Dissociation C-H bond

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... 

This seems reasonable when we think only in terms of normal vibrations, but intuition suggests that, since the dissociation in Equation (6.90) would require something like six times the C—H bond dissociation energy ca 6 x 412 kJ mol ), the process... 

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. 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

Corresponds to the C—H bond dissociation energy in the trimethylsilyl radical, implying a K bond strength of 192-193 kJ/mol. 

Energies of a-C—H Bonds Dissociation in Ethers and Enthalpies of Secondary Alkylperoxyl Radical Reactions with Ethers [2]... 

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]... 

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... 

The Values of a-C—H bond Dissociation Energy in Acids (Dc H) and Enthalpies (AH) of Secondary Alkylperoxyl Radical (R1R2CHOO ) Reaction with Acids [2]... 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

Four methods are used for generating pools of these onium ions oxidative C-H bond dissociation, oxidative C-Si bond dissociation, oxidative C-S bond dissociation, and oxidative C-C bond dissociation (Scheme 3). In the following sections, we will discuss the principles and synthetic applications of these methods. 

If we consider the generation of alkoxycarbenium ions by C-H bond dissociation, ethers should be of our first choice as precursors of alkoxycarbenium ions by analogy to carbamates. The oxidation potentials of ethers, especially aliphatic ethers, however, are very positive, and therefore, it is rather difficult to oxidize ethers selectively under usual conditions. The regioselectivity is also a problem. Usually a mixture of two regioisomers of products is obtained because two regioisomeric alkoxycarbenium ions are generated. 

This is related to reaction (X) for propene, but for isobutene this process is unlikely because it involves formation of a 2-methylallyl ion and destruction of a tertiary ion in the gas phase this reaction would be highly endothermic [113] because the ionisation potential of the 2-methylallyl radical [114] is appreciably greater than that of the tertiary butyl radical [115], and the difference in the homolytic C—H bond dissociation energies is in the same... 

Table 6.9 Comparison of experimental C-H bond dissociation energies at 0 K (kJ/mol) with those calculated with wavefunction-based electronic structure methods.

The radical stabilization energy (RSE) of a substituted methyl radical CH2X is generally defined as the difference between the C-H bond dissociation energy in methane and the C-H BDE in the substituted methane CH3X ... 

The value of 169.9 kJ mol" (1.75 eV) corresponds to AE(ch3+) = 14.35 eV which is in good agreement with published values of about 14.3 eV. [28,39] In addition, this is only 40 % of the homolytic C-H bond dissociation enthalpy of the neutral methane molecule, thereby indicating the weaker bonding in the molecular ion. 

Esr-spectroscopic measurements 145 C—H Bond-dissociation energies 151 C—-C Bond-dissociation energies 154 Rotational barriers 159 Isomerization reactions 163 Addition reactions 170 Azoalkane decomposition 171... 

The determination of thermodynamic stability of a radical from C—H bond-dissociation energies (BDE) in suitable precursors has a long tradition. As in other schemes, stabilization has to be determined with respect to a reference system and cannot be given on an absolute basis. The reference BDE used first and still used is that in methane (Szwarc, 1948). Another more refined approach for the evaluation of substituent effects by this procedure uses more than one reference compound. The C—H BDE under study is approximated by a C—H bond in an unsubstituted molecule which resembles most closely the substituted system (Benson, 1965). Thus, distinctions are made between primary, secondary and tertiary C—H bonds. It is important to be aware of the different reference systems if stabilization energies are to be compared. 

In Table 2.4, we have collected background information for discussion in the following chapters. Recommended C—H bond dissociation enthalpies of selected organic compounds are reported in the first two columns, followed by a variety of heteroatom-hydrogen bond strengths including N—H, O—H, S—H, Ge—H, and Sn—H bonds. 

Loss of the first H atom from PH2 gives a free radical PH which is strongly stabilized due to the formation of one fully aromatic unit. The effective 4 —H bond dissociation energy, D(PH—H) assumes a very low value of ca. 47 Kcal/mole (or below) just because of this aromatic stabilization This value of D(PH-H) should be compared with the usual C—H bond dissociation energy, e.g., D((CH3)3C—H) =... 

---