Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions

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

Bruice 8th Edition

Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions

Problem 90

Chapter 7, Problem 90

The reaction of an alkene with diazomethane forms a cyclopropane ring. Propose a mechanism for the reaction. (Hint: It is a concerted reaction.)

Verified step by step guidance

Step 1: Recognize that diazomethane (CH₂N₂) is a highly reactive compound due to the presence of a diazo group (-N≡N) and a negatively charged carbon atom. This makes it a good nucleophile and a source of carbene intermediates.

Step 2: Understand that the reaction is concerted, meaning all bond-making and bond-breaking events occur simultaneously in a single step. The alkene (CH₂=CH₂) acts as an electron-rich species, while diazomethane provides the carbene intermediate.

Step 3: Propose the mechanism: The diazomethane undergoes decomposition to generate a carbene (CH₂:), which is a neutral species with a divalent carbon atom and two nonbonding electrons. This carbene is highly reactive and interacts with the alkene.

Step 4: The carbene reacts with the alkene in a concerted manner. The π-electrons of the alkene attack the carbene, forming two new σ-bonds simultaneously, resulting in the formation of a cyclopropane ring.

Step 5: During the reaction, nitrogen gas (N₂) is released as a byproduct due to the decomposition of diazomethane. This release of N₂ drives the reaction forward energetically.

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Key Concepts

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

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond is a site of reactivity, making alkenes important in organic synthesis. Their ability to undergo addition reactions allows them to react with various reagents, leading to the formation of more complex structures, such as cyclopropanes.

Concerted Reactions

Concerted reactions are processes in which bond-breaking and bond-forming occur simultaneously in a single step, without intermediates. This type of mechanism is characterized by a transition state where all changes happen at once. Understanding concerted mechanisms is crucial for predicting the outcome of reactions, such as the formation of cyclopropanes from alkenes and diazomethane.

Cyclopropane Formation

Cyclopropane formation involves the addition of a reagent across the double bond of an alkene, resulting in a three-membered ring structure. This reaction is typically facilitated by the presence of a reactive species, such as diazomethane, which can insert into the double bond. The unique strain and reactivity of cyclopropanes make them interesting intermediates in organic synthesis.

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General properties of cyclopropanation.

Related Practice

Textbook Question

Dichlorocarbene can also be generated by heating sodium trichloroacetate. Propose a mechanism for the reaction.

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Textbook Question

Draw the products of the following reactions. If the products can exist as stereoisomers show what stereoisomers are formed.

k. (Z)-3,4-dimethyl-3-hexene + H₂, Pd/C

l. (E)-3,4-dimethyl-3-hexene + H₂, Pd/C

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Textbook Question

What alkene gives the product shown after reaction first with ozone and then with dimethyl sulfide?

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Textbook Question

A, a compound with molecular formula C₆H₁₀, contains three methylene units. A reacts with one equivalent of H₂ over Pd/C to yield B. A reacts with aqueous acid to form a single product, C, and undergoes hydroboration/oxidation to form a pair of enantiomers, D and E. Ozonolysis of A followed by reaction with dimethyl sulfide forms F with molecular formula C₆H₁₀O₂. Provide structures for A–F.

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Textbook Question

Of the possible products shown for the following reaction, are there any that will not be formed?

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Textbook Question

Two chemists at Dupont found that ICH₂ZnI is better than diazomethane at converting a C=C bond to a cylcopropane ring. Propose a mechanism for the reaction, now known as the Simmons–Smith reaction in their honor.

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