Carbenes

Carbenes are compounds of divalent carbon . These are unstable highly reactive compounds with six valence electrons and the general formula R ¹ R ² C : (two substituents and an electron pair ). There are two states of carbenes: singlet and triplet . Most carbenes exist for a very short time, although stable carbenes are also known. Carbenes can be fixed in an argon matrix at very low temperatures. The names of carbenes are derived from the name of the corresponding valence-saturated compound with the replacement of the endings "-an", "-en", "-in" with "-ilidene", "-enilidene", "-inilidene", respectively. Substitution nomenclature is also used, for example, diphenylcarbene, and for the particle CH ₂ : the name "methylene" is retained.

Content

Properties

Singlet and triplet carbenes

Singlet carbene is called one in which two non - binding electrons with paired spins are on the same orbital . In the triplet form of carbene, two unpaired electrons with parallel spins are in two orbitals of the same energy. The consequence of various electronic configurations is the diamagnetism of singlet carbene and the paramagnetism of triplet. The structure of triplet carbene can be studied using EPR spectroscopy . In addition, singlet and triplet carbenes have different geometries and different chemical activity.

Getting carbenes

Historically, the first method for generating carbenes is the elimination of hydrogen halide from trihalomethanes under the action of strong bases. Using this method, the first carbene, dichlorocarbene (J. Hein 1950 ) , was obtained as an intermediate .

{\mathsf {CHCl_{3}+C_{4}H_{9}OK}}\rightarrow {\mathsf {CCl_{2}\!\!:+\ C_{4}H_{9}OH+KCl}}

{\ displaystyle {\ mathsf {CHCl_ {3} + C_ {4} H_ {9} OK}} \ rightarrow {\ mathsf {CCl_ {2} \! \!: + \ C_ {4} H_ {9} OH + KCl}}}

In addition, there are other methods for producing carbenes. The simplest carbene can be obtained by decomposition of ketene :

{\mathsf {CH_{2}\!\!=\!\!C\!\!=\!\!O}}\rightarrow {\mathsf {CH_{2}\!\!:+\ CO}}

{\ displaystyle {\ mathsf {CH_ {2} \! \! = \! \! C \! \! = \! \! O}} \ rightarrow {\ mathsf {CH_ {2} \! \!: + \ CO}}}

The most common method for generating carbenes is the photolytic, thermal, or transition metal catalyzed decomposition of aliphatic diazocompounds. So, as a result of the decomposition of diazomethane, a simple carbene is formed:

{\mathsf {CH_{2}\!\!=\!\!N\!\!=\!\!N}}\rightarrow {\mathsf {CH_{2}\!\!:+\ N_{2}}}

{\ displaystyle {\ mathsf {CH_ {2} \! \! = \! \! N \! \! = \! \! N}} \ rightarrow {\ mathsf {CH_ {2} \! \!: + \ N_ {2}}}}

The thermal or photochemical decomposition of tosyl hydrazones, derivatives of carboxylic acids , is a similar method for generating mono- and dialkylcarbenes. In this case, diazo compounds act as intermediates.

Stable Carbenes

Imidazoline derivatives having bulky substituents on two nitrogen atoms were the first free stable carbenes. Later, 1,3-dimethylimidazolinylidene-2 was also obtained without bulky substituents. These compounds are sensitive to moisture and air: they attach water and oxygen molecules to a divalent carbon atom. It has now been proposed to use these carbenes as very strong bases in non-aqueous media.

Links

Carbenes and carbenoids - Chemistry Department of Moscow State University

Literature

Chemical Encyclopedia / Editorial Board: Knunyants I.L. et al. - M .: Soviet Encyclopedia, 1990. - T. 2 (Daf-Med). - 671 p. - ISBN 5-82270-035-5 .