Carbon atoms valence-unsaturated

Cyclohexadienyl- (the unsaturated carbons are given numbers as low as possible, numbering from the carbon atom with the free valence given the number 1)... 

Apparently, the cohesive energy of these clusters shows a very slow convergence with the size of the molecule. This should not be surprising, since the number of unsaturated valences "dangling bonds" per carbon atom is one in 1,1/2 in 2 and 1/3 in 3. 

An alternative method of calculation of unsaturation involves substitution of heteroatoms with hydrocarbon moieties. All the elements with valence 1 are substituted for CH3 groups, all the elements with valence 2 for CH2 groups, all the elements with valence 3 for CH groups, and all the elements with valence 4 for C. The formula obtained is compared with that of the alkane with the same number of carbon atoms. The difference between the number of hydrogen atoms in the alkane and in the sample divided by 2 gives the R value. For the above examples one will get ... 

General. Continuous-chain alkanes are the sole parent hydrides for all compounds, the skeleton of which is composed of chains. Cyclic parent hydrides are more diverse. In nomenclature, they are classified according to their structure as carbo-cycles (composed of carbon atoms only) and heterocycles (composed of carbon atoms and other atoms, such as N, O and Si). They are also classified as saturated and unsaturated. Saturated cycles have the maximum number of hydrogen atoms attached to every skeletal atom, as judged by a prespecified valence unsaturated cycles have fewer hydrogen atoms and multiple bonds between pairs of atoms. 

Hydrocarbons. Compounds consisting of carbon and hydrogen. The number of such compounds is immense and they may be classified into aliphatic (alicyclic) and aromatic (cyclic) compounds. In the former class, the principal carbon atoms are arranged in chains, while in the latter class they are arranged in one or several rings. Hydrocarbons may also be divided into saturated —in which all four valences of C are satisfied, and unsaturated —in which there are one or more double or triple bonds between carbon atoms... 

The names of univalent radicals derived from unsaturated monocyclic hydrocarbons have the endings -enyl , -ynyl , -dienyl , etc., the positions of the double and triple bonds being indicated according to the principles of Rule A-3.3. The carbon atom with the free valence is numbered as 1, except as stated in the rules for terpenes (see Rules A-72 to A-75). 

Names of bivalent radicals derived from saturated or unsaturated monocyclic hydrocarbons by removal of two atoms of hydrogen from the same carbon atom of the ring are obtained by replacing the endings -ane , -ene , -yne , by -ylidene , -enylidene and -ynylidene , respectively. The carbon atom with the free valences is numbered as 1, except as stated in the rules for terpenes. 

At the so-called radical center an organic radical R- has an electron septet, which is an electron deficiency in comparison to the electron octet of valence-saturated compounds. Carbon atoms are the most frequently found radical centers and most often have three neighbors (see below). Carbon-centered radicals with their electron septet occupy an intermediate position between the carbenium ions, which have one electron less (electron sextet at the valence-unsaturated C atom), and the carbanions, which have one electron more (electron octet at the valence-unsaturated C atom). Since there is an electron deficiency present both in C radicals and in carbenium ions, the latter are more closely related to each other than C radicals are related to carbanions. Because of this, C radicals and carbenium ions are also stabilized or destabilized by the same substituents. 

The carbocations considered here have three, five, seven, nine or eleven carbon atoms. In their comprehensive review Leone et al. (1973) point out that carbocations with the empirical formula (CH) , where n is odd, are related to the unsaturated hydrocarbons with the formula (CH) , where m = 1. By the mathematical treatment developed by Balaban (1966), all valence isomers of (CH) hydrocarbons can be derived. Thus through this... 

Trimethylenemethane achieves at its central carbon atom the maximum bond order and minimum free valence possible for an unsaturated carbon atom. This highly unstable molecule can be stabilized as the iron... 

With the aid of these structures we may obtain an approximate evaluation of the relative unsaturation of the various atoms. This is shown in XII where the figures represent the number of valence bond structures in which a particular carbon atom is involved in a Dewar type bond... 

Analogous structures will contribute to the resonance of AlC HgS, PHg GuHal and PH3 AlHalg. In the complexes formed between aluminium chloride and unsaturated hydrocarbons, valence bond structures are possible in which the aluminium becomes negatively charged with respect to a carbon atom, i.e. 

The formation of addition products with ethylene, indicating the unsaturated character of the molecule, would naturally lead to the idea that the structural formula of the compound, in accordance with its relation to ethane and in accordance with our assumption that carbon in organic compounds is tetravalent, should be that of ethane with one valence bond of each carbon atom free and unsatisfied, —CH2—CH2—. 

The reaction between trialkyl phosphites and a,P-unsaturated nitriles has been less thoroughly investigated. Whereas the hydrophosphonylation of unsaturated nitriles with diakyl phosphites proceeds smoothly, the addition of trialkyl phosphites requires more severe conditions. It involves the nucleophilic attack by the trivalent phosphorus reagent at the tenninal carbon atom of the conjugated system followed by valency expansion of phosphorus in agreement with the Michaelis-Arbuzov mechanism (Scheme 6.27). - ... 

A single bond/ shown by a short stroke, or often by a dot, indicates the mutual ci gcmcnt, or satisfying, of one of the four valencies of each of the two carbon atoms concerned. In a double bond, two, and in a triple bond, three, of the valencies of each carbon atom enter into a similar alliance. Double and triple bonds are evidence of intramolecular strain, and hence of enhanced reactivity as compared with single bonds and molecules containing them are said to be unsaturated. ... 

---