Modern inorganic chemistry

196 GROUP IV

are chemically (kinetically) inert and, unlike all other Group IV

element tetrahalides, they are not hydrolysed by water. Carbon

tetrajluoride is a gas. b.p. 145 K. and is made by direct combination of

carbon and fluorine ; it is also the main product of burning fluorine

in benzene vapour. Carbon tetrachloride (tetrachloromethane) is

a liquid, b.p. 350 1, and is prepared by the action of chlorine on

carbon disulphide (p. 201) in the presence of a catalyst, usually

manganese(II) chloride or iron(III) chloride :

CS 2 + 3C1 2 CC1 4 + S 2 C1 2

Further reaction then occurs between the disulphur dichloride and

the carbon disulphide :

2S 2 C1 2 + CS 2 CC1 4 + 6S

Carbon tetrachloride is an excellent solvent for organic substances.

It has been used in dry-cleaning and in fire-extinguishers, but it has

now largely been replaced because it is highly toxic, causing damage

to liver and kidneys. 1,1,1 trichloroethane is the most commonly

used dry-cleaning solvent and fluorocarbons are used in many fire-

extinguishers.

Silicon

Silicon tetrajluoride is formed when hydrogen fluoride reacts with

silica or a silicate :

4HF 4- SiO 2 -» SiF 4 t 4- 2H 2 O

The hydrogen fluoride is conveniently produced in situ by the action

of concentrated sulphuric acid on calcium fluoride :

CaF 2 + H 2 SO 4 -> CaSO 4 + 2HF

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule
having, like the tetrahalides of carbon, a tetrahedral covalent
structure. It reacts with water to form hydrated silica (silica gel, see
p. 186) and hexafluorosilicic acid, the latter product being obtained
by a reaction between the hydrogen fluoride produced and excess
silicon tetrafluoride :
SiF 4 + 2H 2 O -» SiO 2 i + 4HF
SiF 4 + 2HF -> H 2 SiF 6
Silicon tetrachloride is a colourless liquid, b.p. 216.2 K, and again
the molecule has a covalent structure. Silicon tetrachloride is
hydrolysed by water :