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A carbene is defined as a molecule that comprises a neutral carbon atom. Along with this, the carbene molecule consists of two valence electrons as well as two unshared valence electrons.
The general formula of carbene is R-(C:)-R’. There’s another formula for carbene that is R=C:
In the given formula, R is known as the substituents or hydrogen atoms.
A carbene is also defined as a specific compound. The classification of carbenes is done in 2 categories, namely singlets or triplets. This classification is done based on the electronic structure.
Singlet and triplet carbene
Carbenes are classified into two types, namely singlet and triplet carbene. Singlet carbenes are known to be spin-paired. On the other hand, triplet carbene consists of two unpaired electrons. Both of the electrons can be bent or linear.
Mostly, carbenes have a ground state of the nonlinear triplet. However, exceptions are oxygen, nitrogen, or sulphur atoms.
Carbenes are known as singlets or triplets, according to their electronic spins. Triplet carbenes are those carbenes that are paramagnetic. Triplet carbenes, if they remain for a long time, can be seen by electron spin spectroscopy.
How is carbene generated?
In most cases, a carbene is formed from diazoalkanes through photolytic thermal or even transition-metal catalysed paths. Typically, catalysts display copper as well as rhodium. Carbenes are provided in aprotic solvents, and carbenium ions are provided in protic solvents through Bamford – Steven reaction.
Carbenes are generated through the process of base-induced elimination HX under the conditions of phase transfer.
Wolff rearrangement provides carbene as intermediaries.
Carbene insertion reaction
Another kind of carbene reaction includes insertions. The carbene tends to interpose itself into a bond that is already existing. The order of choice is :
- X-H bonds. In this, X is not carbon.
- C-H bonds.
- C-C bond
It is difficult to predict if the insertion will take place in a single step.
Carbene dimerization
Carbenes, as well as carbenoid precursors, are capable of going through the process of dimerization so that alkanes can be created. This reaction is usually considered not required. However, this reaction can be utilised in the form of a synthetic tool. Constant carbenes are present in the equilibrium along with their respective dimers. This process is called Wanzlick equilibrium.
Reactivity of carbenes
It is known that singlet carbenes and triplet carbenes do not possess similar reactivity. Singlet carbenes are involved in cheletropic reactions. In contrast, triplet carbenes involve stepwise radical additions.
Substituent group preparation methods decide the reactivity of a specific carbene. Some of the reactions that carbenes can perform include skeletal rearrangements, C-H bonds, etc.
Carbenes can be classified into different categories, such as electrophilic, nucleophilic, or even amphiphilic. Substituents are responsible for strong reactivity.
Conclusion
A carbene is defined as a molecule that comprises a neutral carbon atom. Along with this, carbene molecules consist of valence of two as well as two unshared valence electrons. Carbenes are provided in aprotic solvents, and carbenium ions are provided in protic solvents through Bamford – Steven reaction. Carbenes are generated through the process of base-induced elimination HX under the conditions of phase transfer.
Carbenes, as well as carbenoid precursors, are capable of going through the process of dimerization so that alkanes can be created. This reaction is usually considered not required. However, this reaction can be utilised in the form of a synthetic tool. A most commonly used example of carbene includes dichlorocarbene CI2C. This can be formed with the help of chloroform along with a strong base. Carbenes are molecules that consist of a carbon atom along with only two single bonds. Usually, carbene consists of four bonds. This is the major reason why carbenes tend to be so unstable. Carbenes can be classified into different categories, such as electrophilic, nucleophilic, or even amphiphilic. Substituents are responsible for strong reactivity.
