Iodine monochloride

Iodine monochloride is the compound with the formula ICl. This red brown compound melts just near room temperature. Because of the difference in the electronegativity of iodine and chlorine, ICl is highly polar and behaves as a source of “I+.”

Preparation of iodine monochloride entails simply combining the halogens in a 1:1 molar ratio, according to this equation:
 * I2 + Cl2 → 2 ICl

When chlorine gas is passed through iodine crystals, one observes the brown vapor of iodine monochloride. Dark brown iodine monochloride liquid is collected. Excess chlorine converts iodine monochloride into iodine trichloride in a reversible reaction:
 * ICl +  Cl2  → ICl3

Polymorphs
Polymorphy is the phenomenon where a solid exists in two or more different crystalline forms. ICl has two polymorphs, α-ICl which are black needles (red by transmitted light) with a melting point of 27.2 °C and β-ICl which are black platelets (red-brown by transmitted light) with a melting point 13.9 °C.

Uses
ICl is a useful reagent in organic synthesis. It is used as a source of electrophilic iodine to synthesise certain aromatic iodides. It also cleaves C-Si bonds.

ICl, will also add to the double bond in alkenes to give chloro-iodo alkanes.
 * RCH=CHR’ +  ICl   →  RCH(I)-CH(Cl)R’

When such reactions are conducted in the presence of sodium azide, one obtains the iodo-azide, RCH(I)-CH(N3)R’.