What Is a Carbene?
Basic Structure of Carbenes
Carbenes is a general term for carbon species that have only six valence electrons, are electronically neutral (uncharged), and have a divalent bond at the carbon center.
The six valence electrons for a divalent bond mean that it has two active electrons in the center. Analogues of the homologous elements include silylene and germylene.
It is often contrasted with carbocation, a three-coordinated chemical species with six valence electrons and a positive charge, and carbanion, a three-coordinated chemical species with eight valence electrons and a negative charge. The simplest structure of carbenes is methylene (Methylene) CH2. This substance can be generated by the decomposition of diazomethane.
Uses of Carbenes
A decade ago, carbenes were considered unstable chemical species, but in recent years there have been an increasing number of reports of carbenes being isolated through the use of ingenious molecular structures, such as electron donation by adjacent substituents and stereo-conformational fixation by bulky substituents. However, since they are not stable enough to be sold as products, precursors are sold and are basically prepared on the fly.
Because of their high reactivity, they are used as synthetic intermediates in organic synthetic chemistry. A typical reaction is the addition to a carbon-carbon double bond to form a cyclopropane ring. Insertion reactions occur for carbon-hydrogen, nitrogen-hydrogen, or oxygen-hydrogen bonds.
Many N-heterocyclic carbenes (NHC), which are cyclic carbene species sandwiched between two adjacent nitrogen atoms, have been synthesized and reported to be stable through the use of molecular structural innovations. These carbenes are widely used as ligands for organometallic complexes because of their strong metal-coordination ability. Examples include those substituted with mesityl or 2,6-diisopropylphenyl groups.
Properties of Carbenes
1. Structural Properties of Carbenes
Carbenes are classified into singlet and triplet carbenes according to the spin state of their valence electrons.
Singlet carbenes are classified into the following two categories according to the form of hybridization on carbon. In most cases, the former sp2 hybridization form is more stable.
- sp2 hybridized: 2 electrons in each of the 3 sp2 orbitals, leaving one empty p orbital
- sp3 hybridized: 2 electrons in each of the 3 sp3 orbitals with one empty sp3 orbital remaining
There are two types of triplet carbenes: sp2 hybridized and sp3 hybridized. In the latter type, two of the four sp3 orbitals are filled with two electrons each, and the remaining two sp3 orbitals are occupied by one electron of the same spin.
Which of these structures is more stable depends on factors such as the electrical and structural nature of the substituents on the carbon. In general, singlet carbenes often exhibit electrophilic reactivity, while triplet carbenes exhibit radical reactivity due to unpaired electrons.
2. Preparation of Carbenes
One of the methods for the formation of carbenes is the desorption reaction of nitrogen molecules from diazo compounds. In most cases, the reaction is triggered by heat, light, or a catalyst.
Dichlorocarbenes are also known to be formed by the action of a strong base on chloroform. The intermediate in this reaction is the trichloromethyl anion.
Imidazolidene, formed by the reaction of an imidazolium salt with a base, is another well-known carbenes species. This molecular species, formed by the desorption of a proton from the imidazole ring, is strongly stabilized by the nitrogen atoms on both sides. For this reason, the systematic species of imidazolidinylidene is used as a ligand in many transition metal catalysts.
Types of Carbenes
Carbenes, as mentioned above, are not stable molecular species and are therefore essentially not commercially available. If one wishes to use them, one must purchase precursors and prepare them for use.
Typical examples used in synthetic chemistry include the following:
- Carbenes species obtained by desorbing N2 molecules from α-diazoketones (used in the Wolf rearrangement)
- Dichlorocarbenes (obtained from chloroform and a strong base via trichloromethyl anion)
- N-Heterocyclic Carbenes (NHC) and their metal complexes (obtained by the reaction of imidazolium salts with a base)