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What increases bond energy?

The bond dissociation energy increases as the difference in the electronegativities of the bonded atoms increases. For example, the bond dissociation energies of carbon–halogen bonds increase in the order C—I < C—Br < C—Cl < C—F. The polarities of the carbon–halogen bond are in the same order.

sciencedirect.com - Bond Dissociation Energy - an overview | ScienceDirect Topics

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6.4.3 Bond Strength

The average bond dissociation energies of various metal alkyls have been redetermined after assessing the validity of available standard heats of formation (ΔH f ).261 An acceptable set of ΔH f ° values is required to meet the following criteria: (a) within a class of compounds (alkyls, halides, hydrides, etc.), −ΔH f ° should increase with increasing chain length with the homologous increment in the −ΔH f ° value being constant starting with R = Et; (b) the heats of formation from class to class (e.g. R 3 Al → R 2 AlCl) must be consistent; and (c) heats of reaction calculated from such ΔH f ° values must give acceptable agreement with experimentally observed heats. A compilation of such consistent ΔH f ° values for liquid (l) and gaseous (g) monomers of R 3 Al and R 2 AlH and for dimers of R 2 AlCl is given in Table 12. Table 12. Calculated Enthalpies of Formation for Organoaluminum Compounds1 Compound −ΔH f ° (M, g) a (kJ mol−1) −ΔH f ° (M, l) b (kJ mol−1) −ΔH f ° (l) c (kJ mol−1) Me 3 Al 87.78 110.46 151.20 Et 3 Al 117.18 157.08 192.78 Bu 3 Al 179.34 232.68 265.02 [CH 3 (CH 2 ) 4 ] 3 Al 241.50 308.28 339.36 [CH 3 (CH 2 ) 5 ] 3 Al 303.66 383.88 414.54 [CH 3 (CH 2 ) 6 ] 3 Al 365.82 455.28 490.14 [CH 3 (CH 2 ) 7 ] 3 Al 427.98 535.08 565.74 490.14 610.68 640.92 Me 2 AlH 26.88 45.36 114.66 Et 2 AlH 46.62 77.70 147.00 Pr 2 AlH 88.20 128.94 198.24 Bu 2 AlH 129.36 180.18 249.48 Me 2 AlCl 335.58 d 355.74 d Et 2 AlCl 357.00 386.82 Pr 2 AlCl 399.42 437.22 Bu 2 AlCl 441.42 487.62 The average methyl–aluminum bond dissociation energy D ¯ (Me Al) is considered to be one-third of the enthalpy change for the reaction shown in equation (110). The ΔH f ° (M, g) of trimethylaluminum is then used with the heat of atomization of aluminum104 and a value of ΔH f ° for methyl (142.8 ± 4.2 kJ mol−1) to yield a D ¯ (Me Al) of 281.4 kJ mol−1. Previously reported D ¯ (Me Al) values have ranged from 270.9 ± 8.4 to 277.2 ± 5 kJ mol−1. Experimental ΔH f ° (l) values of −120.54,313 −151.2314 and −152.5315 kJ mol−1 have been obtained from combustion or acetolysis of trimethylaluminum and a value of −151.2 kJ mol−1 has been judged to meet the three criteria most acceptably. (110) Use of such internally consistent and experimentally acceptable −ΔH f ° (l) values provides the following values of D ¯ (Me M) for Group IIIA derivatives (in kJ mol−1): B (369.6), Al (281.4), Ga (256.2), In (218.4) and Tl (151.2). Similar calculations lead to a value of 290.2 kJ mol−1 for D ¯ (H Al).

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Based upon the criteria of monotonic increases in a homologous series, internal consistency and the expected agreement between calculated and experimental ΔH f ° (l), most of the literature values derived from heats of combustion measurements are found unacceptable.316 Related to the methyl–aluminum bond energy is the ionization of trimethylaluminum under electron impact either in the mass spectrometer or in the photoelectron spectrometer. Under varying pressures and ionization voltages the most abundant peak by far is that due to the Me 2 Al+ fragment. For example, at 70 eV monoaluminum peaks and their abundances are as follows: Me 3 Al+ (4.9), Me 2 Al+ (100), MeAl+ (6.3), Al+ (36.5).158 The ionization potentials of the neutral Me 3 Al, Me 2 Al and MeAl fragments are estimated to be 9.09, 6.6 and 7.8 eV, respectively. For comparison, the PE spectrum of monomeric trimethylaluminum yields an ionization potential of 9.76 eV for an electron in the highest occupied molecular orbital, which involves primarily the carbon–aluminum bond. Similar PE spectral studies of monomeric and dimeric dimethylaluminum halides conclude that electrons in the bridging aluminum–halogen bonds require a higher ionization potential than those in the Al X orbitals of the monomer.317,318

sciencedirect.com - Bond Dissociation Energy - an overview | ScienceDirect Topics

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