Carbon-silicon bonds dissociation energies

TABLE 17. Derived silicon-carbon and silicon-silicon bond dissociation energies (kJmoD1)... 

Some typical bond dissociation energies (kj mol 1) for bonds to silicon and the corresponding bonds to carbon are given below. 

Steele, W. C., L. D. Nichols, and F.G. A. Stone The Determination of Silicon-Carbon and Silicon-Hydrogen Bond Dissociation Energies by Electron Impact. J. Amer. chem. Soc. 84, 4441—4445 (1962). 

Silicon-carbon bond dissociation energies are less than those of C—C bonds but are still quite high, in the region 250 to 335 kJ mol 1. The tetraalkyls and -aryls are hence thermally quite stable Si(QH5)4, for example, boils unchanged at 530°C. 

Selected bond dissociation energies are compiled in Table 7. Although bond lengths of sihcon compounds are longer than the corresponding carbon compounds, silicon forms stronger bonds than carbon. ... 

The a-bonds between silicon and electronegative elements such as the halogens are quite strong. A comparison of the bond dissociation energies between carbon/halogen and silicon/halogen is listed in Table I. ... 

The relative reactivities of organometallics containing group 14 elements are closely associated with the electronegativity and dissociation energy of the M—C bonds of the elements. For example, some organotin compounds such as allylstannanes are easily decomposed to form radical species whereas homolysis of the carbon-silicon bond of allylsilanes is rarely achieved, which reflects the dissociation energy of the respective M—C bonds. 

Relative to hydrogen, all the first-row substituents, with the exception of fluorine, stabilize the silicenium ion. The greater stability of a-substituted silicenium cations versus their corresponding carbocations can be attributed in part to the lower electronegativity of silicon (1.7) compared to carbon (2.5). Equation 3 also takes into account the relative stability of alkanes versus silanes, i.e. the stability of C—H versus Si—H bonds. The Si—H bond is weaker than the C—H bond by ca 13 kcalmoU in the gas phase, e.g. the bond dissociation energy D//2Qg(C—H) in CH4 is 104.9 kcalmoU, compared... 

If, in silane, one hydrogen is replaced by a phenyl group, the silicon-hydrogen bond dissociation enthalpy is only reduced by about 9 7 kJ/mol (19). This is in sharp contrast with the decrease observed for the carbon analogues, where the large stabilization energy of benzyl radical makes D(Me-H)-D(PhCH -H) = 71 4 kJ/mol. The effect of a second phenyl group in the hydrocarbon family is also known, D(Ph CH-H) = 340 8 kJ/mol (27,28) and there is a recent estimate for D(Ph C-H), 338 13 kJ/mol (28). 

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