Carbon-hydrogen bonds stability

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

Thermogravimetric data indicate that the structure of a polymer affects stability in a neutral environment (HI). A polymer such as Teflon, with carbon-carbon bonds which are (by comparison) easily broken, and with strong carbon-fluorine bonds, is quite stable thermally. However, polyethylene, also with carbon-carbon bonds but containing carbon-hydrogen bonds which are broken relatively easily in comparison with the carbon-fluorine bond, is less stable than Teflon. In turn, polyethylene is more stable than polypropylene. This difference in stability is probably caused by tertiary carbon-hydrogen bonds in polypropylene. Polypropylene is more stable than polyisobutylene or polystyrene, which decompose principally by unzipping mechanism. 

Only limited success was achieved in determining the relative reactivity of primary, secondary, and tertiary carbon-hydrogen bonds to sulphonyl nitrenes 8>. Insertion of p-toluenesulphonyl nitrene into 2-methylbutane gave a mixture of products which could not be completely resolved. The ratio of (primary) (secondary + tertiary) = [38 + 39 40 + 41] was 1.53, compared to a ratio of 5.6 for carbethoxynitrene58>, indicating the lowered selectivity of the sulphonyl nitrene relative to the carbethoxynitrene, as might be expected from the possible resonance stabilization of the latter species. 

Low stability organic compounds with unsaturated carbon hydrogen bonds, e.g., cyanines ... 

Molecular modeling of the reaction predicts attack of the CN" ion on the re face of the iV-allyl benzaldimine carbon to provide an (5)-adduct. The aromatic ring of the imine and the quinuclidine hydrogen bond stabilizes the iminium above the pyridazine, blocking the rear face of the imine bond. Nucleophilic attack by CN is... 

Chlorofluorocarbon (CFC) replacements have recently been used for their lower stability and because they have carbon-hydrogen bonds, which means that their atmospheric lifetime is expected to be much shorter than those of CFCs. The adsorption properties of l,l,2-trichloro-l,2,2-trifluoroethane (CFC 113) and its replacement compounds, l,l-dichloro-2,2,2-trifluoroethane (HCFC123), 1,1-dichloro-l-fluoroethane (HCFC141b), and l,l-dichloro-l,2,2,3,3-pentafluoropropane (HCFC225ca) on four kinds of activated carbons were investigated. The adsorption isotherms of inhalational anesthetics (halothane, chloroform, enflurane, isoflurane, and methoxyflurane) on the activated carbon were measured to evaluate the action mechanism of inhalational anesthesia. The anesthesia of CFC replacements can be estimated by the Freundlich constant N of the adsorption isotherms (Tanada et al., 1997). 

Apparently, the discrepancies detected for the substitution data are largely the consequence of a multiplicity of minor influences operative in the transition state. The deviations are sufficiently diverse in character to require the significance of additional influences on the stability of the transition state. Four other important factors are complexing of the substituent with the electrophilic reagent or catalyst, the involvement of 7r-complex character in the transition state for the reaction, rate effects originating in the rupture of carbon-hydrogen bonds, and differential solvation of the electron-deficient transition states. 

The stability of carbon-carbon and carbon-hydrogen bonds at standard temperature and pressure means that stable organic compounds are easily available to forms of life in such environments. It also means that compounds built exclusively from carbon and hydrogen do not easily react and—in biological terms—that organic compounds containing only carbon and hydrogen atoms are not easily metabolized. 

Of prime importance to the continued growth of polypropylene are the stabilizer systems which must be used to protect the resin during processing, and during exposure of finished products to various environmental and use conditions. The weak tertiary carbon—hydrogen bonds in polypropylene make it particularly susceptible to degradation caused by heat, oxidation, process shearing, and ultraviolet radiation (24). 

The phenolic initially gives up its labile hydrogen, which in turn reacts with the various radicals produced in chain reactions then the phenoxy radical becomes stabilized owing to its ability to form resonance structures. The resonance-stabilized forms of the phenoxy radical will not attack tertiary carbon—hydrogen bonds in the polypropylene chain but will react with other radicals such as a peroxide, resulting in the elimination of a second free radical. 

According to the Hammond postulate, the transition state for abstraction by chlorine resembles the reactant because this is an exothermic reaction. In contrast, the transition state for abstraction by bromine resembles the product because it is an endothermic reaction (see Figure 21.2). In the case of abstraction by chlorine the carbon-hydrogen bond is only slightly broken in the transition state, and the stability... 

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