**2. Nomenclature of carbenes**

Today, the term carbene is used for divalent carbon species, and they are commonly referred to together with substituents. Substituents are given first, and the word carbene is added to the end.

If the divalent carbon atom is in a ring or the carbene electrons are on a carbon– carbon double bond carbon atom, these carbenes are named by the suffix -*yilidene* [7].

## **3. Structure and reactivity**

Generally, carbenes have two bonding electrons (both in sp<sup>2</sup> -orbitals) and two non-bonding electrons. There are two classes of carbenes called singlet or triplet carbene depending on whether the non-bonding electrons are in the same or different orbitals, respectively as shown in **Figure 1**. Carbenes usually contain sp<sup>2</sup> hybridized carbon atoms according to the valance bond theory. Two of the three sp2 -hybrid orbitals bond with their carbene substituents by covalent bonds and two vacant orbitals remain, consisting of the sp<sup>2</sup> -hybrid orbital and the p orbital. Two non-bonding carbene electrons must be placed in these vacant orbitals. If two electrons are placed in the same orbital, this carbene is called a singlet carbene, since the electron spins will be in the opposite directions. When electrons are placed in different orbitals, parallel spin will be preferred according to Hund's law, and the formed carbene is called triplet carbene [7–11].

**Figure 1.** *Electronic structures of singlet and triplet carbene.*

#### *Basic Information about Carbenes DOI: http://dx.doi.org/10.5772/intechopen.100425*

Triplet carbenes have an angular structure, as well as a linear structure. In carbene, which has a linear structure, the carbon atom to which the substituents are attached makes the sp. hybridization and there are two vacant p orbitals in the molecule. Since the energy levels of these orbitals will be equal, according to Hund's law, carbene electrons are placed in these orbitals one by one and carbene gains the triplet property [7–11].

Generally in most of the organic compounds, the singlet state is more stable than the triplet state. As a result the ground states of these molecules are singlet. In these molecules triplet state occurs only as excited or high energy level. On the contrary, studies show that carbenes usually have an angular structure and their electronic structure is triplet in the ground state because triplet carbenes have lower energy so they are more stable than singlet carbenes. Energy difference between singlet and triplet carbene is 8 kcal/mol.

The nature of substituents affects the electronic properties of carbenes. If the substituents attached to the carbene carbon are electron withdrawing groups, the carbene prefers the singlet structure. Electron withdrawing groups inductively stabilize the σorbital attached to the carbene carbon, increasing the energy difference between the σ- and π-orbital. Thus, electrons place in the σ-orbital, leaving the π-orbital empty. Conversely, if the substituents attached to the carbene carbon are electron donating groups via the σ-bond, the carbene prefers the triplet structure [7–11].

In addition, if the atoms attached to the carbene carbon have non-bonding electron pairs (nitrogen, oxygen, sulfur, halogen, etc.), these atoms easily donate their electrons to the vacant p-orbital of carbene. Thus, this π-donor atoms stabilize the singlet state by resonance structure and then carbenes prefer the singlet configuration in the ground state. For example, dichlorocarbene is singlet in the ground state. As shown in **Figure 2**, electrons on the chlorine atom are conjugated with the carbene atom, increasing the stability of the carbene [7–12].

**Figure 2.** *Stabilization of dicholorocarbene singlet state by π-donation (mesomeric effect).*

Unstabilized carbenes usually have triplet ground states due to their stability, [9] but lone pair donating substituents can reverse this situation. As a consequence, electronic substituent effects, typically π-donation (such as -NR2, -OR, -SR, -F, -Cl, -Br, -I) and π -acceptance (such as -COR, -SOR, -SO2R, -NO, -NO2) [13], as well as hyperconjugation (by alkyl groups) [14] and electronegative substituents [15] mainly stabilize the singlet state. Moreover, electropositive substituents with at least one atom having non-bonding electron pair give singlet carbene [16].

The nature of the substituents affect the chemical reactivity of carbenes as well as their electronic structure. Since carbenes are electron deficient intermediates (the carbon atom having only six electron in its outer shell), they show electrophilic behavior in their reactions. Naturally, when electron withdrawing groups are attached to these carbenes, the electrophilicity of the carbene increases. Nevertheless, if very strong π-donor substituents are attached to the carbene intermediate then it behaves as nucleophile in its reactions. For example, diaminocarbenes (**Figure 3**) are nucleophilic singlet carbenes because of π-donation of substituent [1, 7].

**Figure 3.** *Example of diaminocarbenes (nucleophilic singlet carbene).*

Another example of nucleophilic singlet carbenes is the cycloheptatrienylidene molecule. Since the vacant p-orbital located on the carbene atom participates in the delocalization of π-system of the seventh ring, the carbene electrons have to be placed in the sp<sup>2</sup> hybrid orbital on the ring plane. Therefore, cycloheptatrienylidene is a singlet and nucleophilic carbene [1, 7].

It is not possible to observe carbenes under normal conditions. However, by using the matrix isolation method carbenes formed by photolysis of diazo compounds at 4–77 K in frozen argon or cyclohexane can be observed by IR or ESR (Electron Spin Resonance) spectroscopy [1]. Since ESR is a spectroscopic method based on electron spin, triplet carbenes can be observed with this method. Triplet carbenes, which act like diradicals, can be observed with ESR due to these properties. ESR not only clearly defines triplet carbenes, but also gives information about the molecular and electronic structures of carbenes [17–20]. For example, with this method, it was determined that triplet methylene and diphenylcarbene were angular and these angles were 136° and 142°, respectively.

Carbenes can be stabilized by steric or electronic effects [9]. As a result of the studies carried out in the light of this idea, some stable carbene molecules can be isolated at room temperature as shown in **Figure 4**. In 1988, Bertrand [21] et al reported the synthesis of the (phosphino)(silyl)carbene (first isolated carbene) **1** which can remain stable for weeks at room temperature. This compound, obtained by the decomposition of diazo compounds, was isolated by vacumm distillation (10<sup>2</sup> Torr) at 75–80°C as a red oily material in 80% yield. It has all the typical reactivity associated with "classic" carbenes [22]. A few years later the first crystalline carbene was reported by Arduengo and co-workers [23]. This discovery catalyzed research activities of carbene. The 1,3-diadamantylimidazol-2-ylidene **2** can easily synthesized in 96% yield under an inert atmosphere even at room temperature. Bond angle of 1,3-diadamantylimidazol-2-ylidene, a colorless and crystalline compound with a melting point of 240°C, was determined as 102° by X-ray analysis. There are two main factors that stabilize this molecule. Nitrogen atoms attached to the carbene atom stabilize the carbene electronically, while adamantane groups attached to the nitrogen atoms make it sterically stable.

**Figure 4.** *The first isolated carbenes. iPr = isopropyl, Ad = adamantyl.*
