Bond dissociation enthalpies acetylene

Values for bond lengths and bond strengths (bond dissociation enthalpies) for ethane, ethylene, and acetylene are given in Table 1.11. 

By comparison, the length of the carbon-carbon double bond in ethylene is 134 pm (1.34 A), and that of the carbon-carbon single bond in ethane is 153 pm (1.53 A). Thus, triple bonds are shorter than double bonds, which, in turn, are shorter than single bonds. The bond dissociation enthalpy of the carbon-carbon triple bond in acetylene [966 kj (231 kcal)/mol] is considerably larger than that for the carbon-carbon double bond in ethylene [727 kJ (174 kcal)/mol] and the carbon-carbon single bond in ethane [376 kJ (90 kcal)/mol]. The difference in bond dissociation enthalpies between the carbon-carbon triple bond in acetylene and the carbon-carbon double bond in ethylene is only 239 kJ (57 kcal)/mol. This difference indicates that a tt bond in an alkyne is weaker than a tt bond in an alkene. 

Ervin et al. [27] have determined the electron affinity of the acetylide radical, HC = C-, to be equal to 2.969 + 0.010 eV and the enthalpy of the acid dissociation of acetylene in the gas phase to be equal to 377.8 + 0.6 Kcal mol Use these data, together with the ionization potential of the hydrogen atom, 13.595 eV, to calculate the enthalpy for the dissociation of the CH bond in acetylene. The ionization potentials are properly applied at 0 K, but a good approximation is to assume that they are equal to enthalpy changes at 298.15 K, the temperature at which the enthalpy of the acid dissociation was measured. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

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