Halogenkationen

Halogenkationen, also known as halonium ions, are cationic species in which a halogen atom bears a positive formal charge. The most common examples involve the elements fluorine, chlorine, bromine, and iodine. These ions are generally unstable and highly reactive, often existing as transient intermediates in chemical reactions. The positive charge is typically located on the halogen atom, which is bonded to one or more other atoms.

The simplest halonium ions are diatomic, such as $\text{F}_2^+$, $\text{Cl}_2^+$, $\text{Br}_2^+$, and $\text{I}_2^+$. In these species, the positive charge is delocalized across both halogen atoms, which are held together by a single bond. These diatomic ions are primarily observed in the gas phase or under conditions of extreme low temperature.

More prevalent are cyclic halonium ions, particularly in organic chemistry. These arise when a halogen atom, having attacked an unsaturated system like an alkene or alkyne, forms a three-membered ring with the carbon atoms. This results in a positively charged halogen atom bridging two carbon atoms. For instance, a bromonium ion is formed when bromine reacts with an alkene. These cyclic halonium ions are crucial intermediates in reactions such as the electrophilic addition of halogens to alkenes, leading to vicinal dihalides. The bridging halonium ion explains the observed stereochemistry of these additions, often proceeding via trans-addition.