Page 278 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HALOGENS
258
H H H
−
At O + + O At O +
H H H

Sometimes, a given nucleophile is not strong enough to cleave a halogen molecule
but does form a “charge-transfer complex,” shown below for acetonitrile (CH CN) and
3
bromine:
H H

C C N Br Br N C C
H H
H H

The dotted lines indicate a weak interaction, that is, possibly not full-fledged bonds. The
term “charge transfer” implies that, although electron transfer or bond formation does not
happen in the ground state, an electron does jump from one component to the other in an
excited state, giving rise to characteristic features in the optical spectrum. At low tempera-
tures, charge-transfer complexes with even stronger nucleophiles such as phosphines may
be isolated:
Ph
P Br Br
Ph
Ph

7.2 ALKALI-INDUCED DISPROPORTIONATION OF MOLECULAR
HALOGENS

The reaction of halogen molecules (X = Cl, Br, I) with cold aqueous alkali to yield hypo-
−
−
halite (OX ) and halide (X ) may be viewed as a classic inorganic S 2 displacement, rather
N
similar to the reaction with pyridine mentioned above:
X + 2NaOH → NaX + NaOX + H O (7.9)
2
2
The halogen–halogen bond breaks easily because of its weakness:
−
−
− X (7.10)
HO X X HO X
Hydroxide then deprotonates the hypohalous acid to produce a hypohalate anion:
− − HOH −
HO H X O X (7.11)
O

With hot alkali, however, the same reactants give different products. The hypohalite
−
anions, the initial products of the reaction, disproportionate on warming to halate (XO )
3
−
and halide (X ,X = Cl, Br, I):