Textbook Question
Predict the carbenoid addition products of the following reactions.
(b)
10. Addition Reactions
Carbene
2:40 minutesProblem 79e
Textbook Question
In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2 .
(e) Carbenes have great synthetic utility, especially in the synthesis of cyclopropanes from alkenes. Based on your answers to (a)–(d), show a mechanism for the cyclopropanation of cyclohexene.
Verified step by step guidance1
Understand the structure of a carbene: A carbene is a neutral species with a divalent carbon atom that has two nonbonding electrons. The simplest carbene is methylene (CH₂), which can exist in either a singlet or triplet state. Singlet carbenes have paired electrons, while triplet carbenes have unpaired electrons.
Recognize the reactivity of carbenes: Carbenes are highly reactive intermediates due to their electron deficiency and the presence of nonbonding electrons. They can insert into C-H bonds or react with π-bonds, such as those in alkenes, to form cyclopropanes.
Analyze the reaction: In this case, the carbene will react with cyclohexene, an alkene, to form a cyclopropane ring. The π-electrons of the alkene will interact with the carbene to form two new C-C bonds, resulting in a three-membered cyclopropane ring.
Draw the mechanism: (1) The carbene approaches the double bond of cyclohexene. (2) The π-electrons of the double bond interact with the empty p-orbital of the carbene, while the lone pair of electrons on the carbene forms a bond with the other carbon of the double bond. This concerted reaction forms the cyclopropane ring.
Verify the product: The final product is a cyclopropane derivative of cyclohexene, where the double bond has been replaced by a three-membered ring. Ensure that the stereochemistry of the product is consistent with the reaction conditions and the nature of the carbene (singlet or triplet).
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2mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Carbenes
Carbenes are neutral reactive intermediates that contain a carbon atom with only six valence electrons, making them highly reactive. The simplest carbene, methylene (CH₂), can participate in various chemical reactions, including cyclopropanation. Their unique structure allows them to act as electrophiles, enabling them to add across double bonds in alkenes.
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Cyclopropanation
Cyclopropanation is a chemical reaction that involves the formation of a cyclopropane ring from an alkene. This process typically occurs through the addition of a carbene to the double bond of the alkene, resulting in a three-membered ring. The reaction is significant in organic synthesis due to the strain and reactivity of cyclopropanes, which can lead to further transformations.
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01:49General properties of cyclopropanation.
Reaction Mechanism
A reaction mechanism is a step-by-step description of the pathway taken during a chemical reaction, detailing the formation and transformation of intermediates. Understanding the mechanism of cyclopropanation involves identifying the key steps, such as the generation of the carbene and its subsequent addition to the alkene. This knowledge is crucial for predicting the products and optimizing reaction conditions in synthetic chemistry.
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