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Free radicals bond dissociation energies

Dissociation energies D values) of R—H bonds provide a measure of the relative inherent stability of free radicals Table 5.4 lists such values. The higher the D value, the less stable the radical. Bond dissociation energies have also been reported for the C—H bond of alkenes and dienes and for the C—H bond in radical precursors XYC—H, where X,Y can be H, alkyl, COOR, COR, SR, CN, NO2, and so... [Pg.243]

Bond dissociation energies such as those in Table 12.6 are also useful for estimation of the energy balance in individual steps in a free-radical reaction sequence. This is an... [Pg.697]

FIGURE 4.20 The bond dissociation energies of methylene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals. The secondary radical is more stable than the primary. [Pg.171]

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... [Pg.58]

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... [Pg.69]

The hydroxyl group of alcohol weakens the a-C—H bond. Therefore, free radicals attack preferentially the a-C—H bonds of the secondary and primary alcohols. The values of bond dissociation energy (BDE) of C—H bonds in alcohols are presented in Table 7.1. The BDE values of C—H bonds of the parent hydrocarbons are also presented. It is seen from comparison that the hydroxyl group weakens BDE of the C—H bond by 23.4 kJ mol 1 for aliphatic alcohols and by 8.0 kJ mol 1 for allyl and benzyl alcohols. [Pg.288]

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. [Pg.39]

As stated above, the thermochemistry of free radicals can also be estimated by the group additivity method, if group values are available. With the exception of a few cases reported in Benson (1976), however, such information presently does not exist. Therefore, we rely on the model compound approach (for S and Cp) and bond dissociation energy (BDE) considerations and computational quantum mechanics for the determination of the heats of formation of radicals. [Pg.122]

Loss of the first H atom from PH2 gives a free radical PH which is strongly stabilized due to the formation of one fully aromatic unit. The effective 4 —H bond dissociation energy, D(PH—H) assumes a very low value of ca. 47 Kcal/mole (or below) just because of this aromatic stabilization This value of D(PH-H) should be compared with the usual C—H bond dissociation energy, e.g., D((CH3)3C—H) =... [Pg.78]

The similarity of the structure of peroxynitrous acid to the simplest peroxy acid, per-oxyformic acid, immediately raised the question as to its relative reactivity as an oxygen atom donor. This became particularly relevant when it was recognized that the 0—0 bond dissociation energy (AG° = 21 kcalmoR ) of HO—ONO was much lower than that of more typical peroxides. Consequently, peroxynitrous acid (HO-ONO) can be both a one- and two-electron oxidant. Since the 0-0 bond in HO-ONO is so labile, its chemistry is also consistent in many cases with that of the free hydroxyl radical. [Pg.14]

Bond Dissociation Energies in the Phenyl Benzoate Molecule and in Related Free Radicals... [Pg.292]

The standard heat of formation of phenyl benzoate as the gaseous species at 25°C. has been determined as AHt°(PhC02Ph) = — 35 1 kcal. per mole. Seven distinct bond dissociation energies are immediately related to this reference basis. Values for these, together with values for the heats of formation of related free radicals, are discussed, and a provisional set is presented. They include the following estimates (kcal. per mole) D(PhC02—Ph) = 94 D(Ph—C02Ph) = 96, D( C02—Ph) = 62 D(PhCO—OPh) = 64. Errors are likely to be around 5 kcal. per mole. [Pg.292]

The approach taken in our laboratory combines both of these trends. Specifically, we have developed a new experiment that allows us to study, for the first time, the photodissociation spectroscopy and dynamics of an important class of molecules reactive free radicals. This work is motivated in part by the desire to obtain accurate bond dissociation energies for radicals, in order to better determine their possible role in complex chemical mechanisms such as typically occur in combustion or atmospheric chemistry. Moreover, since radicals are open-shell species, one expects many more low-lying electronic states than in closed-shell molecules of similar size and composition. Thus, the spectroscopy and dissociation dynamics of these excited states should, in many cases, be qualitatively different from that of closed-shell species. [Pg.730]

Of fundamental importance to free-radical chemistry are bond dissociation energies and radical heats and entropies of formation. Bond dissociation energy is defined as the energy required to break a particular bond to form two radicals. More precisely, bond dissociation energy of the R—X bond, D(R—X), is the enthalpy change of Reaction 9.7.37... [Pg.471]

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. [Pg.327]

The kinetics of thermolysis processes can also be used to obtain important thermodynamic information regarding free-radicals in solution (e.g., bond dissociation energies, etc.) and, as is the case with conventional solvents, sc C02 can be used in this regard. For example, Roth et al. (1996) examined... [Pg.69]

Table V shows the efficient organization of this reaction chemistry into five reaction families. Bond fission, for example, is the elementary step that creates two free radicals from a parent molecule. In chain processes this will often be the initiation step. Thermochemical estimates often show that the logarithm of the Arrhenius A factor (logioA) is of the order 14-17, whereas the activation energy is essentially equivalent to the bond dissociation energy (19,42). This equality is the result of the essentially unactivated reverse reaction step, radical recombination. Table V shows the efficient organization of this reaction chemistry into five reaction families. Bond fission, for example, is the elementary step that creates two free radicals from a parent molecule. In chain processes this will often be the initiation step. Thermochemical estimates often show that the logarithm of the Arrhenius A factor (logioA) is of the order 14-17, whereas the activation energy is essentially equivalent to the bond dissociation energy (19,42). This equality is the result of the essentially unactivated reverse reaction step, radical recombination.

See other pages where Free radicals bond dissociation energies is mentioned: [Pg.275]    [Pg.219]    [Pg.375]    [Pg.14]    [Pg.31]    [Pg.728]    [Pg.146]    [Pg.149]    [Pg.153]    [Pg.23]    [Pg.225]    [Pg.65]    [Pg.146]    [Pg.152]    [Pg.175]    [Pg.72]    [Pg.24]    [Pg.394]    [Pg.840]    [Pg.219]    [Pg.129]    [Pg.129]    [Pg.169]    [Pg.88]    [Pg.155]    [Pg.206]    [Pg.933]   
See also in sourсe #XX -- [ Pg.383 , Pg.384 , Pg.385 , Pg.386 ]




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Bonds bond dissociation energies

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Dissociative bond energy

Energy, bond radicals

Free radical bonding

Radicals bond dissociation energies

Radicals bonding

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