Single 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... 

Robert Curl, Richard Smalley, and Harold Kroto were awarded the Nobel prize in chemistry in 1996 for the discovery of the soccer-ball-shaped molecule C60. This fundamental molecule was the first of a new series of molecular allotropes of carbon. The enthalpy of combustion of C60 is —25 937 kj-mol, and its enthalpy of sublimation is +233 kj-mol There are 90 bonds in C60, of which 60 are single bonds and 30 are double bonds. Like benzene, C60 has... 

Ketones are generally not polymerizable, despite claims that acetone can be polymerized at low temperatures (1 ). A simple explanation for the lack of polymerizability of ketones compared to vinyl monomers can be deduced from consideration of Pauling (Z) average bond energies as shown in Equations 1 and 2, where AHP°l(est) is the estimated enthalpy of polymerization based upon the difference in bond energies of the two single bonds formed in the polymer compared to the double bond in the monomer ... 

It is interesting to note that the strongly polar B—F and Si—F bonds have the largest known single-bond enthalpies, which must be a reflection of the nature of the bonds, as we will discuss in Chapters 8 and 9. 

These assumptions are consistent with the very large bond strength of the BF bond in BF3, which is larger than that of any other single bond. It has a bond dissociation enthalpy... 

As a result of the small, but apparent single bond character of the triafulvene C3/C4 bond and the good stabilization of the transition state of the rotation established earlier, rotation around this bond should be lower in energy in comparison to simple ethylene derivatives183. In fact, 1H-NMR spectra of several types of asymmetrically substituted triafulvenes 219-224 proved to be temperature-dependent and showed reversible coalescence phenomena at definite temperatures diagnostic for internal rotation processes. These were characterized by the free enthalpy of activation AG at the coalescence point of appropriate substituent signals61. ... 

In balance, the small decrease in enthalpy (AH < 0) is more than offset by a large decrease in entropy (AS < 0) so that the overall reaction is unfavorable. Thus, one would not expect to see the formation of single hydrogen bonds between two peptides in water. This is what is found. 

We have summed the enthalpy of formation of five ethylenes for the five formal double bonds and 6(5) for the six formal single bonds by analogy to our discussion of [18]annulene. 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

TABLE 2. Enthalpies of formation of hydroperoxides and peroxides containing single-bonded oxygen functional groups (kJ mol )... 

The amount 463 kJ mol 1 represents the enthalpy released when an O-H bond is formed it is the bond ciuhalpx term for O—H single bonds. 

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