Double bonds alternating

Another of Kelule s revelations that supposedly came to him in a dream was his famous structure of benzene. This related to how a carbon chain can close into a ring. To satisfy the four valenee of carbon, this, of course, raised the need to involve alternating double bonds. 

The naphthenes and aromatics both have cyclic (or ring-like) molecular structures and both possess high octane numbers. Napthenes are saturated and aromatics contain alternate double bonds on their ring. They are typically found in gasoline. The naphthenes also are an important part of kerosene. 

Because they are molecular compounds, most polymers are electrical insulators. However, polymers that have alternating double bonds along the chain can be used to conduct electricity (Box 19.1). These conducting polymers tend to have long chains with few branches. 

Canonical forms of benzene that are calculated to contribute about 22% to the resonance stabilization of benzene. Such resonance structures have no separate physical reality or independent existence. For the case of benzene, the two Kekule structures with alternating double bonds i.e., cyclohexatriene structures) contribute equally and predominantly to the resonance hybrid structure. A dotted circle is often used to indicate the resonance-stabilized bonding of benzene. Nonetheless, the most frequently appearing structures of benzene are the two Kekule structures. See Kekule Structures... 

Canonical forms of benzene with alternating double bonds Le., cyclohexatriene ), structures which contribute equally and predominantly to benzene s resonance hybrid structure. 

Acetylene and its derivatives can be polymerized by chain growth in the presence of suitable transition metal catalysts to give high molecular weight (MW) polymers (Equations (l)-(4)). The monomers include acetylene, mono- and disubstituted acetylenes, and a,tv-diynes. The polymers possess carbon-carbon alternating double bonds along the main chain and exhibit unique properties (e.g., metallic conductivity) that are not expected with vinyl polymers. 

Benzene s molecular formula is C6H6, but it does not behave like hexane, hexene, or any of their isomers. One would expect it to be similar to these other six-carbon hydrocarbons in its properties. Table 4 provides a comparison between benzene, hexane and 1-hexene. The table shows that there are major differences between benzene and the straight-chain hydrocarbons of die same carbon content. Hexene s ignition temperature is very near to hexane s. The flash point difference is not great, however, there are significant differences in melting points. The explanation for these differences is structure which in the case of benzene is a cyclical form with alternating double bonds. 

All the molecules contain alternating double bonds (are highly conjugated). 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

Upon exposure to air, animal and vegetable fats and oils become rancid (i.e., develop color changes and a musty, rank taste and odor). Here, the hydrogen atoms of the —CH2—groups located between alternating double bonds (i.e., —CH=CH—CH2—CH=CH—) of a polyunsaturated phospholipid or fatty acid (LH) are very susceptible to abstraction by free radicals. This process can then lead to a general reaction known as autoxidation, which results in the formation of a lipid hydroperoxide (LOOH) and the generation of a new free radical hence, an autocatalytic reaction results (lipid peroxidation). 

Oxidation and hydrolysis of nitroso compound 5. readily tivailable from the tiddition of nitrosyl fluoride to pcrfluoro(2-methylprop-2-enoyl) fluoride, leads to the corresponding alcohol 6. Alternatively, double bonds arc hydrobora ted and subsequently treated w iih hydrogen peroxide to yield the alcohol, c. g. conversion of compound 7 into the c.vo-product 8 (< xo einlo 86 14). ... 

Unlike vinyl polymers, polyacetylenes which have alternating double bonds along the main chain often show the following unique properties i) electrical conductivity (semiconductivity), ii) paramagnetism, iii) chain stiffness, iv) geometrical isomerism, and v) color. Thus it seems interesting to elucidate the properties of polyacetylenes and develop their functions. 

This review describes the synthesis and properties of polyacetylenes with substituents (substituted polyacetylenes) mainly on the basis of our recent studies At first, Sections 2 and 3 survey the synthesis of substituted polyacetylenes with group 6 (Mo, W) and group 5 (Nb, Ta) transition metal catalysts respectively, putting emphasis on new, high-molecular-weight polyacetylenes. Then, Section 4 refers to the behavior and mechanism of the polymerization by these catalysts. Further, Section 5 explains the alternating double-bond structure, unique properties, and new functions of substituted polyacetylenes. Finally, Section 6 provides detailed synthetic procedures for substituted polyacetylenes. 

NMR and IR spectra of the polyacetylenes formed support that their main chain consists of alternating double bonds. No evidence for monomer isomerization prior... 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

Crystalline and amorphous silicons, which are currently investigated in the field of solid-state physics, are still considered as unrelated to polysilanes and related macromolecules, which are studied in the field of organosilicon chemistry. A new idea proposed in this chapter is that these materials are related and can be understood in terms of the dimensional hierarchy of silicon-backbone materials. The electronic structures of one-dimensional polymers (polysilanes) are discussed. The effects of side groups and conformations were calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, and UV photospectroscopy (UPS) measurements. Finally, future directions in the development of silicon-based polymers are indicated on the basis of some novel efforts to extend silicon-based polymers to high-dimensional polymers, one-dimensional superlattices, and metallic polymers with alternating double bonds. 

Polysilene with alternating double bonds (3) has not been synthesized, and the synthesis may be very difficult. The calculated band structure of polysilene (28) is shown in Figure 18. The most striking feature is the overlapping of two bands, which is accompanied by double crossing at the Fermi... 

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