Carbon Bivalent

The mechanism of the reaction probably involves the production of bivalent carbon during the initial loss of nitrogen the group R shifte from an adjacent position to this carbon leading to the production of a keten the latter then reacts with the solvent to give an acid, an amide or an ester. 

In many of its reactions hydrocyanic acid behaves as the nitrile of formic add H.CN. Many facts, in particular its great chemical and pharmacological similarity to the isonitriles >C = NR suggest another constitution, namely, that of carbimide >C =NH with bivalent carbon. The addition reactions of the nitriles (see above), which reactions are also characteristic of hydrocyanic acid, can equally well be explained on the basis of this second structural formula. In the nitrile form it is at the triple bond between carbon and nitrogen that addition takes place. In the methylene form this occurs at the two free valencies of the bivalent carbon atom, e.g. ... 

In a similar way fulminic acid, another derivative of bivalent carbon, which has been accurately investigated and for which the carboxime >C NOH constitution has been established, is decomposed into formic acid and hydroxylamine (see experiment on p. 159). 

The bond-energy values are not designed for use with a molecule as unconventional as carbon monoxide, containing bivalent carbon. It seems probable, however, that the error involved in this application is not great. 

The first of these formulae seems less suitable than the second for these war gases, which like the mono- and di-halogenated derivatives of acetylene (see p. 45), have properties more in keeping with the presence of a divalent carbon atom. It may be concluded that it is the presence of this divalent carbon atom rather than that of the nitrogen atom which accounts for the toxicity of this radicle. The bivalent carbon atom has in fact great chemical reactivity and is the point of attack in all chemical and biochemical reactions. 

Bivalent Carbon.—Recent work on some derivatives of methane containing phenyl groups, related to the hydrocarbon benzene to be considered in Part II, have led to the view that carbon is not always tetravalenl but that it may be trivalent or bivalent. If in the isocyanides nitrogen remains trivalent then carbon must become bivalent or else... 

This relationship has already been discussed in connection with its bearing on the question of the existence of bivalent carbon in the isocyanides. 

Bivalent Carbon.—This reaction supports the constitution as given and the view that in this compound we have a bivalent carbon atomr. 

Whether we consider, in this reaction, that the bivalent carbon of carbon monoxide is changed by the addition of two chlorine atoms to tetravalent carbon of carbonyl chloride, or that carbon monoxide is an unsaturated compound of tetravalent carbon which by addition of two chlorine atoms forms the saturated compound carbonyl chloride, or that in carbon monoxide oxygen is tetravalent and becomes bivalent in carbonyl chloride, whichever view is held, it is undoubtedly the fact that in carbonyl chloride the carbon is tetravalent and the constitution is... 

Dichlorocarbene is a highly reactive intermediate of bivalent carbon with only six valence electrons around the carbon. It is electrically neutral and a powerful electrophile. As such it reacts with alkenes, forming cyclopropane derivatives by cis-addition to the double bond. 

Calorimetric data indeed confirm this interaction (66). Since the adsorption of carbon dioxide does not modify the electrical conductivity of the oxygen-precovered sample, it does not seem that a bivalent carbonate species [COj fads)] is formed. Moreover, the reactivity toward carbon monoxide of the products of interactions (5a) or (8) between preadsorbed carbon monoxide and oxygen or between preadsorbed oxygen and carbon dioxide, being identical, we believe that, in both cases, the same surface species C08-(ads) is formed. During the catalytic reaction, however, interaction between adsorbed oxygen and gaseous carbon dioxide is unimportant since all carbon dioxide in the gas phase is condensed in a cold trap placed near the catalyst. 

Isonitriles are a unique class of organic compounds, as they are the only stable derivatives (apart from carbon monoxide) containing formally a bivalent carbon. Their discovery by Lieke, Gautier, and Hofmann dates back more than 130 years, but up to the 1960s only a relatively small number of studies concerning their chemistry had been reported [1], This was probably because of the virtual absence of convenient, generally applicable synthetic methods, as well as their very penetrating, unpleasant smell. 

