Structural carbon atoms

As seen in the above structures, carbon atoms with exocyclic C=W bonds (where W symbolizes an electron-withdrawing group such as O, NR, or S) count as Z-type atoms. Indeed, 4//-pyrone 56 and tropone 57 (Scheme 27) may be considered to be normal... 

In view of its electronic structure, carbon atoms in graphite are strongly bonded in planes, and only weak van der Waals forces hold the planes together. This leads... 

Very hard carbides result from strongly heating the oxides with carbon. They are empirically MC and have the NaCl structure. Vanadium also forms V4C3 which has a defect structure, carbon atoms being missing from some of the lattice positions. 

Compare the information provided by (a) the space-filling model and (b) the Haworth structure. Carbon atoms are green, oxygen atoms are red, and hydrogen atoms are white. 

Figure 3. p-structures of hexagonal graphite C3N4 (a) odd-phase structure, (b) the even-phase structure, and (c) the C2N4-C4N4 planar structure. Carbon atoms are shown in black. 

Figure 3.4. Structural representations of glucose, the major sugar and the primary source of energy of higher animals that gives rise to 36 ATP molecules during its oxidation to carbon dioxide and water. In the Fischer projection, the central structure, carbon atoms 2, 3, 4, and 5 each contains four different substituents, making them asymmetrical centers.

In bond-line structures, carbon atoms and most hydrogen atoms are not drawn. 

This produces a capability for covalent bonding that is unique. In all organic compounds—from the very simple to those that possess a complex network structure—carbon atoms always bond four times. While there may be single bonds, double bonds, and triple bonds involved, the total number of bonds to any carbon atom is always four—-no more and no less—because of the four unpaired electrons resulting from hybridization. 

Fig. 10.17. Left is the fullerene structure of C 60 and right shows the face-centered unit cell with fullerene structured carbon atoms at the eight corners and at the center of the six faces...

Fig. 3.1 A schematic representation of the structure of a bulk hexagonal graphite crystal showing the bulk unit cell. Side insets. Top view of the basal plane of graphite and a schematic representation of the surface structure (carbon atoms) of graphite, where every other atom is enhanced (right-side inset) and viewed under ideal conditions, and where every single atom is seen (left-side inset). Figure reproduced from Ref. [3]...

ZrC is typical transition metal carbide with the NaCl ground-state crystal stmcture. In the cubic close packed structure, carbon atoms are filled in the octahedral interstices formed by the Zr atoms. The Zr and C atoms are located at the positions of the 4a (0 0 0) and 4b (1/2 1/2 1/2), respectively. If all the octahedral sites are occnpied by C atoms, the stoichiometric monocarbide is formed. In reality, however, stoichiometry... 

A molecule is chiral if it cannot be superimposed on its mirror image (or if it does not possess an alternating axis of symmetry) and would exhibit optical activity, i.e. lead to the rotation of the plane of polarization of polarized light. Lactic acid, which has the structure (2 mirror images) shown exhibits molecular chirality. In this the central carbon atom is said to be chiral but strictly it is the environment which is chiral. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

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. 

These hydrocarbons contain cyclic (or ring) structures in all or part of the skeleton. The number of carbon atoms in the ring thus formed can vary. Refer to Table 1.2. 

Because of the existence of numerous isomers, hydrocarbon mixtures having a large number of carbon atoms can not be easily analyzed in detail. It is common practice either to group the constituents around key components that have large concentrations and whose properties are representative, or to use the concept of petroleum fractions. It is obvious that the grouping around a component or in a fraction can only be done if their chemical natures are similar. It should be kept in mind that the accuracy will be diminished when estimating certain properties particularly sensitive to molecular structure such as octane number or crystallization point. 

With regards to the overall balance of combustion, the chemical structure of the motor or heating fuel, e.g., the number of carbon atoms in tbe chain and the nature of the bonding, does not play a direct role the only important item is the overall composition, that is, the contents of carbon, hydrogen, and — eventually— oxygen in the case of alcohols or ethers added to the fuel. 

The various arrangements of carbon atoms can be categorised into senes , which describe a common molecular structure. The series are based on four main categories which refer to... 

Although benzene contains three carbon-carbon double bonds, it has a unique arrangement of its electrons (the extra pairs of electrons are part of the overall ring structure rather than being attached to a particular pair of carbon atoms) which allow benzene to be relatively unreactive. Benzene is, however, known to be a cancer-inducing compound. 

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... 

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

It also forms compounds known as carbonyls with many metals. The best known is nickel tetracarbonyl, Ni(CO)4, a volatile liquid, clearly covalent. Here, donation of two electrons by each carbon atom brings the nickel valency shell up to that of krypton (28 -E 4 x 2) the structure may be written Ni( <- 0=0)4. (The actual structure is more accurately represented as a resonance hybrid of Ni( <- 0=0)4 and Ni(=C=0)4 with the valency shell of nickel further expanded.) Nickel tetracarbonyl has a tetrahedral configuration,... 

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... 

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