Ch. 11 - Organometallic Compounds

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 11 - Organometallic Compounds

Problem 39

Chapter 12, Problem 39

Dimerization is a side reaction that occurs during the preparation of a Grignard reagent. Propose a mechanism that accounts for the formation of the dimer.

Verified step by step guidance

Step 1: Begin by understanding the formation of the Grignard reagent. The reaction involves the insertion of magnesium (Mg) into the carbon-bromine (C-Br) bond of cyclohexyl bromide in the presence of a solvent like tetrahydrofuran (THF). This generates cyclohexylmagnesium bromide (a Grignard reagent).

Step 2: Recognize that the Grignard reagent is highly reactive and can act as a nucleophile. In the dimerization side reaction, the Grignard reagent reacts with another molecule of cyclohexyl bromide. This occurs because the Grignard reagent can attack the electrophilic carbon attached to the bromine atom in cyclohexyl bromide.

Step 3: Propose the mechanism for the dimerization. The nucleophilic cyclohexyl group from the Grignard reagent attacks the electrophilic carbon of cyclohexyl bromide, displacing the bromide ion (Br⁻). This results in the formation of a new C-C bond between two cyclohexyl groups.

Step 4: Consider the role of magnesium bromide (MgBr₂) as a byproduct. After the dimerization reaction, the bromide ion released combines with the magnesium ion from the Grignard reagent to form MgBr₂.

Step 5: Summarize the overall process. The dimerization side reaction leads to the formation of a dimer (a molecule with two cyclohexyl groups connected by a single bond) alongside the intended Grignard reagent. This occurs due to the high reactivity of the Grignard reagent and its ability to act as a nucleophile.

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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 formed by the reaction of an alkyl or aryl halide with magnesium metal in anhydrous ether. They are highly reactive nucleophiles used in organic synthesis to form carbon-carbon bonds. Understanding their structure and reactivity is crucial for analyzing side reactions like dimerization.

Dimerization Mechanism

Dimerization refers to the process where two molecules combine to form a dimer, which can occur through various mechanisms. In the context of Grignard reagents, dimerization typically involves the coupling of two Grignard species, leading to the formation of a new carbon-carbon bond. This process can be facilitated by the presence of reactive intermediates or through radical pathways.

Radical Intermediates

Radical intermediates are highly reactive species with unpaired electrons, often formed during chemical reactions. In the case of Grignard reagent dimerization, the formation of radical intermediates can occur through homolytic cleavage of the C-Mg bond, allowing for the coupling of two radicals to form a dimer. Understanding the role of these intermediates is essential for proposing a detailed mechanism.

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Textbook Question

Which of the following compounds cannot be prepared by a Heck reaction?

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Textbook Question

A student added an equivalent of 3,4-epoxy-4-methylcyclohexanol to a solution of methylmagnesium bromide in diethyl ether, and then added dilute hydrochloric acid. He expected that the product would be 1,2-dimethyl-1,4-cyclohexanediol. He did not get any of the expected product. What product did he get?

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Textbook Question

Using the given starting material, any necessary inorganic reagents and catalysts, and any carbon-containing compounds with no more than two carbons, indicate how each of the following compounds can be prepared:

Using the given starting material, any necessary inorganic reagents, and any carbon-containing compounds with no more than two carbons, indicate how each of the following compounds can be prepared:

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