Modern inorganic chemistry

156 THE ELEMENTS OF GROUP III

Bromides and iodides

The tribromide and triodide of both boron and aluminium can be

made by the direct combination of the elements although better

methods are known for each halide. The properties of each halide

closely resemble that of the chloride.

Both aluminium tribromide and triodide are dimeric in the solid

state. As expected the solids dissolve in non-polar solvents without

the break-up of these dimeric units.

Nitrides

When boron and aluminium burn in air small quantities of nitride
are formed.
Boron nitride can be prepared by allowing ammonia to react
with boron trichloride. The first product is boron amide which
decomposes on heating to give the nitride:

BC1 3 + 6NH 3 -> B(NH 2 ) 3 + 3NH 4 C1

boron amide

B(NH 2 ) 3 -» BN + 2NH 3 |

Boron nitride is chemically unreactive, and can be melted at 3000 K
by heating under pressure. It is a covalent compound, but the lack
of volatility is due to the formation of 'giant molecules' as in graphite
or diamond (p. 163). The bond B—N is isoelectronic with C—G
By subjecting boron nitride (a white powder) to high pressure and
temperature small crystals of a substance harder than diamond,
known as borazon, are obtained. This pressure-temperature treat-
ment changes the structure from the original graphite-like layer'
structure (p. 163) to a diamond-like structure; this hard form can
withstand temperatures up to 2000 K.
Aluminium nitride can also be prepared by heating a mixture of
aluminium oxide and carbon in nitrogen in an electric arc furnace:

A1 203 + N 2 + 3C -> 2A1N + 3CO

It is stable up to 2000 K and melts under pressure at 2500 K. The
crystal structure of aluminium nitride resembles that of boron
nitride and diamond, but unlike both of these it is rapidly and
exothermically hydrolysed by cold water:

A1N + 3H 2 O -> A1(OH) 3 + NH 3