Carbon hydrogen bonds dissociation energies

Problem 10.3 The carbon-hydrogen bond dissociation energy for benzene (112 kcal) is considerably larger than for cyclohexane. On the basis of the orbital picture of benzene, what is one factor that ntay be responsible for this What piece of physical evidence tends to support your answer Hint Look at Fig. 10.4 and )see Sec. 5.4.)... 

The thermodynamic stabilization was determined from the barrier to rotation, by thermochemical methods,and from thermolysis studies of bis(cyclopropylalkyl)diazenes, and found to be approximately 1.9 kcal mol . The value of the carbon-hydrogen bond dissociation energy, DH (cyclopropylmethyl-hydrogen), deduced from this is 98.5 kcal mol , which may be compared with the DH (ethyl-hydrogen) value of 100 kcal mol This demonstrates that there is a small but chemically significant interaction between the orbital containing the unpaired electron (SOMO) and orbitals of the C -C, bond. 

Direct hydrogen atom abstraction occurs less frequently from the nucle-obases, despite the expected modest carbon—hydrogen bond dissociation energy of the carbon—hydrogen bonds in the methyl groups of thymidine and 5-methyl-2 -deoxycytidine due to resonance stabilization of the incipient radicals. The respective radicals are also formed by deprotonation of the nucleobase radical cations, intermediates involved in electron transfer that are produced via one-electron oxidation. Amine radicals are also postulated as intermediates produced from the spontaneous decomposition of chloramines that arise from reactions of nucleosides with hypochlorous acid." " However, the majority of nucleobase radical intermediates arise from the... 

Further research into the reaction mechanism revealed that the reaction rate was correlated with the electron structure of the sulfoxide the more electropositive sulfoxides were the better oxygen donors. Excellent correlation of the reaction rates with the heterolytic benzylic carbon-hydrogen bond dissociation energies indicated a hydride abstraction mechanism in the rate-determining step to yield a carbocation intermediate. The formation of 9-phenylfluorene as by-product in the oxidation of triphenylmethane supports this suggestion. Further kinetic experiments and NMR showed the formation of a polyoxometalate-sulfoxide complex before the oxidation reaction, this complex being the active oxidant in these systems. Subsequently, in a similar reaction system, sulfoxides were used to facilitate the aerobic oxidation of alcohols [29]. In this manner, benzylic, allyUc, and aliphatic alcohols were all oxidized to aldehydes and ketones in a reaction catalyzed by Ke jn-type... 

Singleton, D.L., R.S. Irwin, and R.J. Cvetanovid (1977), Arrhenius parameters for the reactions oxygen( P) atoms with several aldehydes and the trend in aldehydic carbon-hydrogen bond dissociation energies. Can. J. Chem., 55, 3321-3327. 

Carbon-Hydrogen and Carbon-Chlorine Bond Dissociation Energies of Selected Compounds... 

Table 1.1 Carbon-Hydrogen and Heteroatom-Hydrogen Bond Dissociation Energies (D in kJ morI)a °...

Steele, W. C., L. D. Nichols, and F.G. A. Stone The Determination of Silicon-Carbon and Silicon-Hydrogen Bond Dissociation Energies by Electron Impact. J. Amer. chem. Soc. 84, 4441—4445 (1962). 

It is reasonable to postulate that the main driving force for the reaction of (MeC5H4)3U(t-Bu) with hexafluorobenzene is thermodynamic. A weak uranium-carbon bond and a carbon-fluorine bond have to be broken. This is offset by the formation of a strong uranium-fluorine bond and either a carbon-carbon or a carbon-hydrogen bond. As illustrated in equation 8, the uranium-fluorine bond energy can be estimated to be on the order of 150 kcal/mol based on known thermochemical data for uranium fluorides (21), The carbon-fluorine bond dissociation energy for hexafluorobenzene is reported to be 154 kcal/mol (17), The... 

Thermal Properties. Thermodynamic stabiUty of the chemical bonds comprising the PPS backbone is quite high. The bond dissociation energies (at 25°C) for the carbon—carbon, carbon—hydrogen, and carbon—sulfur bonds found in PPS are as follows C—C, 477 kj/mol (114 kcal/mol) ... 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

Table 1. Dissociation Energies of Carbon—Hydrogen Bonds ...

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

Caibon has eight electrons in its valence shell in both methane and carbon tetrafluoride. By forming covalent bonds to four other atoms, carbon achieves a stable electron configuration analogous to neon. Each covalent bond in methane and carbon tetrafluoride is quite strong—comparable to the bond between hydrogens in Fl2 in bond dissociation energy. 

Figure 8. 5is-allylic carbon-hydrogen bond. The dissociation energy of this bond is lower than allylic and alky] carbon-hydrogen bonds, and therefore the iiy-allylic bond is favored for attack by LOO which explains the susceptibility of polyunsaturated lipids. 

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