Some Notes on Elemental Halogens

Born of poor parents in Montpellier, France, Balard was adopted and educated by his godmother. At age 17, he became a laboratory assistant ( preparateur ) at the local Ecole 

In general, the X-X bonds in the elements are weak (bond dissociation energies (BDEs) F-F 159 Cl-Cl 243 Br-Br 192,1-I 151, all in kJ/mol) and easily broken. Both homolysis and nucleophile-induced heterolysis are important reaction pathways for the diatomic 

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  

Although transition metals are outside the scope of this book, some indication of the mechanism 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 CI2 molecules. 

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

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