Acetylene bond dissociation energies

Bond dissociation energies of the C-C bonds in ethylene (one o and one n bond) and acetylene (one c and two n bonds) can be used to estimate the strength of the second n bond of the triple bond. If we assume that the a bond and first n bond in acetylene are similar in strength to the G and n bonds in ethylene (88 and 64 kcaiymol, respectively), then the second n bond is worth 48 kcaiymol. 

Cottrell (1), gives an upper limit for the C-H bond dissociation energy in acetylene, A H (HCC-H) <121 kcal. This was... 

Bridged species such as 76 are well documented in rhodium porphyrin chemistry.240-241 An acetylene bonded to one metal-centered radical is presumed to be trapped by addition of a second metal-centered radical. Lower bond dissociation energies of cobalt relative to rhodium would disfavor species such as 76 and facilitate the reaction with metal—hydride intermediates to form a trans product. 

Table 1-1 Calculated bond dissociation energies for acetylene (I>o> kcal/mol) ...

A two-dimensional Httckel molecular orbital (HMO) theory approach to acetylenic systems yielded n-bond orders of P = 0.894 for the central C—C bond and P = 1.788 for the C C triple bonds in 1,3-butadiyne (209, 210). For comparison, P = 1 for ethylene and P = 2 for acetylene. A different criterion for determining the relative strengths of chemical bonds was used by Politzer and Ranganathan (17). Starting from STO-3G geometries and force constants, they calculated a bond order of 1.34 for the central C-C bond in diacetylene. This corresponds to a bond dissociation energy of 150 kcal/mol [211], which compares with bond orders and bond dissociation energies of 1.14 and 88 kcal/mol for ethane and 1.85 and 163 kcal/mol for ethylene. 

Bond dissociation energy, 13, 151—153, 155 acetylene, 343 aryl halides, 918 benzene, 918 ethane, 151, 343, 918 ethylene, 171, 343, 918 ethyl halides, 918 and halogenation of methane, 155 2-methylpropane, 151,152, 414 peroxides, 220 propane, 151 propene, 370, 414 table, 151 vinyl hahdes, 918... 

In complex 33b, where the Cr-Cr bond lacks obvious polarization (<3 (Cr) = —0.36e and <3 (Cr ) = —0.33e), the Co-COpseq bond dissociation energies are nearly identical (110.5 and 111.7 kJ/mol, respectively). In this case, the steric effects, as opposed to electronic ones, should play the decisive role. Therefore, the COpseq ligands should dissociate with equal facility and ultimately lead to the larger substituent of acetylene at the a position in the cyclopentenone. 

In this section, studies on the reactions of naphthyl radicals with unsaturated aliphatic hydrocarbons toward the production of three fused rings and of aliphatic residues attached to naphthalene are described. This is an effort to examine the process of polycyclic aromatic hydrocarbon (PAH) growth [88-99]. The source of naphthyl radicals for the experimental study was naphthyl iodide, in view of its low C—I bond dissociation energy. It dissociates very fast following the reflected shock heating. The unsaturated aliphatic hydrocarbons that have been studied were ethylene [28] and acetylene [29]. 

The arguments used for ethylene imply that the CH bonds in acetylene should be even shorter and stronger. This is the case (length 1.057 A and bond dissociation energy greater than in ethylene). Furthermore, here there are... 

The problem is avoided by modification of the characteristic atomic radii to match the condition of strain that exists in molecules. As a typical example the variation of calculated dissociation energy with characteristic radius is shown in the table below for the C-C bonds in ethane, ethene and acetylene. The discrepancy approaches experimental uncertainty for all three bonds at the same value of rc = 1.85 A, which also models diatomic C2 correctly. 

The bond energies of acetylene and its mono and di-fluorinated derivatives can be studied in a related manner. Table VII presents the results where D and D are the "uncorrected" and "Simonized" dissociation energies corresponding to formation of the doublet o2ir1 27r and quartet o n2 4E methylidynes as shown in reactions (13) and (14). 

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