C-H BDEs with a-OH

The broken bonds (boldface = dissociated atom), AfH°(R), kcal/mol (kj/mol) BDEs (boldface = recommended data reference in parentheses) Methods (reference in parentheses)  

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From this linear correlation, and entering the experimental E value determined for 4-Me0C6H4CH20H (Table 7), a BDE value of 77 1 kcalmoH could be extrapolated for the benzylic C—H bond bearing a geminal OH group. As a matter of fact, the BDEc h of benzyl alcohols was not experimentally available or reported with reasonable confidence the extrapolated value compares well with a BDEc h of 81 1 kcalmol" that could be extrapolated for PhCH20H from data of Espenson and coworkers. Because the BDEc h of toluene is 88.5 kcalmol", the extrapolated value... 

It is seen that the weaker the attacked a-C—H bond the higher the partial rate constant. Therefore, the strength of the bond of alcohol attacked by the peroxyl radical is a very important factor of alcohol reactivity. It is seen from the data in Table 7.7 that peroxyl radicals of different structures react with the same alcohol with different rate constants. The comparison of the rate constants of reactions RiOO + HR1 OH with the strength of the formed —H bond in hydroperoxide shows the higher the BDE of the —H bond in RjOOH the more rapidly peroxyl radical R 00 reacts with cyclohexanol (T= 348 [52]). 

The HO-H bond dissociation energy (BDE) is 499 kj mol-1, while the C-H bonds in saturated hydrocarbons are much weaker (BDE = 376-410 kj mol-1 Berkowitz et al. 1994 for a compilation, see Chap. 6). Thus, there is a considerable driving force for H-abstraction reactions by -OH. On the other hand, vinylic hydrogens are relatively tightly bound, and an addition to the C-C double bond is always favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the case of ethene, no vinylic radicals are formed (Soylemez and von Sonntag 1980), and with benzene and its derivatives the formation of phenyl-type radicals has never been conclusively established. 

The BDEs in the series NO2CXH2 (X = H, F, OH, NH2, CH3, CH=0 and CH=CH2) have been calculated (MP4/6-31G //HF/3-21G), and compared with those previously calculated and the experimental values. The BDE were analyzed in terms of IBE (Intrinsic Bond Energies) and radical stabilization energies by comparing the BDE of C—H and C—NO2 in a series of related compounds. They are almost constant (59-62 kcalmoD ) due to a near cancellation of the increase in IBE (more positive)... 

In many cases electrochemistry proved to be a powerful tool to activate the SnAt reactions. The cathodic reduction in aromatic compounds, prior to their interaction with nucleophilic reagents, is a new essential step of the S Ar reactions. Moreover, a basic thermodynamic study explains (BDEs values of C-Nu vs. C-H), why F, OH, OR, and SR nucleophiles do not react with arenes or heteroarenes. 

Determination of the acidities of eight enols R1R2C=C(OH)R3 in DMSO and the oxidation potentials of then conjugate bases have permitted the estimation of the corresponding H—O bond dissociation enthalpies since BDEha = 1.37pXwA + 23.1 ox(A ) + 73.3 kcal mol-1.129 The results have been discussed in terms of steric and electronic effects. For MeS2C=C(OH)R, pAfrA increases, E0X(A ) becomes more negative, and BDE decreases with the bulk of R. 

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