C-H bond energy

Procedure. Run one or more simultaneous equation programs to determine the C—C and C—H bond energies and interpret the results. The error veetor is the veetor of ealeulated values minus the veetor of bond enthalpies taken as tme from an aeeepted source. Caleulate the enor veetor using a standard souree of bond enthalpies (e.g., Laidler and Meiser, 1999 or Atkins, 1994). Expand the method for 2-butene (2-butene) = —11 kJ mol ] and so obtain the C—H, C—C,... 

Radicals derived from hydrofluorocarbons (HFCs) as well as hydrofluo-roethers (HFE) are often destabilized with respect to the methyl radical [51, 57,68,70,79-82], The low stability of these radicals implies that the C-H bonds in the corresponding closed shell parent compounds are comparatively strong and thus rather unreactive towards attack of oxidizing reagents. This latter property is of outstanding importance for the use of these compounds in a variety of technical applications, in which thermally stable, non-oxidizable, non-flammable compounds are needed. However, with respect to the environmental fate of these compounds high C-H bond energies... 

Szwarc, M. The C—H-Bond Energy in Toluene and Xylene. J. chem. 

The most satisfactory species fulfilling these requirements is a half-hydrogenated radical. This is linked to the surface preferably through the tertiary carbon atom (which has the lowest C—H bond energy). The loss of a second hydrogen atom from its vicinity may lead to 3-methyl-2-pentene... 

Besides having smaller oxidation potential values than substituted benzyl alcohols (E° > 1.4 V/NHE), the DMAs have larger energy values (90-92 kcalmoD ) for the NC—H bond with respect to C—H bond energies around 75-85 kcalmoD of the benzyl alcohols (Scheme 12). Both factors disfavour the operation of the radical HAT route for PINO with the DMAs, and cause a mechanistic changeover to the ET route, as opposed to the reactions with the benzylic substrates listed in Table 4. 

As a result, a rich catalytic combustor exhibits better light-off performance than a lean catalytic combustor. Veser et al. [136] also found that the ignition temperature correlates well with the C-H bond energy of the hydrocarbon, in line with the crucial role of activation of the first C-H bond proposed in the literature [140, 141[. 

This requires sufficient energy inserted into the relevant bond vibration for the bond to break or for bonding locations to move. C-C and C-H bond energies in stable alkanes are greater than 80 kcal/molc, and these processes are very infrequent. As we wiU see later, hydrocarbon decomposition, isomerization, and oxidation reactions occur primarily by chain reactions initiated by bond breaking but are propagated by much faster abstraction reactions of molecules with parent molecules. 

MNDO or ab initio calculations (Table 5.3). Further confirmation for the preference of 1,2-addition was established by ab initio calculation of the C-H bond energy in hydrogenated fullerenes [35]. Hybrid density functional theory using the B3LYP functional with the 6-31 G(d,p) basis set leads to the bond energies shown in Table 5.3. The most stable bond is found in 1,2 adducts with a bond energy of 2.86 eV, followed by a bond energy of 2.69 eV in 1,4-adducts. All the other addition patterns such as 1,3 addition or addition to a [5,6] bond lead to less stable C-H bonds (Table 5.3). 

Table 5.6 C-H bond energies for some isomers of the series CgoHi2n n = 1-5 [35. ...

The different behavior of the alcohols probably arises from differences in bond dissociation energies. Experiments show that radical attack on methanol (4) and ethanol (27) leads to rupture of the C—H rather than the O—H bond. There appear to be no direct measurements of C—H bond energies in alcohols. However, D(R—OH) has been determined as 102 kcal. and does not appear to vary greatly with changes in R, provided R is a simple alkyl radical (16). Moreover, the heat of rearrangement of alkoxy radicals to hydroxyalkyl radicals has been determined from electron impact data (12). Considering, for example, 2-propanol and the following reactions... 

Somewhat analogous reactions would be expected for the reaction of ethylene with 02 ions but the observed reaction rate is lower than for propene, suggesting that the reaction pathway may be controlled by the C—H bond energies. For reactions of propane and 1-butene with 02, oxygenated compounds of the same carbon number as the reactants were produced. The initial step is thought to involve a hydrogen atom abstraction from a secondary carbon atom. 

In comparing the activation energies for reactions of hydroxyl with ethane, ethylene, and acetylene it may be concluded that a similar reaction occurs in all three cases, namely, H atom abstraction. The activation energy increases from ethane to acetylene depending upon the C-H bond energy. 

The order of reactivity of different alkyl-substituted benzenes is toluene > ethylbenzene > isopropylbenzene, which is opposite to the general reactivity expected from the C—H bond energies. This was explained in terms of an initiating electron transfer equilibrium between Co(OAc)3 and the arene as the rate-determining... 

The initiating step in the oxidation of methane is the first abstraction of a hydrogen atom. However, because of the tetrahedral molecular structure with comparatively high C-H bond energies, the methane molecule is extremely stable, and at lower temperatures the initiation step may be rate limiting for the overall conversion. In methane-oxygen systems, the chemistry is generally initiated by reaction of CH4 with O2,... 

Effect of hybridization on overlap and bond Molecule Hybridization C—H bond energy (Id not"1) C—H bond length (pm)... 

It is important io recognize that the bond energies listed in Table 4-3 for all molecules other than diatomic molecules are average values. That the C-H bond energy is stated to be 98.7 kcal does not mean that, if the hydrogens of methane were detached one by one, 98.7 kcal would have to be put in at each step. Actually, the experimental evidence is in accord with quite different energies for the separate dissociation steps ... 

From AH° and any other required bond energies in Table 4-3, compute a second C-H bond-energy value for methane. 

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