Atomic carbon

Nomenclature

The trivial name monocarbon is the preferred IUPAC name. The systematic names, methanediylidene and carbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively.

Methanediylidene is viewed as methane with all four hydrogen atoms removed. By default, this name pays no regard to the radicality of the atomic carbon. Although, in even more specific context, it can also name the non-radical excited states, whereas the radical ground state with two unpaired electrons is named methanediylylidene.

Radicality

Many of atomic carbon's electronic states lie relatively close to each other, giving rise to varying degrees of radical chemistry. The ground state is a triplet radical with two unpaired electrons (X³P₀), and the first two excited states are a singlet non-radical (a¹D₂) and a singlet radical with two unpaired electrons (b¹S₀). A sample of atomic carbon exists as a mixture of electronic states even at room temperature, giving rise to complex reactions. For example, reactions of the triplet radical with non-radical species generally involves abstraction, whereas reactions of the singlet non-radical involves not only abstraction, but also insertion or addition.

[C]^2•(X³P₀) + H
2O → [CH]^• + [HO]^• [C](a¹D₂) + H
2O → [CO] + H
2 or H
2CO

Amphotericity

Atomic carbon accept an electron-pair donating ligand into the molecule by adduction:

[C] + :L → [CL]

Because of this acceptance of the electron-pair donating ligand, atomic carbon has Lewis-acidic character. Atomic carbon can accept two such ligands. An example is C(CO)
2.

The methylylidyne group can also donate an electron-pair to an accepting centre by adduction:

M + [C] → [MC]

Because of this donation of the electron-pair, atomic carbon has Lewis-basic character. An example is [Au
6C(PPh
3)
6]2+
. Since a Lewis base is also Brønsted base, atomic carbon can in theory be protonated to form a conjugate acid, which is methyliumylidene or hydridocarbon(+) (CH+
). Atomic carbon is not stable in aqueous solution, as it is rapidly oxidized to form carbon monoxide or formaldehyde.

Chemical reactions

Atomic carbon is highly reactive, most reactions are very exothermic. They are generally carried out in the gas phase at liquid nitrogen temperatures (77 K). Typical reactions with organic compounds include:

Insertion into a C-H bond in alkanes to form a carbene Deoxygenation of carboxyl groups in ketones and aldehydhdes to form a carbene, 2-butanone forming 2-butanylidene. Insertion into carbon -carbon double bonds to form a cyclopropylidene which undergoes ring-opening, a simple example being insertion into an alkene to form a cumulene.

With water insertion into the O-H bond forms the carbene, H-C-OH that rearranges to formaldehyde, HCHO.

Normally, a sample of atomic carbon exists as a mixture of excited states in addition to the ground-state in thermodynamic equilibrium. Each state contributes differently to the reaction mechanisms that can take place. A simple test used to determine which state is involved is to make use of the diagnostic reaction of the triplet state with O₂, if the reaction yield is unchanged it indicates that the singlet state is involved. The diradical ground-state normally undergoes abstraction reactions. Atomic carbon has been used to generate "true" carbenes by the abstraction of oxygen atoms from carbonyl groups:

R₂C=O + :C: → R₂C: + CO

Carbenes formed in this way will exhibit true carbenic behaviour. Carbenes prepared by other methods such as diazo compounds, might exhibit properties better attributed to the diazo compound used to make the carbene (which mimic carbene behaviour), rather than to the carbene itself. This is important from a mechanistic understanding of true carbene behaviour perspective.

Production

This very short lived species is created by passing a large current through two adjacent carbon rods, generating an electric arc. Atomic carbon is generated in the process. Professor Phil Shevlin has done the principal work in the field based at Auburn University in the USA.

The way this species is made is closely related to the formation of fullerenes C₆₀, the chief difference being that a much lower vacuum is used in atomic carbon formation.

Atomic carbon is generated in the thermolysis of 5-diazotetrazole upon extrusion of 3 equivalents of dinitrogen:

CN₆ → :C: + 3N₂