Page 275 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

7.1 SOME NOTES ON ELEMENTAL HALOGENS 255
de Pharmacie. While studying salt marsh flora, he concentrated sea water, crystallized out
sodium sulfate, which precipitated first, and attempted to find uses for the remaining mother
liquor. With certain reagents, he found that the brine turned brown. Saturating the liquors
with chlorine and distilling, he isolated a pungent, dark-red liquid, with a boiling point of
∘
47 C and a density three times that of water. He attempted to decompose it into simpler
substances, but soon recognized that he had discovered an element similar to chlorine and
iodine. The results were communicated to the Académie des Sciences and eventually pub-
lished under the title “Sur une substance particulière contenue dans l’eau de la mer (About a
particular substance present in sea water)” in the Annales de Chimie et de Physique. Balard’s
findings caused a sensation, not least because of his tender age; he was 23 years old at the
time of his discovery. Life thereafter was kind to Balard. Despite the trauma of childhood
poverty, which affected him deeply, he became a highly successful chemist, making many
significant discoveries and ultimately becoming a professor at the prestigious Collège de
France.
Others also played a role in the discovery of bromine. A young German student named
Carl Löwig discovered bromine a year before Balard, but did not publish his findings after
he came across Balard’s paper. The great German chemist Justus von Liebig also prepared
bromine but apparently did not recognize its elementary nature, viewing it instead as a
chloride of iodine. Did Liebig’s comment referring to bromine’s powerful stench, cited at
the beginning of this chapter, reflect a tinge of envy or bitterness? We don’t know; it may
well have been pure, good-natured humor.
In general, the X–X bonds in the elements are weak (bond dissociation energies (BDEs):
F–F 159; Cl–Cl 243; Br–Br 192, I–I 151, all in kJ/mol) and easily broken. Both homo-
lysis and nucleophile-induced heterolysis are important reaction pathways for the diatomic
elements:
−
Nu −
Nu X + X
X X (7.1)
Δ
X + X
or h
A classic reaction for preparing a molecular halogen (other than fluorine)—indeed, the
reaction that led to the discovery of chlorine as an element—involves oxidation of a hydro-
halic acid with manganese dioxide:

MnO + 4HX → MnX + X + 2H O (7.2)
2
2
2
2
Although transition metals are outside the scope of this book, some indication of the mech-
anism can be given. In essence, the acid protonates some of the oxide ligands, thereby
increasing the electrophilicity of the Mn(IV) centers, which are then reduced by electron
−
transfer from Cl . The chlorine atoms so formed couple to form Cl molecules.
2
− Mn(IV)
X X + Mn(III)
−
X Mn(III) X + Mn(II) (7.3)
X
X X X