Double-bond character Subject

The question of electronic conductivity in the polyphosphazenes inevitably raises questions regarding the electronic structure of the phosphazene linkage.7-12 This matter has been the subject of controversy in the literature, but experimentally the situation is now well known.4,13 In spite of the fact that the phosphazene backbone is fully conjugated, bond equalized and possesses bond lengths which are indicative of partial double bond character, the evidence suggests that these are localized systems. 

The oxadiazole ring has little aromatic character. Both C—N bond distances suggest conjugated double bond character in the only oxadiazole (90) that appears to have been subjected to X-ray analysis (79AX(B)2256). The C—N bond lengths in the aminopyridine... 

The addition of dihalocarbenes to double bonds occurs stereospecifically to give cyclopropane derivatives [68J. The reaction appears to have electrophilic character even though the electronic state of the carbene is uncertain (cf. p. 341). Again the direction of attack on the double bond is subject to steric control. The diene (2) for example, reacts with di-bromocarbene to give first the 2ct,3[Pg.294]

The polypeptide backbone can only bend in a very restricted way. The peptide bond itself is a hybrid of two resonance structures, one of which has double bond character, so that the carboxyl and amide groups that form the bond must, therefore, remain planar (see Fig. 7.3.). As a consequence, the peptide backbone consists of a sequence of rigid planes formed by the peptide groups (see Fig. 7.3). However, rotation within certain allowed angles (torsion angles) can occur around the bond between the a-carbon and the a-amino group and around the bond between the a-carbon and the carbonyl group. This rotation is subject to steric constraints that maximize the... 

This subject has been investigated for the case of piperazine polymers and diacoyl piperazines as model substances. The N—CO bond of these compounds possesses partial double-bond character. Consequently, the rotation about this bond is comparatively slow. Various absorption bands will be observed in the proton resonance spectrum according to whether neighboring groups are in the cis or the trans position about the N—CO bond. The Gibbs energy of activation AG" can be determined from the temperature dependence of the band intensities. 

Of the series of heavier Group 14 double-bonded species, the silyl derivatives are somewhat unique in retaining their double-bond character while in solution. As such, they are subject to a plethora of cycloaddition reactions with unsaturated organic molecules, forming both [2-1-2] and [2-1-3] cycloadducts with much enhanced reactivity over their carbon analogs (Scheme 14.3) [65, 80]. 

The mechanism for the addition of diazoalkanes to a C=—O double bond is generally written along lines similar to those discussed so far (Scheme 1 X" = N2+). The initial adduct is also the progenitor of the various rearrangement pathways. However, the subject of mechanism is by no means settled, with 1,3-dipolar cycloadditions and carbonyl ylide formation" considered to be prominent alternatives. In general, successful epoxidation of carbonyl compounds improves with increasing electron-poor character of the C—O bond. When the diazoalkane is electron poor, yields of epoxide diminish. 

Diverse types of unsaturated sugars are described in the literature on carbohydrates excellent articles by Ferrier have appeared on this subject in this Series. The chemical reactivity of unsaturated pyranosides and furanosides is strongly influenced by the character of the double bond. 

In contrast to the living character of the polymerization of EO, the anionic polymerization of substituted epoxides like POx initiated by alkali metal derivatives is subject to transfer reactions to the monomer. Indeed, the highly basic alkoxide propagating species can pull out a proton of the monomer substituent, leading to chain termination and the formation of a new growing chain bearing a terminal double bond, as indicated in Scheme 12 in the case of POx. 

Why is deshielding so pronounced for alkenyl hydrogens Although the electron-withdrawing character of the p -hybridized carbon is partly responsible, another phenomenon is more important the movement of the electrons in the ir bond. When subjected to an external magnetic field perpendicular to the double-bond axis, these electrons enter into a circular motion. This motion induces a local magnetic field that reinforces the external... 

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