Modern inorganic chemistry

GROUPIV 175

Cyclic hydrocarbons

Carbon also forms numerous cyclic hydrides of which benzene,
QH 6. is a well-known example. This has a planar, regular hexagonal

(c)

Figure 8.3

structure often represented as a resonance hybrid between the struc-

tures in Figure 83(d). Overlaps of the p orbitals (b) gives the structure

shown in (c). All the C—C bond lengths are equal, as are all the

C—H bond lengths, and the double bonds are "delocalised'.

Silicon

Silicon, unlike carbon, does notform a very large number of hydrides.
A series of covalently bonded volatile hydrides called silanes analog-
ous to the alkane hydrocarbons is known, with the general formula
SinH2n+2' ^ut less than ten members of the series have so far been
prepared. Mono- and disilanes are more readily prepared by the
reaction of the corresponding silicon chloride with lithium aluminium
hydride in ether:

SiCl LiAlH 4 -> SiH 4 t + LiCl| + A1C1

and

2Si 2 Cl 6 + 3LiAlH 4 -> 2Si 2 H 6 t

The Si — Si bond is weaker than the

3LiCli 3A1C1
C bond (mean thermo-
chemical bond energies are C — C in diamond, 356 kJ mol~ j, Si — Si