represented as RMgX, where R is an organic group and X is a halogen. They are highly nucleophilic and react with electrophilic centers such as carbonyl groups.

Recognize the nature of the carbonyl group: The carbonyl group (C=O) in ketones is electrophilic due to the polarization of the C=O bond, where the carbon atom is partially positive and the oxygen atom is partially negative.

Identify the nucleophilic attack: In the reaction, the Grignard reagent provides a methyl carbanion (CH₃⁻) that attacks the electrophilic carbon of the carbonyl group, breaking the C=O π bond and forming a new C-C bond.

Consider the formation of the alkoxide intermediate: The nucleophilic attack results in an alkoxide intermediate, which is a negatively charged oxygen species. This intermediate is planar and can be attacked from either side, leading to the formation of two enantiomers.

Explain the formation of a racemic mixture: Since the alkoxide intermediate is planar, the subsequent protonation step can occur from either side, resulting in equal probability of forming both enantiomers of the 3° alcohol. This leads to a racemic mixture, which is a 50:50 mixture of two enantiomers.

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Key Concepts

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

Grignard Reagents

Grignard reagents are organomagnesium compounds used in organic synthesis to form carbon-carbon bonds. They are highly nucleophilic, allowing them to attack electrophilic centers such as carbonyl groups in ketones, leading to the formation of alcohols. The reactivity of Grignard reagents is crucial for understanding their role in the nucleophilic addition mechanism.

Nucleophilic Addition Mechanism

Nucleophilic addition involves the attack of a nucleophile, such as a methyl carbanion from a Grignard reagent, on an electrophilic center, like the carbonyl carbon in ketones. This process breaks the C―Oπ bond, forming an alkoxide intermediate. Understanding this mechanism is essential for predicting the outcome of reactions involving Grignard reagents and carbonyl compounds.

Racemic Mixture Formation

A racemic mixture consists of equal amounts of two enantiomers, which are mirror-image isomers. In the context of Grignard reactions with ketones, the planar nature of the carbonyl group allows the nucleophilic attack to occur from either side, leading to the formation of both enantiomers of the 3° alcohol. This symmetry results in a racemic mixture, which is key to understanding the stereochemical outcome of the reaction.

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Related Practice

Textbook Question

Predict the products of the following reactions.

(a) allyl bromide + cyclohexyl magnesium bromide

Textbook Question

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons.

c. 1-cyclopentylpent-2-ene

Multiple Choice

Predict the major, organic product for the following reaction.

Multiple Choice

Which of the following reactions will NOT produce a 3° alcohol as the major, organic product?

Textbook Question

We discuss the reaction of Grignard reagents (organomagnesium compounds) to ketones in Chapter 17. Mechanistically, the reaction proceeds by the nucleophilic addition of a methyl carbanion to the electrophilic carbon of the carbonyl, breaking the C―Oπ bond, resulting in an alkoxide intermediate that is subsequently protonated to produce the 3° alcohol.

(b) Why, when the substrate is modified slightly, does the reaction result in an excess of one stereoisomer?

Textbook Question

In Chapters 8 and 17 we learn two reactions for the synthesis of the alcohol shown. (a) Show a mechanism for each of the reactions. (b) If you were designing a synthetic route, which would be considered more sustainable? Consider all factors. [Assume the starting organic molecules are equally green.]

Textbook Question

Starting from bromobenzene and any other reagents and solvents you need, show how you would synthesize the following compounds. Any of these products may be used as starting materials in subsequent parts of this problem.

b. 1-phenylpropene

Textbook Question

Which of the following compounds are suitable solvents for Grignard reactions?

(a) n-hexane

(b) CH₃–O–CH₃

(d) cyclohexane

(e) benzene

(f) CH₃OCH₂CH₂OCH₃

(g)

(h)

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