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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10. Addition Reactions
Carbene
3:58 minutesProblem 52
Textbook Question
When dichloromethane is treated with strong NaOH, an intermediate is generated that reacts like a carbene. Draw the structure of this reactive intermediate, and propose a mechanism for its formation.
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Dichloromethane (CH₂Cl₂) is treated with a strong base like NaOH. The first step involves the abstraction of a proton from dichloromethane by the hydroxide ion (OH⁻), forming a negatively charged intermediate called the dichloromethide anion (CHCl₂⁻).
The dichloromethide anion (CHCl₂⁻) is unstable and undergoes further reaction. One of the chlorine atoms leaves as a chloride ion (Cl⁻), resulting in the formation of a neutral, highly reactive intermediate known as dichlorocarbene (:CCl₂).
The structure of dichlorocarbene (:CCl₂) is a divalent carbon species with two lone pairs of electrons. It is neutral but highly reactive due to the incomplete octet on the carbon atom, making it an electrophile.
The mechanism for the formation of dichlorocarbene involves two key steps: (1) deprotonation of dichloromethane by the strong base to form the dichloromethide anion, and (2) elimination of a chloride ion from the dichloromethide anion to generate the carbene.
To summarize, the reactive intermediate formed is dichlorocarbene (:CCl₂), and its formation mechanism involves deprotonation followed by elimination. This intermediate is often used in organic reactions such as cyclopropanation due to its high reactivity.
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3mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Carbenes
Carbenes are neutral species containing a carbon atom with only six valence electrons, making them highly reactive. They can be classified as singlet or triplet carbenes based on the spin state of their electrons. Carbenes often act as intermediates in organic reactions, participating in addition reactions or rearrangements due to their electron-deficient nature.
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Nucleophilic Substitution Reactions
Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. In the context of dichloromethane and NaOH, the hydroxide ion (OH-) acts as a nucleophile, attacking the carbon atom bonded to the leaving group (Cl), leading to the formation of an intermediate. Understanding the mechanism of these reactions is crucial for predicting the products formed.
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Mechanism of Carbene Formation
The formation of a carbene intermediate from dichloromethane and NaOH typically involves the deprotonation of a chloromethyl group, resulting in the loss of a chloride ion and the generation of a carbene. This process can be facilitated by strong bases like NaOH, which can stabilize the carbene through resonance or inductive effects. Recognizing the steps in this mechanism is essential for accurately drawing the structure of the intermediate.
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