Olefin bond-enthalpy

An extensive computational analysis expanded the range of the c-d distances for reactive cyclic enediynes to 2.9-3.4 A.38 By comparing unsubstituted enediynes with dialkyl-substituted enediynes, it was found that the activation enthalpy is dependent on factors other than the c-d distance and that reactivity hinges on a subtle interplay of steric and electronic effects that accompany distortion caused by incorporation into a macrocycle. For example, since alkyl substituents stabilize acetylenic bonds to a greater extend than olefinic bonds,39 such substituents stabilize the starting material, thus increasing both the activation barrier and the reaction endothermicity. 

Table 17. Enthalpy of sublimation/vaporization, A//sub/vac, enthalpy of disruption, A/fo, and olefin-iron bond enthalpy contribution for [Fe(CO)n(olefin),j] compounds. All values are in kJ mol-1...

It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. 

D. L. S. Brown, J. A. Connor, M. Y. Leung, M. I. Paz-Andrade, H. A. Skinner. The Enthalpies of Thermal Decomposition of Iron-Olefin Complexes, and the Strengths of Iron-Olefin Bonds. J. Organometal. Chem. 1976, 770,79-89. 

Experimental information on the energetics of silver-olefin bonds comes from a 1973 study by Partenheimer and Johnson26. Using titration calorimetry with the inert dichloromethane as solvent, these authors measured the enthalpies of reaction 14 for several olefin complexes (hfacac = 1, 1, 1, 5, 5, 5-hexafluoro-2,4-pentanedionate) whose structure (1) is illustrated below for olefin = cyclohexene. 

The data, collected in Table 4, show the trend in Ag-olefin bond dissociation enthalpies in solution, the weakest bond being between Ag+ and cyclopentene and the strongest between Ag+ and cyclooctene. 

The heat of reaction, AH3, for the binding of cw-cyclooctene to the coordinatively unsaturated intermediate 3 to generate cis-4 was found to be -14.3 0.9 kcal/mol. This is somewhat more exothermic than the -12.2 kcal/mol olefin-metal bond enthalpy found in (1-hexene)-Cr(CO)5 (2,4). Perhaps this stronger bonding interaction can be attributed to the 6 kcal/mol ring strain found in cw-cyclooctene (22). A much more dramatic effect is seen in the... 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

The enthalpy of the R02 + RH reaction is determined by the strengths of disrupted and newly formed bonds AH= Z>R H—Droo—h- For the values of O—H BDEs in hydroperoxides, see the earlier discussion on page 41. The dissociation energies of the C—H bonds of hydrocarbons depend on their structure and vary in the range 300 - 440 kJ mol-1 (see Chapter 7). The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

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