Bond dissociation energy values

The radical stabilization provided by various functional groups results in reduced bond dissociation energies for bonds to the stabilized radical center. Some bond dissociation energy values are given in Table 12.6. As an example of the effect of substituents on bond dissociation energies, it can be seen that the primary C—H bonds in acetonitrile (86 kcal/mol) and acetone (92kcal/mol) are significantly weaker than a primaiy C—H... 

TABLE 5. NO—H bond dissociation energy values of X-aryl-substituted Ai-hydroxyphthaUmides (X-HPIs), listed with the Hammett p values (vs. ct+) and h/ d ratios obtained in the oxidation of substituted benzyl alcohols using the X-HPIs/Co(II)MCBA/02 system in MeCN solution at 25 °C... 

R. Walsh, Acc. Chem. Res., 14, 246 (1981). Bond Dissociation Energy Values in Silicon-Containing Compounds and Some of Their Implications. 

The Y(C5)C0J 0 acid radical which results from the dissociation of Y(C5H5)C(0H)=0 (Y(C=CC=CCH—)C(0H)=0 to Y(C=CC=CCH-)C(0 )=0), in which the carboxyl group is attached to an a sp carbon has a BE of 110.01 kcal mof. The bond dissociation energy value of benzoic acid reported by Blanksby [137] at 111 kcal mof is slightly higher. A second comparison acid 0—H bond occurs in the dissociation of prop-2-enoic acid, CH2=CHC(0— H)=0 to CH2=CHC(0 )=0, where we find 110.14 kcal mol, in agreement with the three values above. 

The experimental bond dissociation energy values of halogen are ... 

Selected Bond Dissociation Energy Values for Bonds in Specific Molecules... 

However, they also reported extremely comparable bond dissociation energies values for the O-H group of phenol (about 334 8 kj mol 0. which is in contradiction with the observed difference between rate constants towards POO° radicals (more than 2 decades). 

For many purposes, for example the estimation of approximate heats of formation (p. 63), it is sufficient to have an average value. This average of the bond dissociation energies is called the average thermochemical bond energy or (more commonly) simply the bond energy. ... 

Even within a particular approximation, total energy values relative to the method s zero of energy are often very inaccurate. It is quite common to find that this inaccuracy is almost always the result of systematic error. As such, the most accurate values are often relative energies obtained by subtracting total energies from separate calculations. This is why the difference in energy between conformers and bond dissociation energies can be computed extremely accurately. 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

Bromination of methane is exothermic but less exothermic than chlorination The value calculated from bond dissociation energies is AH° = -30 kJ Al though bromination of methane is energetically fa vorable economic considerations cause most of the methyl bromide prepared commercially to be made from methanol by reaction with hydrogen bromide... 

Because these stability measurements pertain to the gas phase, it is important to consider the effects that solvation might have on the structure-stability relationships. Hydride affinity values based on solution measurements can be derived from thermodynamic cycles that relate hydrocarbon p T, bond dissociation energy and electrochemical potentials. The hydride affinity, AG, for the reaction... 

From this value and known C—H bond dissociation energies, pK values can be calculated. Early application of these methods gave estimates of the p/Ts of toluene and propene of about 45 and 48, respectively. Methane was estimated to have a pAT in the range of 52-62. Electrochemical measurements in DMF have given the results shown in Table 7.3. These measurements put the pK of methane at about 48, with benzylic and allylic stabilization leading to values of 39 and 38 for toluene and propene, respectively. The electrochemical values overlap with the pATdmso scale for compounds such as diphenyl-methane and triphenylmethane. 

The latter equation contains constants with well-known values and can therefore be used to predict the fracture stress of most polymers. For example, the bond dissociation energy Do, is about 80 kcal/mol for a C-C bond. For polystyrene, the modulus E 2 GPa, A. 4, p = 1.2 g/cm, = 18,000, and we obtain the fracture stress, o A1 MPa, which compares well with reported values. Polycarbonate, with similar modulus but a lower M. = 2,400 is expected to have a fracture stress of about 100 MPa. In general, letting E 1 GPa, p = 1.0 g/cm, and Do — 335 kJ/mol, the tensile strength is well approximated by... 

AH and AS to various notional subprocesses such as bond dissociation energies, ionization energies, electron affinities, heats and entropies of hydration, etc., which themselves have empirically observed values that are difficult to compute ab initio. 

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

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. 

---