Hydrides of The group 14 elements

     Although the extensive chemistry of hydrocarbons (i.e. carbon hydrides) lies outside this book, we note several points for comparisons with later group 14 hydrides:
•     Table 1.1 illustrated the relative strength of a C^_H bond compared with C^_Cl and C^_O bonds, and this trend is not mirrored by later elements;

•  CH₄ is chlorinated with some difficulty, whereas SiH₄ reacts violently with Cl₂;
• CH₄ is stable with respect to hydrolysis, but SiH₄ is readily attacked by water;
• SiH₄ is spontaneously inflammable in air and, although it is the kinetic stability of CH4 with respect to reaction with O₂ at 298K that is crucial, values of ΔcHo show that combustion of SiH4 is more exothermic than that of CH₄;
• catenation is more common for C than the later group 14 elements, and hydrocarbon families are much more diverse than their Si, Ge, Sn and Pb analogues.

Table 1.1 Some experimental covalent bond enthalpy terms (kJ mol^-1); the values for single bonds refer to the group 14 elements in tetrahedral environments.

Worked example 13.3 Bond enthalpies and group 14 hydrides

Suggest why catenation is more common for C than for Si, Ge and Sn. Why is this relevant to the formation of families of saturated hydrocarbon molecules?

The much higher C^_C bond enthalpies (see Table 1.1) compared with those of Si^_Si, Ge^_Ge and Sn^_Sn bonds means that the formation of compounds containing bonds between carbon atoms is thermodynamically more favourable than analogous compounds containing Si^_Si, Ge^_Ge and Sn^_Sn bonds. On descending group 14, orbital overlap becomes less efficient as the valence orbitals become more diffuse, i.e. as the principal quantum number increases. The backbones of saturated hydrocarbons are composed of C^_C bonds, i.e. their formation depends on catenation being favourable. An additional factor that favours the formation of hydrocarbons is the strength of the C^_H bonds (stronger than Si^_H, Ge^_H or Sn^_H (see Table 1.1). On descending group 14, the hydrides become thermodynamically less stable, and the kinetic barriers to reactions such as hydrolysis of E^_H bonds become lower.a

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