Page 272 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

7

The Halogens

Balard did not discover bromine, rather bromine discovered Balard.
Comment by Justus von Liebig about Antonie Jerome Balard

Halogens (often given the generic symbol X) are ubiquitous in all areas of chemistry, so you
have already encountered a good deal of halogen chemistry by now. They occur most com-
monly as monovalent groups or substituents and as mononegative halide anions; the latter
act as both nucleophiles and leaving groups. Beyond these stereotypes, however, halogen
chemistry is considerably more diverse. A few general remarks should help set the stage
for our in-depth survey.
• With the exception of fluorine, which is essentially always monovalent, with an oxi-
dation state of –1, the other halogens exhibit a variety of valences, particularly 3,
5, and 7. The higher-valent states typically occur in the company of strongly elec-
tronegative atoms, notably oxygen and fluorine. The various relevant species will be
discussed in due course, as we work through the chapter; it’s useful, however, to intro-
duce the trivial names of the various oxoacids and oxoanions (for X = Cl, Br, I) right
at the outset. These traditional names are widely used, and it’s important that you
become familiar with them as quickly as possible.

X Oxidation
State, Valence Oxoacids Oxoanions
−
+I, 1 HOX Hypohalous acid XO Hypohalite
−
+III, 3 HXO 2 Halous acid XO 2 Halite
+V, 5 HXO 3 Halic acid XO 3 − Halate
+VII, 7 HXO 4 Perhalic acid XO 4 − Perhalate

• Halide ions may be oxidized to the molecular halogens, with the larger anions more
easily oxidized:
−
−
−
I > Br > Cl > F −
Arrow Pushing in Inorganic Chemistry: A Logical Approach to the Chemistry of the Main-Group Elements,
First Edition. Abhik Ghosh and Steffen Berg.
© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.

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