Substitution bond dissociation energies

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzyiic position). Explain, based on the bond dissociation energies in Table 5.3 on page 156. 

Also pertinent to Step 1 is the material in Table II, which includes bond dissociation energies and kinetic data at conversion temperatures for a series of C-C bonds. For the purposes of this discussion it can be assumed that substitution of -0- for -CH2-... 

More than just a few parameters have to be considered when modelling chemical reactivity in a broader perspective than for the well-defined but restricted reaction sets of the preceding section. Here, however, not enough statistically well-balanced, quantitative, experimental data are available to allow multilinear regression analysis (MLRA). An additional complicating factor derives from comparison of various reactions, where data of quite different types are encountered. For example, how can product distributions for electrophilic aromatic substitutions be compared with acidity constants of aliphatic carboxylic acids And on the side of the parameters how can the influence on chemical reactivity of both bond dissociation energies and bond polarities be simultaneously handled when only limited data are available ... 

Compared with GeHq, the bond dissociation energies of these hydrides are not affected by alkyl substitution and are in the range of 81.6-82.6 kcal mol-1. Aryl substitution results in a slightly weakened Ge—H bond (79.2-80.2 kcal mol-1). In the cases of phenyl-, diphenyl- and triphenylgermane10, the Ge—H bond dissociation energies have been found to be identical within experimental error (Table 2). The Ge—H BDE value... 

The bond dissociation energies (BDE) of several alkyl- and aryl-substituted germanium hydrides were measured by the laser-induced photoacoustic effect22. The alkyl-substituted compounds exhibited similar BDEs to that of GeEU (81.6-82.6 kcal mol 1) while aryl substitution results in a slight weakening of the Ge—H bond (BDEs 79.2-80.2 kcal mol 1). 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

The reaction of pyridine (py) with bare metal ions (except Fe+) has not been studied widely. The reaction of Fe+ produced by electron ionization of Fe(CO)5 with a mixture of two pyridines (108) was used to compare the proton affinities with the Fe+ affinity. A good correlation was observed. The absolute Fe+ affinity of py was determined to be 49 3 kcal mol-1, which is higher than the value of 44 3 kcal mol 1 for the Fe+-NH3 bond dissociation energy (46). Steric problems with ortho substituted pyridines gave lower than expected affinities. The reaction of py and substituted pyridines showed a maximum addition of four pyridines, similar to the GIB experiments with ammonia (46). 

Local HSAB principle can also be used to calculate the relative homolytic bond dissociation energies (BDE). For the homolytic dissociation of para-substituted phenols ... 

Figure 1 Bond dissociation energies for some methyl and substituted methyl halides...

Substitution has no significant effect on the geometry of the n complex and RuHCl(PH3)2(CH2=CH2) and RuHCl(PH3)2(H2C=CH(OMe)) have very similar shape. Of interest the OMe group cannot reach the second empty coordination site (trans to H) of Ru. Remarkably, the Ru-alkene bond dissociation energy is also not affected by the presence of OCH3 (less than 3 kcal.mol 1 difference in binding dissociation energy). The methyl vinyl ether... 

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. 

Bond dissociation energies (BDEs) provide a measure of both the reactivity of a compound (with respect to homolytic bond rupture) and the stability of the corresponding radical. There have been many theoretical investigations of BDEs for a wide variety of species [36], In particular, the C-H BDE for a substituted methane is given by the enthalpy change for the reaction ... 

Bond dissociation energies for a selection of substituted methanes, calculated at a range of levels [23], are compared with experimental values [37] in Tables 6.9 and 6.10. Also listed are mean absolute deviations (MADs) and mean deviations (MDs) from experimental values [e.g. MAD(Exp.)] and from CBS-RAD [e.g. MD(CBS-RAD)]. 

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 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. 

The pKi, values of a series of para- and meffl-substituted benzaldoximes and phenyl methyl ketoximes, ArCR=NOH (R=H, Me), have been measured in DMSO. The aldoximes exhibit pK. = 20.05 + 3.21ap. The homolytic bond dissociation energy of the O-H bond has been estimated as 88.3 (aldoximes) and 89.2kcal mol" (ketoximes) by relating the pK to the oxidation potential of the conjugate base (i.e. ox for ArCR=NO- ArCR=NO ). 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

TABLE 5. NO—H bond dissociation energy values of X-aryl-substituted Ai-hydroxyphthaUmides (X-HPIs), listed with the Hammett p values (vs. ct+) and h/ d ratios obtained in the oxidation of substituted benzyl alcohols using the X-HPIs/Co(II)MCBA/02 system in MeCN solution at 25 °C... 

Similar decomposition is observed in p-bromoacetophenone, o-bromo-, p-bromo, and p,p -dibromobenzophenone, and p-iodobenzophenone44 but not in the fluoro- and chloro-substituted compounds. This order of reactivity follows the bond dissociation energies for aromatic halides which are about 90 kcal/mole for chlorobenzene, 70 kcal/mole for bromobenzene, and 60 kcal/ mole for iodobenzene. The lowest-lying triplet of p-bromoacetophenone is 71.2 kcal45 while that of the substituted benzophenones is slightly lower since benzophenone itself has a lower triplet energy than acetophenone. p,p Dibromobenzophenone was the least reactive of the compounds that photoeliminated halogen atoms. 

Similarly, if we look at the H-C bond-dissociation energies of the hydrocarbons shown in Table 4-6, we would infer that Cl- would remove a hydrogen most rapidly from the carbon forming the weakest C-H bond and, again, this is very much in accord with experience. For example, the chlorination of methylbenzene (toluene) in sunlight leads to the substitution of a methyl hydro-... 

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