Page 276 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HALOGENS
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Note that in the “mechanism” above we have only shown electrons that are involved in the
electron transfer. The +IV, +III, and +II states of manganese are typically associated with
3, 4, and 5 unpaired d electrons, respectively.

REVIEW PROBLEM 7.3
Explain Balard’s discovery with the help of equations/mechanisms. Would it have
been reasonable to formulate the red liquid as iodine chloride?

The elemental halogens are important electrophiles. An important and familiar example
is the electrophilic addition of bromine to carbon–carbon double bonds. As mentioned in
Section 1.12, the first product of electrophilic addition is a cyclic bromonium ion.
Molecular iodine is less reactive and does not add to a carbon–carbon double bond in
the absence of a catalyst. It does, however, react with somewhat stronger nucleophiles such
as pyridine:
+
2I + 2py → [I(py) ] [I ] − (7.4)
2
2
3
The first step of the reaction is probably a straightforward nucleophilic displacement:
+ −
N I I N I + I (7.5)

Both the ions produced react further with nucleophiles, as shown below:

+ + − +
N I N N I N
(7.6)

− −
I
I I I I I
−
The triiodide anion, [I ] , is a well-known species, yellow in dilute aqueous solution and
3
brown when concentrated. Indeed, the formation of the triiodide anion underlies the solu-
bility of elemental iodine in aqueous potassium iodide but not in pure water.

REVIEW PROBLEM 7.4
+
−
Note that the central iodine atom in the above final products, [I(py) ] and [I ] ,are
2 3
hypervalent. Use valence shell electron pair repulsion (VSEPR) theory to predict the
structures (linear/bent) of these two ions.
+
As with pyridine, the interaction of cryptand and I leads to a cationic I complex
2
(depicted below), with I 3 − as the counterion: