Bond dissociation energy values carbon-hydrogen

The thermodynamic stabilization was determined from the barrier to rotation, by thermochemical methods,and from thermolysis studies of bis(cyclopropylalkyl)diazenes, and found to be approximately 1.9 kcal mol . The value of the carbon-hydrogen bond dissociation energy, DH (cyclopropylmethyl-hydrogen), deduced from this is 98.5 kcal mol , which may be compared with the DH (ethyl-hydrogen) value of 100 kcal mol This demonstrates that there is a small but chemically significant interaction between the orbital containing the unpaired electron (SOMO) and orbitals of the C -C, bond. 

Alternatively, the BDE values may be reported relative to the C-H bond dissociation energy in methane (3) as the reference. This is quantitatively described in Equation 5.3 as a formal hydrogen transfer process between methane (3) and a substituted carbon-centered radical 2. The reaction enthalpy for this process is often interpreted as the stabilizing influence of substituents Rj, R2, and R3 on the radical center and thus referred to as the radical stabihzation energy (RSE). When defined as in Equation 5.3, positive values imply a stabilizing influence of the substituents on the radical center. The RSE energies are connected to the BDE values in Equations 5.land 5.2 as described in Equation 5.4. 

Here AHf (A ) is the heat of formation of radical A, AHf (B ) is the heat of formation of radical B, and AHf (A-B) is the heat of formation of A-B. DH° (A-B) is also called the bond dissociation energy of A-B. Table 1.10 gives a list of standard bond dissociation enthalpies for bonds involving hydrogen atoms, and Table 1.11 gives a list of DH° values for bonds between carbon atoms in various alkyl groups and a number of common organic substituents. ... 

The carbon-hydrogen bond in methane is very strong its bond dissociation energy is 105 kcal/mol. This means that 105 kcal/mol must be applied in order to break one of the carbon-hydrogen bonds in methane homolytically. This reaction is endothermic by 105 kcal/mol (Fig. 2.8). Compare this value to the 104 kcal/mol bond strength of the very strong H—H bond in H2 (p. 36). The C—H bond in methane and the H—H bond in H2 have very similar strengths. 

The standard enthalpy change for bond dissociation is usually given by the symbol DH . A hst of some bond dissociation energies is given in Table 3.5. The bond cleaved is indicated by a dash. For CHj—H, the DH° value for cleaving the carbon—hydrogen bond refers to the following process. 

Figure 21-1 Energy of H2 as a function of the internuclear distance between the hydrogens. The zero of energy is that of two hydrogen atoms at an infinite distance from one another. The corresponding curve for dissociation into H and H e would reach about+295 kcal mole-1 at infinite separation. Similar curves are expected for other electron-pair bonds, but the re values are much larger. Thus bonds to carbon commonly fall in the range of 1,1 A (C-H bonds) to 2.2 A (C—I bonds). The upper dashed curve is for the approach of two hydrogen atoms whose electrons have parallel spins.

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