23 4 Divalent carbon compounds carbenes

Definition of Carbene: A divalent carbon atom bonded to two hydrogen atoms and possessing a lone pair.
Charge: The formal charge of a carbene is zero.
Reactivity: Carbenes are highly reactive species that participate in significant reactions with alkenes.

From Diazomethane:
- Resonance Structures: Multiple resonance structures exist, but the most useful shows an electron transfer to nitrogen, forming dinitrogen gas.
- Leaving Group: The departure of nitrogen gas (N2) acts as an excellent leaving group, leading to the formation of a neutral carbene complex.
- Method: Carbenes can be generated by heating or exposing diazomethane to UV light.
From Chloroform:
- Reaction with Potassium t-Butoxide:
  - This method involves deprotonation of chloroform by a strong base (potassium t-butoxide).
  - Alpha Elimination: A rare example because it lacks beta hydrogens; results in the formation of dichloro carbene.

Formation of Cyclopropane Compounds:
- The carbene donates electrons to one end of the alkene, leading to a cyclic structure (cyclopropane) through a stereospecific mechanism.
- Stereospecificity: The reaction can only bond to one face of the double bond, preventing bridging to the opposite side.

Addition Mechanism:
- The reaction involves a syn addition where both substituents add to the same face of the double bond, ensuring high stereoselectivity.

Dichloro Carbene Addition to Alkenes:
- Trans-2-butene:
  - Addition leads to a product where circumferential methyl groups remain trans to one another.
- Cis-2-butene:
  - Addition results in a product where the two methyl groups are cis to one another.

Carbenes exhibit unique stereospecific reactions with alkenes, highlighting their importance in organic synthesis, particularly in cyclopropane formation and other stereoselective reactions.

Definition of Carbene: A carbene is defined as a divalent carbon atom that forms two covalent bonds with other atoms, typically involving two hydrogen atoms while having a lone pair of electrons. This unique electronic structure contributes to the carbene's reactivity.
Charge: The formal charge of a carbene is zero, which indicates a neutral species.
Reactivity: Carbenes are highly reactive intermediates in organic reactions. Their reactivity arises from the presence of the lone pair and the ability to engage in diverse chemical transformations, particularly with alkenes.

From Diazomethane:
- Resonance Structures: Carbenes generated from diazomethane can be understood through resonance structures; the most relevant structure involves an electron transfer to nitrogen, resulting in the emission of dinitrogen gas (N2).
- Leaving Group: The formation of nitrogen gas as a leaving group during the reaction enhances the generation of a stable neutral carbene complex.
- Method: Carbenes from diazomethane can be effectively generated by heating or upon exposure to ultraviolet (UV) light, which initiates the decomposition reaction.
From Chloroform:
- Reaction with Potassium t-Butoxide: This method requires the deprotonation of chloroform by a strong base, such as potassium t-butoxide.
- Alpha Elimination: Although this method is less common due to chloroform's lack of beta hydrogens, it results in the formation of dichloro carbene, which has potential applications in various synthetic pathways.

Formation of Cyclopropane Compounds: The interaction between carbenes and alkenes typically leads to the formation of cyclopropane derivatives. In this reaction, the carbene donates electrons to one end of the alkene, facilitating the closure of a three-membered ring structure (cyclopropane) in a stereospecific manner.
Stereospecificity: This reaction is noted for its stereospecific nature, where the carbene can only add to one face of the double bond, preventing alternate bonding configurations and resulting in unique structural outcomes.

Addition Mechanism: The mechanism of addition is characterized by a syn addition process. In this context, both substituents from the carbene add across the double bond on the same face, leading to a highly stereoselective outcome in the resultant product.

Dichloro Carbene Addition to Alkenes:
- Trans-2-butene: When dichloro carbene reacts with trans-2-butene, the product produced maintains a configuration where the circumferential methyl groups remain trans to each other, illustrating the preferred stereochemical outcome.
- Cis-2-butene: Conversely, the reaction with cis-2-butene results in a product where the two methyl groups are positioned cis to one another, showcasing how initial configurations influence final product stereochemistry.

Carbenes exhibit distinct and unique stereospecific reactions with alkenes, playing a pivotal role in organic synthesis processes. Their ability to form cyclopropanes and engage in other stereoselective reactions highlights their significance in synthetic organic chemistry, frequently serving as an essential tool in the synthesis of complex organic molecules.