Saturated carbon bonds

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. 

Intramolecular hydrogen abstraction is an important synthetic reaction because it provides a method of introducing a functional group at a saturated carbon bonded to only hydrogen and other carbons, a difficult task to accomplish by other means.196 The abstracting radical need not be carbon the Barton reaction (Scheme 15) is a standard method in which the abstracting radical is oxygen.197 This technique has been used extensively for selective introduction of functionality in steroid synthesis.198... 

The fatty acids are first converted to CoA derivatives at the expense of the hydrolysis of ATP and then transported into mitochondria where they are broken down sequentially, two carbon units at a time, by a pathway known as -oxidation (see Fig. 9). The fatty acyl CoA derivatives undergo oxidation at the carbon that is p to the carboxyl carbon from that of a saturated carbon-carbon bond to that of an oxo-saturated carbon bond. Enzymes that contain FAD or use NAD" as the electron acceptors catalyze these reactions. As is the case in the oxidation of carbohydrates, the NADH and FADH2 generated by the 0-oxidation of fatty acids are converted to their oxidized forms by the mitochondrial electron transport chain, which results in the formation of ATP by oxidative phosphorylation. 

Alkanes from CH to C4gFlg2 typically appear in crude oil, and represent up to 20% of the oil by volume. The alkanes are largely chemically inert (hence the name paraffins, meaning little affinity), owing to the fact that the carbon bonds are fully saturated and therefore cannot be broken to form new bonds with other atoms. This probably explains why they remain unchanged over long periods of geological time, despite their exposure to elevated temperatures and pressures. 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

Hydrocracking. Hydrogenolysis of a carbon—carbon bond in an organic molecule produces smaller, hydrogen-saturated molecules and is termed hydrocracking when 50% or more of the feedstock is converted into smaller molecules. 

We consider first the Sn2 type of process. (In some important Sn2 reactions the solvent may function as the nucleophile. We will treat solvent nucleophilicity as a separate topic in Chapter 8.) Basicity toward the proton, that is, the pKa of the conjugate acid of the nucleophile, has been found to be less successful as a model property for reactions at saturated carbon than for nucleophilic acyl transfers, although basicity must have some relationship to nucleophilicity. Bordwell et al. have demonstrated very satisfactory Brjinsted-type plots for nucleophilic displacements at saturated carbon when the basicities and reactivities are measured in polar aprotic solvents like dimethylsulfoxide. The problem of establishing such simple correlations in hydroxylic solvents lies in the varying solvation stabilization within a reaction series in H-bond donor solvents. 

Free rotation around each of the carbon-carbon bonds makes saturated fatty acids extremely flexible molecules. Owing to steric constraints, however, the fully extended conformation (Figure 8.1) is the most stable for saturated fatty acids. Nonetheless, the degree of stabilization is slight, and (as will be seen) saturated fatty acid chains adopt a variety of conformations. 

One large and structurally simple class of hydrocarbons includes those substances in which all the carbon-carbon bonds are single bonds. These are called saturated hydrocarbons, or alkanes. In the alkanes the carbon atoms are bonded to each other in chains, which may be long or short, straight or branched. 

In an unsaturated hydrocarbon, at least one of the carbon-carbon bonds in the molecule is a multiple bond. As a result, there are fewer hydrogen atoms in an unsaturated hydrocarbon than in a saturated one with the same number of carbons. We will consider two types of unsaturated hydrocarbons—... 

Since a similarity between the rates of decomposition of thiirene dioxide complexes and those of thiirane dioxides was found, it was suggested103 that upon coordination the carbon-carbon bond order of thiirene dioxides decreases and the ligand becomes thiirane dioxide-like. The role of the metal is thus to saturate the carbon-carbon double bond so that the reactivity of the coordinated thiirene dioxide approaches that of the thermally less stable thiirane dioxide. 

A saturated hydrocarbon is an aliphatic hydrocarbon with no multiple carbon-carbon bonds an unsaturated hydrocarbon has one or more double or triple carbon-carbon bonds. More hydrogen atoms can be added to compounds in which there are multiple bonds, but compounds with only single bonds are saturated with hydrogen. Compound (3) is saturated compounds (4) and (5) are unsaturated. 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

All the mechanisms so far discussed take place at a saturated carbon atom. Nucleophilic substitution is also important at trigonal carbons, especially when the carbon is double bonded to an oxygen, a sulfur, or a nitrogen. Nucleophilic substitution at vinylic carbons is considered in the next section at aromatic carbons in Chapter 13. 

A saturated carbon atom usually forms at least a bond on one side of the plane with the carbon on it and three bonds on the other side (Scheme 15). However, there are some molecules (e.g., tricyclo[1.1.1.0 ]pentane), where the carbon atom has an... 

The free radicals combine to form a carbon-to-carbon bond and give a saturated polymer molecule with initiator fragments on both ends. Termination by disproportionation produces two polymer molecules, one of which will contain a double bond ... 

From the two-dimensional, graphite-like clusters, the extension to three-dimensional structures is obvious. Symmetric structures developed in a similar fashion to the planar systems would grow in three dimensions with increasing N, and the number of atoms would increase faster. In this work clusters of T symmetry were studied, resembling a small fragment of a diamond structure. Only systems with saturated external bonds were considered. The number of carbon and hydrogen atoms in such a structure is given by... 

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