Multiple bonds bond enthalpies

Bond enthalpies for a variety of single and multiple bonds are listed in Table 8.4. Note that bond enthalpy is always a positive quantity heat is always absorbed when chemical bonds are broken. Conversely, heat is given off when bonds are formed from gaseous atoms. Thus... 

You will note from Table 8.4 that the bond enthalpy is larger for a multiple bond than for a single bond between the same two atoms. Thus... 

Besides the weak bonds listed in the previous table, there are other multiple bonds that endow the molecules in which they are situated with a positive enthalpy of formation. Such compounds are termed endothermic compounds. The danger they represent does not necessarily come from the fact that they are unstable, but is related to the exothermicity of their decomposition reaction. The most convincing examples are the acetylenic compounds, and in particular, acetylene. It is also the case for ethylene, aromatic compounds, imines and nitriles. 

Table 2.9 Mean Bond Enthalpies (kJ mol-1) Multiple Bonds ...

R. F. C url, R. F. Smalley, and 11. W. Kroto were awarded the Nobel prize m chemistry in 199fe for the discovery of the soccer ball-shaped molecule C, . The enthalpy of combustion of (J o is 25 937 kj-mol 1 and its enthalpy of sublimation is +233 kj-mol 1 There are 90 bonds in of which 60 are single and 30 are double bonds. is like benzene in that it has a set of multiple bonds for which resonance structures may be drawn, (a) Determine the enthalpy of formation of Cfj) from its enthalpy of combustion, (h) Calculate the expected enthalpy of formation of from bond enthalpies, assuming the bonds to be isolated double and single bonds, (c) Is CMI more or less stable than predicted on the basis of the isolated bond model ... 

We have already acknowledged our intent to use relevant estimation approaches to enthalpies of vaporization and sublimation to maximize the usefulness of the data available. That dienes and polyenes have multiple double bonds that are potentially hydrogenatable to the totally saturated aliphatic or alicyclic hydrocarbons allows the employment of two other assumptions. The first assumption argues that the enthalpy of hydrogenation, AHn measured in a nonpolar solvent is essentially equal to that which would be obtained in the gas phase. The second assumption, implicitly employing the first, legitimizes the use of estimation techniques and even molecular mechanics to derive the enthalpy of formation of the totally saturated species. From this last number, the enthalpy of formation of the unsaturated diene or polyene of interest can be derived by equation 3 and simple arithmetic. 

The driving force for such reactions is the loss of multiple-bond enthalpy. Addition reactions of this type may be involved in the formation of sulfur-nitrogen oligomers. 

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