THE CARBON ATOM

The carbon atom is central to all organic compounds. The atomic number of carbon is 6, and its electron structure is ls 2s 2p. Two stable isotopes of carbon exist C-12 and C-13. In addition, carbon has several radioactive isotopes, C-14 being the most widely known of these because of its use in radiocarbon dating. 

With four valence electrons, the carbon atom ( C ) forms four single covalent bonds by sharing electrons with other atoms. The structures of methane and carbon tetrachloride illustrate this point  

Carbon-carbon bonds are formed because carbon atoms can share electrons with other carbon atoms. One, two, or three pairs of electrons can be shared between two carbon atoms, forming a single, double, or triple bond, respectively  

Each dash represents a covalent bond. Carbon, more than any other element, has the ability to form chains of covalently bonded atoms. This bonding ability is the main reason for the large number of organic compounds. Three examples are shown here. It s 

Electrons are found in regions around the nucleus in an atom, and those regions are called orbitals. The orbitals can be defined and differentiated by size, shape, and orientation. Valence electrons are electrons that are found in the outermost shell. The carbon atom has four valence electrons. These valence electrons are involved in chemical reactions and bonding. 

Ionic bond - Ionic bond is formed between an electropositive and electronegative atom (ion), or generally we can define it as an attractive force between a positive and a negative ion (e.g., KCl). 

Covalent bond - Covalent bond is formed by the sharing of a pair of electrons between two atoms. Carbon compounds generally contain covalent bonds. 

For a more detailed discussion of this topic see the general chemistry section of this book - Chapter 5. 

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Later findings have modified the rule, especially when the carbon atoms concerned are connected with carbonyl- or carboxyl-groups, but it still holds good for monohydric unsaturated alcohols. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Covalent bonds are formed by the sharing of electrons. Thus the carbon atom, with four equivalent electrons shares with the electrons from four hydrogen atoms. 

The carbon atom has a share in eight electrons (Ne structure) whilst each hydrogen atom has a share in two electrons (He structure). This is a gross simplification of covalent bonding, since the actual electrons are present in molecular orbitals which occupy the whole space around the five atoms of the molecule. 

Using Infrared Spectrometry to Characterize Petroleum Fractions according to the Nature of the Carbon Atoms... 

Figure 5.13 shows the way in which the molecules are visualised, their chemical symbol, and the names of the first three members of the series. The carbon atom has four bonds that can join with either one or more carbon atoms (a unique-property) or with atoms of other elements, such as hydrogen. Hydrogen has only one bond, and can therefore join with only one other atom. 

Beyond propane, it is possible to arrange the carbon atoms in branched chains while maintaining the same number of hydrogen atoms. These alternative arrangements are called isomers, and display slightly different physical properties (e.g. boiling point, density, critical temperature and pressure). Some examples are shown below ... 

Fig. XVII-18. Contours of constant adsorption energy for a krypton atom over the basal plane of graphite. The carbon atoms are at the centers of the dotted triangular regions. The rhombuses show the unit cells for the graphite lattice and for the commensurate adatom lattice. (From Ref. 8. Reprinted with permission from American Chemical Society, copyright 1993.)...

Coulomb potential felt by a 2p orbital s electron at a point r in the ls 2s 2p 2p configuration description of the carbon atom is ... 

The above mean-field potential is used to find the 2p orbital of the carbon atom, which is then used to define the mean-field potential experienced by, for example, an electron in the 2s orbital ... 

To improve upon die mean-field picture of electronic structure, one must move beyond the singleconfiguration approximation. It is essential to do so to achieve higher accuracy, but it is also important to do so to achieve a conceptually correct view of the chemical electronic structure. Although the picture of configurations in which A electrons occupy A spin orbitals may be familiar and usefiil for systematizing the electronic states of atoms and molecules, these constructs are approximations to the true states of the system. They were introduced when the mean-field approximation was made, and neither orbitals nor configurations can be claimed to describe the proper eigenstates T, . It is thus inconsistent to insist that the carbon atom... 

This is a relatively rare structure, diamond being probably the best known example. Here, the carbon atoms are not close-packed. Each carbon is surrounded tetrahedrally by four other carbon atoms (Figure 2.1). Clearly, each carbon is exerting a tetrahedrally directed... 

