Toluene bond dissociation enthalpy

In principle, the pK of the toluene radical cation can be estimated from the one-electron reduction potential of the radical cation and the C-H bond dissociation enthalpy for toluene (368 kj mor )[62] using equation (5). The resulting pR is ca -10, i.e., considerably more acidic than the phenol radical cation. Nicholas and Arnold have estimated the pfC of the toluene radical cation to between -9 and -13 in acetonitrile which is weU in line with the estimate given here. [32] Since the C-H bond dissociation enthalpies of substituted toluenes seem to be almost invariant with substituent,[63-66] the substituent effect on the pK of... 

To ensure thermochemicai consistency, the H-H and C-H bond dissociation enthaipies were aiso quoted from Ref 124. In the cases of ethane, butane, benzene, and toluene, these are not the most updated values (see, for instance, Blanksby, S. J. Ellison, G. B. Acc. Chem. Res. 2003, 36, 255), but the (upward) adjustments are smaller than the uncertainties of the metal-carbon bond dissociation enthalpies. 

The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C—H bond, like an allylic one, is weaker than a C—H bond of an alkane, as the bond dissociation enthalpies of toluene, propene, and 2-methylpropane attest ... 

As in Section 10.3 where we counted the decreased bond-dissociation enthalpy in propene compared with 2-methylpropane as evidence for stabilization of allyl radical by delocalization of the unpaired electron, we attribute the decreased C—H bond strength in toluene versus propene as reflecting better electron delocalization in benzyl radical versus allyl. 

The driving force for such a reaction is the difference between the free energy of bond formation for YO-H (-AGbf), and the enthalpy of bond dissociation for R-H (AHdbe)- Table 12 summarizes the YO-H bond-formation energies for oxygen radicals (Y0-), as well as dissociative bond energies for several R-H substrates. Reference to these data indicates that t-BuO- will react exothermally with toluene (PhCH3), but will be essentially unreactive with methane. 

To gain more information concerning the C-C bond cleavage of the pinacolate 8a, a variable-temperature UV-Vis spectroscopic study in toluene was carried out. The derived dissociation enthalpy DHc1iss for 8a to give 7c is 11 kcal/mol (Scheme 8) [7]. This value is much smaller than that reported for silicon benzo-... 

First of all the BDE itself may depend on the solvent as a result of contributions of solvation to the enthalpy of the bond dissociation process, i.e., differences between the solvation enthalpies of the reactant (L M-R) and products (L M. + R.) of Equation 1. There is not much direc information about tRis. Most of the applications of ghe kinetic method to dat have involved low spin octahedral d complexes (notably of Co ) where the product of homolysis is a 17 electron low spin five-coordinate d complex. Such complexes generally exhibit little tendency to add a sixth ligand so that solvent influences due to coordination are likely to be small. Support for this is provided by invariance of the Co-C BDE of PhCH(CH )-Co(DH)2Py in solvents of considerably different polarity and coordinating ability, e.g., toluene and acetone. Bond dissociation processes that are accompanied by substantial changes in charge distribution (unexpected for the homolytic dissociation of neutral radicals) are, of course, expected to exhibit significant medium effects. 

The enthalpy change was measured for the first two reactions in toluene, with R = Cy and Ph. When these equations are combined, one obtains the third equation, so that AH = AH - AH . With the standard assumptions that the bond energies in the (Cp XMe3P)Ir firagment are the same in all the species and that the Ir—bond energy is independent of the other ligands, AH is related to the bond dissociation energies, D, by... 

As expected, the activation enthalpy, AH, is close to the (CO)3Ni-CO BDE of 25 2 kcal/mol. Also as predicted for a dissociative mechanism, the activation entropy, AS, is positive (8 1 eu in toluene and 13 1 eu in hexane). The positive AS lies in the range expected for a reaction in which the M-L bond is stretched to two or three times the length of the bond in the ground state " and experiences additional rotational freedom in the transition state. " Because the starting complex and the transition state are both single particles, the contribution of translational entropy to AS is zero," and the absolute value of the entropy term is smaller than that for an associative process. 

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