Carbon Hydrogen Bond Dissociation

Several groups have studied photoreactions of dicyano-aromatic compounds with alkylbenzenes as the electron donors [8]. Efficient proton transfer from the benzylic position of the alkylbenzene radical-cation, formed by electron transfer to excited DCN, to the counter anion (DCN -) is reported to produce a benzylic radical and 

Until recently the intermolecular photoreduction of ketones by amines has not provided much synthetic utility. In general, coupling of the radicals formed by PET and proton transfer produces a mixture of a-amino alcohols, diamines and/or 

The same process has been observed by irradiating phthalimides that are N-substituted with alkylamines. In particular, the photochemical cyclization of to- 

Further investigations on the acceptor-donor arene-amine system have been made. Direct irradiation of 9-(co-anilinoalkyl)phenanthrenes give spirocyclic pyrrolidine derivatives by NH addition to the phenanthrene C(9)-C(10) bond [19]. 

It is worthy of note that the photoreduction of [60]fullerene by tertiary amines, such as triethylamine, results in the formation of a coupling product. This product is likely to be formed by a PET-proton transfer mechanism [20]. 

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Because the cis-decalin molecule extends its two methine carbon-hydrogen bonds on the same side in contrast to frans-decalin, the carbon-hydrogen bond dissociation of adsorbed decalin would be advantageous to the cis-isomer on the catalyst surface (Figure 13.17). A possible reaction path by octalin to naphthalene in dehydrogeno-aromatization of decalin will be favored to the cis-isomer, since its alkyl intermediate provides the second hydrogen atom from the methine group to the surface active site easily. 

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. 

Carbon-Hydrogen Bond Dissociation (Deprotonation Reaction)... 

Table V. Silicon-Hydrogen and Carbon-Hydrogen Bond Dissociation Enthalpies in Phenylsilanes and Phenylmethanes Data in kJ/mol...

This implies that the reaction will be favorable (A,G (60)<0) only if A, (60) = Z) (Me-H)— DIP (M-H) - DIP (M-Me) < —44 kJ mol, that is, the sum of metal—carbon and metal—hydrogen bond dissociation enthalpies must exceed the carbon-hydrogen bond dissociation enthalpy in methane, 439 kJ moF , by that amount. Taking a middle range value for Z)//°(M-H), 250kJmoF (Table 1), one concludes that Z) (M-Me) should be higher than ca. 230kJ moF. ... 

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... 

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 ...

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

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

The spectra of C3H6 and C3D6 show that chemisorption of propylene is dissociative, but they fail to identify which carbon-hydrogen bond is broken on adsorption. To this end the spectra of a number of deuterium-labeled propylenes were studied and compared. These results are summarized in abbreviated form in Table VI, which specifies the hydrogen fragment formed on adsorption the fragment was identified as an OH if a band appeared near 3593 cm-1 or as an OD if a band appeared near 2653 cm-1. In those cases where the spectrum changed with time the summary... 

In sulfonation on the other hand, a tritium isotope effect is observed.287 Sulfonation is a reversible reaction and the fact that it is less exothermic is compatible with a slow, rate-determining dissociation of the intermediate. The transition state for the slow second step has a less covalent carbon-hydrogen bond than the ground state and hence the reaction is faster for deprotonation than for detritonation. 

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. 

This value of the C-H bond enthalpy does not correspond to the enthalpy of dissociation of the carbon-hydrogen bond in methane, as represented in Equation (4.35). 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

Flanagan and Rabinovitch were able to establish another point of general interest. They deduced from the relative rates of exchange and isomerization of isotope effect in the rupture of the carbon- hydrogen bond when adsorbed ethyl radicals dissociate to form adsorbed ethylene molecules. The ratio of the rupture probabilities of C—H and C—D decreased from 15.9 at —78° to 1.4 at 429°. More evidence of this kind would obviously be valuable because it suggests that some revision may be necessary of the theory for calculating initial distributions of... 

Methylene attacks the double bond and the carbon-hydrogen bonds of olefins. These reactions lead to excited adducts which may undergo isomerization, collisional deactivation, or dissociation ... 

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