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

Only the carbon atom can gain four electrons this only happens when it is combined with extremely electropositive elements and this state may be regarded as exceptional. Bonding in carbides is almost invariably predominantly covalent. 

Pure carbon occurs naturally in two modifications, diamond and graphite. In both these forms the carbon atoms are linked by covalent bonds to give giant molecules (Figure S.2). 

The carbon atoms of the double bond have a trigonal planar configuration and free rotation about the C—C bond is prevented by the n bond. The inability to rotate means that geometrical isomers can be produced, with substituents a and b, thus ... 

This structure indicates that carbon monoxide should have donor properties, the carbon atom having a lone pair of electrons. Carbon... 

The interstitial carbides These are formed by the transition metals (e.g. titanium, iron) and have the general formula M, C. They are often non-stoichiometric—the carbon atoms can occupy some or all of the small spaces between the larger metal atoms, the arrangement of which remains essentially the same as in the pure metal (cf. the interstitial hydrides). 

Alternatively the ion exchanger may be a synthetic polymer, for example a sulphonated polystyrene, where the negative charges are carried on the —SO3 ends, and the interlocking structure is built up by cross-linking between the carbon atoms of the chain. The important property of any such solid is that the negative charge is static—a part of the solid—whilst the positive ions can move from their positions. Suppose, for example, that the positive ions are... 

Cobalt compounds have been in use for centuries, notably as pigments ( cobalt blue ) in glass and porcelain (a double silicate of cobalt and potassium) the metal itself has been produced on an industrial scale only during the twentieth century. Cobalt is relatively uncommon but widely distributed it occurs biologically in vitamin B12 (a complex of cobalt(III) in which the cobalt is bonded octahedrally to nitrogen atoms and the carbon atom of a CN group). In its ores, it is usually in combination with sulphur or arsenic, and other metals, notably copper and silver, are often present. Extraction is carried out by a process essentially similar to that used for iron, but is complicate because of the need to remove arsenic and other metals. 

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

Usually only the indices of the carbon atoms are written, not the symbols hydrogen atoms can have, but do not need, an atom number... 

In this simplified example of phenylalanine, in the first iteration the methyl groups arc given a value of I in the first classification step because they contain a primaiy C-atom, The methylene group obtains a value of 2, and the methine carbon atom a value of 3. In the second step, the carbon atom of the methyl group on the left-hand side obtains an extended connectivity (EC) value of 2 because its neighboring atom had a value of 2 in the first classification step. 

The c arlrnn atoms of the other two methyl groups (on the right-hand side) obtain an EC value of 3 because they arc adjacent to the racthinc carbon atom. The carbon atom of the methylene group obtains an EC value of 4 in the second rcla.xation process, as the sum (1 + 3) of the eonncctivity values of its neighboring atoms in the first iteration. 

Hendrickson (14,15] concentrated mainly on C-C bond-forming reactions because the construction of the carbon atom skeleton is the major task in the synthesis of complex organic compounds. Each carbon atom is classified according to which kind of atoms are bonded to it and what kind of bonds ([Pg.184]

This example again emphasizes that Hendridcson only considered the bond changes at the carbon atoms of a reaction. 

Figure 3-22 shows a nucleophilic aliphatic substitution with cyanide ion as a nucleophile, i his reaction is assumed to proceed according to the S f2 mechanism with an inversion in the stereochemistry at the carbon atom of the reaction center. We have to assign a stereochemical mechanistic factor to this reaction, and, clearly, it is desirable to assign a mechanistic factor of (-i-1) to a reaction with retention of configuration and (-1) to a reaction with inversion of configuration. Thus, we want to calculate the parity of the product, of 3 reaction from the parity of the... 

Once these approximations have been made, HMO theory becomes very simple. Using 1,3-butadiene, 1, as an example, we can work through an HMO calculation in order to outline the process involved. Firstly, we assign numbers to the carbon atoms, as shown in Figure 7-19. 

We can now assign the four carbon p-orbitals, one to each carbon. For simplicity, we will label them with the subscript corresponding to the number of the carbon atom to which the AO belongs. We will use the symbol p to denote AOs and P for MOs. We can now write the Hiickel matrix as a square matrix involving the AOs as shown in Figure 7-20. 

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