Double bond characteristics

It must be emphasized that none of these resonance forms actually exists, but the superposition of all of them for a given molecule serves as a good representation, and shows that each bond possesses both single-bond and double-bond characteristics. 

Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents SN2 reactions. Likewise, phenyl cations are unstable, thus making SN1 reactions impossible. In addition, the carbon-halogen bond is shorter and therefore stronger in aryl halides than in alkyl halides. The carbon-halogen bond is shortened in aryl halides for two reasons. First, the carbon atom in aryl halides is sp2 hybridized instead of sp3 hybridized as in alkyl halides. Second, the carbon-halogen bond has partial double bond characteristics because of resonance. 

Numerous synthetic routes have been utilized to prepare isoprenoid analogs. The classical synthetic route to isoprenoids and isoprenoid analogs utilized iterative Horner-Emmons-Witting coupling reactions to generate the trisubstituted double bonds characteristic of linear isoprenoids... 

It is possible to draw a canonical structure for this molecule that involves a separation of charges, but which in turn confers some double bond characteristics on the central part of the molecule. As a consequence, there are two identifiable conformers of 1,3-butadiene that result from this small energy barrier to free rotation. Suggest what are these two conformers. 

Molecules like 1,3-butadiene may exist in the cisoid or transoid conformers, as a result of the small barrier to free rotation that exists due to the fact that the central single bond has some double bond characteristics. Only the cisoid conformer undergoes the Diels-Alder reaction. Usually, the rate of the Diels-Alder reaction is increased by electron donating substituents on the diene, and by electron withdrawing groups on the alkene, or dienophile. Thus, the diene acts as the nucleophile, while the dienophile acts as the electrophile. Under all normal conditions, the endo adduct is preferred due to favourable overlap of secondary orbitals. This results in syn-addition of an R-R unit. 

Chemically speaking, fullerenes are polyolefins or more or less aromatic systems. As already mentioned in Section 2.2.2, an alternation of (5,6)- and (6,6)-bonds, respectively, is observed. The latter feature double-bond characteristics and the Jt-electrons are just moderately delocalized. Hence the aromatic nature of Cgo as well as of higher fullerenes is limited. 

In the optimized stmcture, Pyl coordinated with the Zr atom consequently, it is coplanar with Zr, C6, and C7, and the dihedral angle Nl-C5-C6-Zr is only —3.3°. This indicates that Pyl can be conjugated with C6. The C5-C6 bond has some double bond characteristics (1.434 A). Py2 is rotated off this plane, and the dihedral angle N2-C20-C8-C7 is —67.0°. The C6-Zr bond length is 2.273 A, even stronger than the C9-Zr bond (2.379 A). The coordination of Pyl with the Zr center indicates that the 2-pyridyl group on the alkyne may be important to the reductive elimination of the zirconacyclopentadienes [1,2]. 

ECPs appears in the course of its doping by counterions because of formation of delocalized n-electrons or holes and their transport under the action of electric field through the system of polyconjugated double bonds characteristic of any ECPs. ECPs include polyacetylene (Pac), polyaniline (PAni), poly(p-phenylene) (PPh), polythiophene (PT), polypyrrole (PPy), polyporphyrin (PP), and their derivatives. Eigure 28.3 shows structural formulas for some ECPs used in ECSCs. 

Stiffness and planarity of the chain because of the partial double bond characteristics of the amide groups, and... 

The expected length of a single C-N bond is 1.45 A, as in the C -N bond, and that of a C = N double bond is 1.25 A. The actual length of the C -N peptide bond is 1.33 A, showing that it has partial double bond characteristics (40% double bond). Rotation can occur, in principle, around all three bonds [j/, q>, and w, where j/ = (p = w= %Q°. This means that for a protein of 100 residues there are 2 x 10 possible conformations, far more possible conformations than there would be protein molecules, even in a large sample. However, we know that a folded protein has a relatively stable conformation. This is due to many factors, one being the partial double bond characteristics of the C -N peptide bond that limits it to a trans conformation (with the exception of proline), the atoms of the side chains restrict bond rotation due to excluded volume effects that dictate... 

Polyethylene and polypropylene are the major members of the class of polymers known as polyolefins see Section 14.1. The term olefin derives from the double-bond characteristic of the alkene series. 

The use of d as well as p orbitals gives the C-S bond 28% double bond character without the molecule t ing planar. The double bond characteristic is estimated from the measured C-S bond length of 1.76 A, reduced from the singlebond value of about 1.81 A. The bond length is in good agreement with C-S values in aromatic substances such as phenyl sulfide. 

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