Carbon-1 3 molecular structure

FIGURE 7.19 Comparison of average carbon molecular structures of BP HC-4 molecule (shaded circles) and decene trimer (open circles). 

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. 

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... 

Molecular Structure and Monomer Addition Orientation. The addition of vinyl monomer to a growing PVC chain can be considered to add in a head-to-tail fashion, resulting in a chlorine atom on every other carbon atom, ie,... 

Molecular Structure. The chain stmcture is as shown in Table 1 and molecular weights of 300,000—500,000 are achieved. The Mooney viscosities are in the range of 40—70 leading to a soft elastomer, which requires carbon black reinforcement for higher modulus. 

Adsorbents are natural or synthetic materials of amorphous or microcrystalhne structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

The classifications in Table 16-3 are intended only as a rough guide. For example, a carbon molecular sieve is truly amorphous but has been manufactured to have certain structural, rate-selective properties. Similarly, the extent of hydrophobicity of an activated carbon will depend on its ash content and its level of surface oxidation. 

The C NMR spectrum displays five instead of ten carbon signals as expected from the empirical formula C107//0O4. To conclude, two identical halves C5//5O2 build up the molecular structure. 

Fullerenes are described in detail in Chapter 2 and therefore only a brief outline of their structure is presented here to provide a comparison with the other forms of carbon. The C o molecule, Buckminsterfullerene, was discovered in the mass spectrum of laser-ablated graphite in 1985 [37] and crystals of C o were fust isolated from soot formed from graphite arc electrodes in 1990 [38]. Although these events are relatively recent, the C o molecule has become one of the most widely-recognised molecular structures in science and in 1996 the codiscoverers Curl, Kroto and Smalley were awarded the Nobel prize for chemistry. Part of the appeal of this molecule lies in its beautiful icosahedral symmetry - a truncated icosahedron, or a molecular soccer ball, Fig. 4A. 

The as-spun acrylic fibers must be thermally stabilized in order to preserve the molecular structure generated as the fibers are drawn. This is typically performed in air at temperatures between 200 and 400°C [8]. Control of the heating rate is essential, since the stabilization reactions are highly exothermic. Therefore, the time required to adequately stabilize PAN fibers can be several hours, but will depend on the size of the fibers, as well as on the composition of the oxidizing atmosphere. Their are numerous reactions that occur during this stabilization process, including oxidation, nitrile cyclization, and saturated carbon bond dehydration [7]. A summary of several fimctional groups which appear in stabilized PAN fiber can be seen in Fig. 3. 

The properties of mesophase pitch-based carbon fibers can vary significantly with fiber texture. Inspection of the cross-section of a circular mesophase fiber usually shows that the graphitic structure converges toward the center of the fiber. This radial texture develops when flow is fully developed during extrusion through the spinnerette. Endo [48] has shown that this texture of mesophase pitch-based carbon fibers is a direct reflection of their underlying molecular structure. 

As useful as molecular models are, they are limited in that they only show the location of the atoms and the space they occupy. Another important dimension to molecular structure is its electron distribution. We introduced electrostatic potential maps in Section 1.5 as a way of illustrating charge distribution and will continue to use them throughout the text. Figure 1.6(d) shows the electrostatic potential map of methane. Its overall shape is similar to the volume occupied by the space-filling model. The most electron-rich regions are closer to carbon and the most electron-poor ones are closer to the hydrogens. 

Looking back at the frequency output once again, we note that its symmetry is A", indicating that this is a symmetry-breaking mode. The molecular structure has C, symmetry, indicating that there is a single plane of symmetry (in this case, the plane of the carbon atoms). The structure wants to move down the PES to a lower-energy structure of equal or lower symmetry. 

How can there be so many compounds containing this one element The answer lies in the molecular structures. We shall find that carbon atoms have an exceptional tendency to form covalent bonds to other carbon atoms, forming long chains, branched chains, and rings of atoms. Each different atomic arrangement gives a mole-... 

Using the data given in the last column of Table 18-111, plot the heat released per carbon atom against the number of carbon atoms for the normal alkanes. Consider the significance of this plot in terms of the molecular structures of these compounds. 

The repeat unit in the Phenoxy molecule is longer by two carbon atoms than that of PC. Otherwise the molecular structures of the two polymers are similar. Eq. (7.8) should, therefore, apply also to PC, at least approximately. 8C = 8.9 pm was measured for the crack opening displacement in a Makrolon sheet (Mn = 9.5 kg/mol) by Fraser and Ward [80], An estimate of the molecular mass between entanglements according to Eq. (7.8) yields Mc = 1.7 kg/mol. This result is not very different from the value of 2.5 kg/mol for IVIC in PC as given in Table 3.2. 

Organic chemists have found a way to draw complex molecular structures in a very simple way, by not showing the C and H atoms explicitly. A line structure represents a chain of carbon atoms by a zigzag line, where each short line indicates a bond and the end of each line represents a carbon atom. Atoms other than C and H are shown by their symbols. Double bonds are represented by a double line and triple bonds by a triple line. Because carbon almost always forms four bonds in organic compounds, there is no need to show the C—FI bonds explicitly. We just fill in the correct number of hydrogen atoms mentally compare the line structure of 2-chlorobutane, QT3C1TC1CF12C]T3 (3a), with its structural form (3b). Line... 

C.5 In the following ball-and-stick molecular structures, black indicates carbon, red oxygen, light gray hydrogen, blue nitrogen, and green chlorine. Write the chemical formula of each structure. 

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