Dissociation energy of C H bonds

Table 1.18 Inversion barrier of methyl and fluoromethyl radicals and dissociation energy of C—H bond of fluoromethanes ...

The chemistry of the alkanes is dominated by the abstraction of a hydrogen atom by various free radicals. The bond dissociation energies of C-H bonds decrease in the order primary > secondary > tertiary > benzylic and allylic, and thus free radical reactions tend to occur at tertiary, benzylic or allylic centres. 

Table 8.1. Dissociation energy of C-H bonds and stabilization energy of the corresponding hydrocarbon free radicals...

Figure 6.15 Correlation between the bond dissociation energies of C-H bonds in CH3R molecules and the electrophilicity indexes of the corresponding CHjR radicals.

Bordwell et al., 1988, 1989) and Amett (Amett et al., 1990a,b, 1992 Venimadhavan et al., 1992) have employed thermodynamic cycles consisting of heterolysis of a molecule and redox processes of the resulting ions to evaluate homolytic dissociation energies of C—H, C—C, C—N, C—O and C—S bonds in solution. In a similar way, knowledge of the A//het(R-R ) values allows determination of the heat of homolysis of carbon-carbon bonds [A/fhomo(R"R )] using (27). The results are summarized in Table 4. 

The dissociation energy for C-H bond in methane (E = 436 kj/mol) is one of the highest among all organic compounds. Its electronic structure (i.e., the lack of n- and n-electrons), lack of polarity, and any functional group makes it extremely difficult to thermally decompose the methane molecule into its constituent elements. 

M. Lucarini, P. Pedrielli, G. F. Pedulli, S. Cabiddu, C. Fattuoni. Bond Dissociation Energies of O-H Bonds in Substituted Phenols from Equilibration Studies. J. Org. Chem. 1996, 61, 9259-9263. 

Table 2. Bond dissociation energies of alkenes, alkynes, and aromatics Table 3. Bond dissociation energies of C/H/O compounds Table 4. Bond dissociation energies of sulfur-containing compounds Table 5. Bond dissociation energies of nitrogen-containing compounds Table 6. Bond dissociation of halocarbons...

Dissociation energies for C—H bonds have been obtained from electron impact measurements both by the direct and indirect method. The dissociation energy D CH3—H) in methane was deter-niinecD by measuring the appearance potential of the CHI ion when methane was fed into the ionization chamber. This is taken to be the energy of the process... 

The bond dissociation energies for C—H bonds, and the heats of formation of the alkyl radicals which have been obtained from electron impact measurementsare shown in Table 1, together with the heats of formation of the hydrocarbons on which they are based. 

Figure 1 Bond-dissociation energies of C—H and C—X bonds (experimentai bond en-thaipies, AH298 (kcai/moi)) (2003ACR255).

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

Similarly by comparing the bond dissociation energies of the two different types of C—H bonds m 2 methylpropane we see that a tertiary radical is 30 kJ/mol (7 kcal/ mol) more stable than a primary radical... 

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. 

The Values of C—H bond Dissociation Energies in Ketones [1,2] and Enthalpies AH of Reactions sec-RCV + R1CH2C(0)R2 > sec-ROOH + R1CHC(0)R2... 

C. Wickleder, D. Henseler, and S. Leutwyler, Accurate dissociation energies of O H O hydrogen bonded 1 naphthol solvent complexes. J. Chem. Phys. 116, 1850 1857 (2002). 

The C-H bond-dissociation energies Z>c-h of the acetals 130 and 131, monothioacetal 132, and the dithioacetal 133 (Figure 7) were calculated from experimental reaction kinetic data <2005MI379>. The C-H dissociation energies of the 1,3-dithiane derivative 133 proved to be lowest, followed by the 1,3-oxathiane derivatives 132,... 

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... 

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