154 THE ELEMENTS OF GROUP Ml4BF 3 + 6H 2 O -> 3H 3 O+ + 3BF4 + H 3 BO 3
The BF 4 ion has a regular tetrahedral configuration. The most
important property of boron trifluoride is its great capacity to act
as an electron pair acceptor (Lewis acid). Some examples of adducts
are:
C 2 H 5In each case the configuration around the boron changes from
trigonal planar to tetrahedral on adduct formation. Because of this
ability to form additional compounds, boron trifluoride is an im-
portant catalyst and is used in many organic reactions, notably
polymerisation, esterification, and Friedel-Crafts acylation and
alkylations.Aluminium fluoride is a white solid which sublimes without melting
at 1530 K. Like boron trifluoride, it can be prepared by the direct
combination of the elements but it can also be prepared by reacting
aluminium hydroxide with gaseous hydrogen fluoride. Aluminium
fluoride is chemically unreactive ; it does not react with cold water,
in which it is only sparingly soluble, and it is attacked only slowly
even by fused potassium hydroxide. Hydrofluoric acid dissolves it
forming the octahedral hexafluoroaluminate ion, [A1F 6 ]^3 ". The
sodium salt of this ion, Na 3 AlF 6 , occurs naturally as cryolite (p. 141)
but in insufficient quantities to meet the demand for it. It is produced
industrially in large quantities by the action of hydrogen fluoride
on sodium aluminate :
12HF + A1 2 O 3 .3H 2 O 4- 6NaOH -> 2Na 3 AlF 6 4- 12H 2 OChloridesBoth boron and aluminium chlorides can be prepared by the direct
combination of the elements. Boron trichloride can also be prepared
by passing chlorine gas over a strongly heated mixture of boron
trioxide and carbon. Like boron trifluoride. this is a covalent com-
pound and a gas at ordinary temperature and pressure (boiling point
285 K). It reacts vigorously with water, the mechanism probably
involving initial co-ordination of a water molecule (p. 152). and
hydrochloric acid is obtained :
BC1 3 + 3H 2 O -> H 3 BO 3 + 3HC1