Carbon-hydrogen bonds vinylic

The carbon-halogen bonds of aryl halides are both shorter and stronger than the carbon-halogen bonds of alkyl halides In this respect as well as m their chemical behavior they resemble vinyl halides more than alkyl halides A hybridization effect seems to be responsible because as the data m Table 23 1 indicate similar patterns are seen for both carbon-hydrogen bonds and carbon-halogen bonds An increase m s... 

We can divide commodity plastics into two classes excellent and moderate insulators. Polymers that have negligible polar character, typically those containing only carbon-carbon and carbon-hydrogen bonds, fall into the first class. This group includes polyethylene, polypropylene, and polystyrene. Polymers made from polar monomers are typically modest insulators, due to the interaction of their dipoles with electrical fields. We can further divide moderate insulators into those that have dipoles that involve backbone atoms, such as polyvinyl chloride and polyamides, and those with polar bonds remote from the backbone, such as poly(methyl methacrylate) and poly(vinyl acetate). Dipoles involving backbone atoms are less susceptible to alignment with an electrical field than those remote from the backbone. 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

The hybridization of the carbon in an alkene makes it even more difficult to break the carbon-hydrogen bond of a vinylic carbon than of a saturated carbon. As a consequence, cytochrome P450, rather than abstracting a hydrogen atom, catalyzes the addition of an oxygen atom to the double bond leading to the formation of an epoxide as shown in Figure 4.72. 

The a-carbon-hydrogen bonds of vinyl(phenyl)iodonium ions are relatively acidic, two examples of base catalyzed hydrogen-deuterium exchange in -oxavinyliodonium salts having been reported (equations 227 and 228)79,95. Although vinylidene-iodonium ylides... 

Among a variety of dienophiles the simplest one ethylene, and 1-alkenes react sluggishly.3 Consequently, their equivalents are highly desirable and a few such compounds have been introduced.4 Vinylic organoboranes are good candidates for that purpose, provided they would be sufficiently reactive and the carbon-boron bond of the adduct could be transformed into the carbon-hydrogen bond without isomerization of the double bond. Here, our studies in this direction are described.5... 

In this case, the strongest C-H bond is the vinyl carbon/hydrogen bond, and this bond is practically never broken. It is the allylic hydrogen that is abstracted, because this carbon/hydrogen bond is relatively weak. Furthermore, cleavage of this bond results in a radical that is stabilised by delocalisation. 

The key feature is the presence of a small equilibrium quantity of the vinyl hydride complex Tp Rh(H)(CH=CH2)( i -C2H4) formed by C-H oxidative addition of one of the ethylene ligands. The formation of Tp Rh(C2H5)(C6H5)(L) and its deuterated analog are postulated to arise from coordination of benzene to the ethylene-loss species Tp Rh(jj -C2H4) followed by C-H oxidative addition see Aikane Carbon-Hydrogen Bond Activation)... 

The analysis of the resulting copolymers were established by H NMR, infrared and UV—visible spectroscopies. Table 26 shows the copolymerization results by metal catalysts. The H-NMR spectra of both the monomer and the polymer are shown in Figure 33. As the polymerization proceeded, the acetylenic proton peak at around 1.96 ppm disappeared and a new vinylic proton peak appeared in the aromatic region. Also, the IR spectra of the polymer showed no absorption peaks at 3290 and 2140 cm , which are expected to be present for the acetylenic carbon—hydrogen bond stretching and carbon—carbon triple bond stretching in the monomer, respectively. 

The secondary vinylic carbon-hydrogen bond in 2-propenal-2-yl radical, C CJC 0, resulting from the dissociation of CH2=C(CH=0)—H has a bond strength of 112.72 kcal moT. This BDE is similar to the primary vinyl bond energy of ethylene C=C—H, given by Lineberger et al. [135] to be 111.2 kcal moT, and 7 kcal mol stronger than the secondary vinyl C—bond... 

An iodine-catalyzed intramolecular oxidative thiolation of vinylic carbon-hydrogen bonds via tandem iodocyclization and dehydroiodination has also been developed for the construction of 2-methylene-3-thiophe-nones (14ASC743).The synthetic strategy was extended to the preparation of 2-methylene-3-benzothiophenone via the cyclization of o-methylthio-phenyl vinyl ketones. 

The amount of chain transferring that takes place to monomers is usually low because the reaction requires breaking strong carbon-hydrogen bonds. Monomers, however, such as vinyl chloride and vinyl acetate have fairly large chain transferring constants. In the case of vinyl acetate, this is attributed to the presence of an acetoxy methyl group. This explanation, however, cannot be used for vinyl chloride. 

We see that an allylic carbon—hydrogen bond of propene is broken with greater ease than even the tertiary carbon-hydrogen bond of isobutane and with far greater ease than a vinylic carbon-hydrogen bond ... 

The ease with which an allylic carbon—hydrogen bond is broken means that relative to primary, secondary, tertiary, and vinylic free radicals an allylic radical is the most stable (Pig. 10.3) ... 

Radicals substituted with electron-delocalizing groups, such as the vinyl or phenyl groups, are even more stable. As Table 11.1 shows, breaking a carbon-hydrogen bond to give an allyl radical or benzyl radical is relatively easy. The allyl and benzyl radicals are even more stable than a tertiary radical (Fig. 11.21). 

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