Chapman6 has pointed out that bivalent carbon can fix both COa and Na according to reactions (2) and (3). 

Many reactions of organic compounds can best be interpreted by the assumption that carbon may, under certain circumstances, exist in the bivalent condition. We have a striking case in carbon monoxide. Although the graphic symbol of this compound may be written C = 0, its properties are better in accord with the view that it contains a bivalent carbon atom, C = 0. For example, it reacts with chlorine and forms carbonyl chloride in which the chlorine atoms are joined to carbon —... 

The poisonous properties of the hydrocyanic acid and the isocyanides may be associated, as in the case of carbon monoxide, with the presence of a bivalent carbon atom. 

The results of experimental observations and molecular orbital calculations indicate that many carbenes have a nonlinear triplet ground state. Exceptions are the dihalocarbenes and carbenes with oxygen, nitrogen or sulfiar atoms attached to the bivalent carbon, all of which are singlets. The singlet and triplet states of a carbene do not necessarily show the same chemical behaviour. For example, addition of singlet carbenes to olelinic double bonds to form cyclopropane derivatives is more stereoselective than addition of triplet carbenes. 

There were three main explanations of the constitution of unsaturated compounds (i) bivalent carbon, (ii) carbon atoms with unsaturated units of affinity, (iii) two carbon atoms linked by two combining units (a double bond). Couper proposed (i) and Wurtz thought it possible, but most chemists followed Kekule s assumption that carbon is always quadrivalent. Kolbe thought that carbon C2 is not an absolutely quadrivalent (vierwertig) element but also diatomic (even triatomic in oxalic acid zYLO-C O He formulated ethylene... 

The valency of an atom is equal to the number of atoms in direct union with it. In C2H2 there are two bivalent carbon atoms, in benzene six tervalent carbon atoms, each linked to an atom of hydrogen. In formulae Lossen uses only single bonds between atoms. The valency of a radical is a number expressing how many atoms, not belonging to the radical, are directly bound to the atoms which compose the radical. A radical is an atomic complex contained in a molecule, all the constituents of which are linked either directly or indirectly . Each part of a molecule may be called a radical, but usually only those atomic aggregates are called radicals which have the character just described. Lossen s... 

He showed that sodium salts of aliphatic primary and secondary nitrocompounds react with acids to form carbonyl compounds 2R2CHNO2 = 2R2C0+N2O+ H2O. Nef supported the theory of bivalent carbon. In the preparation of the explosive mercuric salt of nitromethane, he found that mercury fulminate is formed and regarded fulminic acid as C N OH, the reaction being Hg(CH2 N0 0)2==Hg(0-N C)2 + 2H20. He prepared the explosive addition compound of fulminic acid and hydrogen chloride, HON CHCl. In researches on acetylene compounds he prepared explosive di-iodoacetylene, IC-CI (he formulated it IgC.-C). He discovered the reaction between sodium acetylene and a ketone, followed by hydrolysis, to form an acetylenic carbinol ... 

Carbenes are chemical species that possess a bivalent carbon atom with two nonbonding electrons. The divalent carbenes were considered as highly reactive intermediates, and the isolation of stable carbenes has been a challenge for a long time. Based on Wanzlick et al. s work in 1960, Arduengo et al. [1] reported the synthesis of stable imidazolium carbenes in 1991. Independently, Bertrand and coworkers [2] reported the synthesis of phosphinocarbene in 1988. In 1995, Enders et al. [3] reported the synthesis of triazolium carbenes. 

A number of fluorinated silanes have been obtained in the course of work on bivalent carbon species [MeaSi CHj CH CH, —rM CBig-CF.. a ... 

These new agents for transfer of bivalent carbon react with olefins within minutes at 80 °C to give cyclopropanes, generally in high yields. 

---