The O—H bond dissociation energies

Combination of the phenol acidity A//acid(PhOH) = 1458 8 kJmol and the EA value given above yields the gas-phase bond dissociation energy of phenol BDE(PhO—H) = 362 8 kJmor . Photoacoustic calorimetry studies in various solvents having different hydrogen-bond accepting properties provided values ranging from 360 to 369 kJmol A spectroscopic ESR equilibrium method for measuring 

In general, the effect that a substiment exerts on the phenoxyl radical is by far more important than that on the corresponding phenol. An empirical equation relating the differences in phenolic O—H strengths (in kJmoU ) to the sums of the cr-constants for all the ring substituents has been proposed (equation 34), 

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The antioxidant efficiency of phenolic acids, as determined by the accelerated autooxidation of methyl linoleate and scavenging of the free radical 2,2-diphenyl-1-picrylhydrazyl (141) ° methods, was found to be inversely proportional to the maximal detector response potential in the voltammetric determination of these compounds. No similar correlation was found for the flavonoids . A good correlation was found between the O—H bond dissociation energy of a phenolic compound and its effectiveness as antioxidant, expressed as the rate constant of free radical scavenging . The bond dissociation energy of the phenol O—H bond was estimated by a three-dimensional quantitative structme-activity relationship method incorporating electron densities computed using the Austin Method 1 (AMI) followed by correlation of the... 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

The Values of the C—H Bond Dissociation Energies in Aldehydes DC—h and Enthalpies AH of the Reaction of Acylperoxyl Radical (RC(O)OO ) with Aldehydes [2]... 

Wood [127] reported an innovative development of the Barton-McCombie deoxygenation of alcohols allowed to work under tin-free conditions. A trimethylborane-water complex proves to be an efficient reagent for the reduction of xanthates. Complexation of water by trimethylborane induces a strong decrease of O - H bond dissociation energy from 116 kcal/mol (water) to 86 kcal/mol (Me3B-water complex). 

This, initially surprising, finding has been shown (143) to originate in the difference in O-H bond dissociation energies between the organic and inorganic pairs listed in Table VII. The lower limit of the value for CraqOH2+ (bde > 373 kJ/mol) was estimated (143) from the reduction potential of the CraqO )Cra couple ( L6 V), Ka for Craq (10 4 M ), and the oxidation potential for H (2.29 V). 

The difference between the O—11 bond-dissociation energies for H20 and Oil can be ascribed to the stabilization energy of the normal state, 3P, of the oxygen atom. When one O—H bond is broken in the water molecule there is produced, in addition to a hydrogen atom, an... 

The logarithm of the rate constants of H-abstraction by reactive oxygen species decreases with decreasing O-H bond dissociation energy from OH [BDE(HO-H) = 497 kj mol1] > RO [BDE(RO-H) = 439 kj mol1] > ROO [BDE(ROO-H) = 372 kj mol"1], and this sequence also includes the even less reactive metal-oxo complexes, e.g., permanganate (Mayer 1998). 

The dynamics of hydrogen abstraction reactions promoted by F, O, OH and OD with monogermane have been studied as a function of the vibrational and rotational state by infrared chemiluminescence While this technique provides enormous insight on the energy disposal in a reaction, it also led to a value of 326 4 kJmoP for the H3Ge—H bond dissociation energy at 0 K. This value is somewhat lower than the value of 346 10 kJmoP obtained from a kinetic study of reaction 44 and its thermochemistry . ... 

The reported Arrhenius parameters for the diacyl peroxides appear quite reasonable. For example, the heat of formation of the acetoxy radical deduced from the observed activation energies is A/f (CH3COO-) = —49.7 kcal.mole . This gives a bond dissociation energy of the acidic hydrogen in acetic acid of >(CH3C02 -H) = 106 kcal.mole , very comparable to (O-H) bond dissociation energies in alcohols, -factors are also very reasonable. Since two internal rotations become partially restricted in the transition state as shown below... 

Dissociation energy The energy necessary to cleave a particular bond to give the constituent radicals, e g. HO-H to give HO and H requires 495 kJ mol-1 while H-0 to give 0 and H requires 420 kJ mol-1. The average of (approximately) 460 kJ mol-1 is taken as the O-H bond association energy in water. 

From the reduction potentials at pH 0 and estimated values for the free energies of solvation of phenol and phenoxyl in water, gas-phase O—H bond dissociation energies have been calculated. The values derived from such calculations are given in Table 12. They are comparable to values determined by other methods which are discussed in Chapter 3. 

We have also discussed the use of the electrostatic potential for the analysis of substituent effects in aromatic systems. Substituent effects on gas phase and solution acidities of benzoic acids and phenols are dominantly determined by the relative stabilization of the negative charge in the ionized forms of these systems. The oxygen Vmin is an excellent tool for the analysis of this stabilization effect. On the other hand, we have found that the homol5dic O-H bond dissociation energy in phenols depends both on the substituent s ability to stabilize the parent molecule (the phenol) and the radical. The relative stabilization energies of the parent molecule and the radical can be estimated from their computed Vmin and surface maxima in the spin density, respectively. 

The mean O-H bond dissociation energy is hydrogen peroxide, defined as half the standard energy of the reaction... 

In conclusion, vitamin E and 2,6-di-tert-butyl-phenols (AOl, A02 and A04 for example) displays some commonality because of comparable O-H bond dissociation energy in the phenol group. Then, they have the same reactivity at least for the POO° -I- AH POOH -I- A° reaction. However, vitamin E can also trap P° radical which is not the case... 

Kondo, O., and S. W. Benson (1984), Kinetics and equlibria of the reaction between bromine and ethyl ether.The C—H bond dissociation energy in ethyl ether, Int. J. Chem. Kinet, 16, 949-960. 

We will now look at how different types of wave functions behave when the O-H bond is stretched. The basis set used in all cases is the aug-cc-pVTZ, and the reference curve is taken as the [8, 8J-CASSCF result, which is slightly larger than a full-valence Cl. As mentioned in Section 4.6, this allows a correct dissociation, and since all the valence electrons are correlated, it will generate a curve close to the full Cl limit. The bond dissociation energy calculated at this level is 122.1 kcaPmol, which is comparable to the experimental value of 125.9 kcal/mol. 

The influence of an ort/io-imidazole substituent on the bond dissociation energy of the O—H bond in phenol was studied by DFT calculations [00IJQ714]. The imidazole ring is twisted with respect to the phenol ring by 59° and causes a decrease of the bond dissociation energy by about -1 kcal/mol with respect to the unsubstituted molecule only. 

There are two kinds of bond energy. The energy necessary to cleave a bond to give the constituent radicals is called the dissociation energy D. For example, D for H2O—>HO -f H is 118 kcal mol (494kJmol ). However, this is not taken as the energy of the O—H bond in water, since D for H—O H -f O is 100 kcal... 

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