Modern inorganic chemistry

THE ELEMENTS OF GROUP III 155

It forms an ion BClJ only under special circumstances, and never

in aqueous solutions (cf. BF 3 ). Like the trifluoride, it is an electron

pair acceptor, but the adducts formed tend to decompose more

readily. Unlike the corresponding aluminium chloride, boron

trichloride exists only as the monomer.

Aluminium chloride can be prepared not only by the direct com-

bination of the elements but also by the passage of dry hydrogen

chloride over heated aluminium :

2A1 + 3C1 2 -> A1 2 C1 6

2A1 -f 6HC1 -> A1 2 C1 6 4- 3H 2

Pure anhydrous aluminium chloride is a white solid at room

temperature. It is composed of double molecules in which a chlorine

atom attached to one aluminium atom donates a pair of electrons

to the neighbouring aluminium atom thus giving each aluminium

the electronic configuration of a noble gas. By doing so each

aluminium takes up an approximately tetrahedral arrangement

(p. 41). It is not surprising that electron pair donors are able to

split the dimer to form adducts, and ether, for example, forms the

adduct,

CH

C 2 H 5

in which aluminium again has a noble gas electronic configuration
and tetrahedral symmetry.
When heated above 673 K the dimer, A1 2 C1 6 , begins to dissociate
into the monomer in which the aluminium has a regular trigonal
planar configuration.
Aluminium chloride is used extensively in organic chemistry as a
catalyst, for example in the Friedel-Crafts reaction :
C 6 H 6 + C 2 H 5 C1 ^± C 6 H 5 C 2 H 5 4- HC1
It is believed that an intermediate complex ion [A1C1 4 ] ~ is formed
thus:

C 2 H 5 C1 + A1C1 3 ^ C 2 H 5 + 4- A1CU

The C 2 H5 is a carbonitim ion (cf. ammonium NH^) and reacts

with the benzene :

and then hydrogen chloride and aluminium chloride are formed :

H+ + A1C1 4 -*HC1 + A1C1 3