Iodine monochloride

Iodine monochloride is an interhalogen compound with the formula $ICl$ . It is a red-brown chemical compound that melts near room temperature. Because of the difference in the electronegativity of iodine and chlorine, $ICl$ is highly polar and behaves as a source of $I +$ . Owing to the similarity of the properties of the compound with bromine (red-brown liquid at room temperature), its synthesis led early researchers to believe that the reaction between the heaviest and lightest of a 'triad' of elements (three elements with similar chemical properties, now found in the same group of the modern periodic table) would produce the central element of the triad, the so-named 'Law of Averages'.

Iodine monochloride is produced simply by combining the halogens in a 1:1 molar ratio, according to the equation

I 2 + Cl 2 \to 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 + Cl 2 ⇌ ICl 3

Polymorphs

$ICl$ has two polymorphs; $α-ICl$ , which exists as black needles (red by transmitted light) with a melting point of 27.2 °C, and β-ICl, which exists as black platelets (red-brown by transmitted light) with a melting point 13.9 °C.^[1]

In the crystal structures of both polymorphs the molecules are arranged in zig-zag chains. $β-ICl$ is monoclinic with the space group P2₁/c.^[2]

Uses

$ICl$ is a useful reagent in organic synthesis.^[1] It is used as a source of electrophilic iodine in the synthesis of certain aromatic iodides.^[3] It also cleaves C-Si bonds.

$ICl$ will also add to the double bond in alkenes to give chloro-iodo alkanes.

{\ce {RCH=CHR'+ICl->RCH(I)-CH(Cl)R'}}

When such reactions are conducted in the presence of sodium azide, the iodo-azide RCH(I)-CH(N₃)R’ is obtained.^[4]

Wijs solution, which is iodine monochloride dissolved in acetic acid, is used to determine the iodine value of a substance.

It can also be used to make pure iodates, by reacting with a chlorate and releasing chlorine gas as byproduct

References

1 2 Brisbois, R. G.; Wanke, R. A.; Stubbs, K. A.; Stick, R. V. "Iodine Monochloride" Encyclopedia of Reagents for Organic Synthesis, 2004 John Wiley & Sons. doi:10.1002/047084289X.ri014
↑ Carpenter, G. B.; Richards, S. M. (1 April 1962). "The crystal structure of β-iodine monochloride". Acta Crystallographica. 15 (4): 360–364. doi:10.1107/S0365110X62000882.
↑ Wallingford, V. H.; Krüger, P. A. (1943). "5-Iodo-anthranilic Acid". Org. Synth. ; Coll. Vol., 2, p. 349
↑ Padwa, A.; Blacklock, T.; Tremper, A. "3-Phenyl-2H-Azirine-2-carboxaldehyde". Org. Synth. ; Coll. Vol., 6, p. 893

Iodine compounds

Iodine(-I)	HI AtI ICN I⁻

Iodine(I)	HIO ICl IF IBr

Iodine(III)	HIO₂ ICl₃ IF₃

Iodine(V)	HIO₃ IF₅ I₂O₅

Iodine(VII)	HIO₄ H₅IO₆ IF₇

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