iodination

Iodination is the chemical process of introducing iodine into a molecule or biological substance. In organic synthesis it is used to place an iodine atom into an organic framework, producing organoiodine compounds that function as versatile intermediates in further transformations.

In electrophilic iodination, molecular iodine (I2) is converted into an electrophilic iodine species (I+) by an oxidant or catalyst and adds to substrates such as arenes or alkenes. Electrophilic iodination of arenes (aryl iodides) proceeds with typical directing effects from substituents and provides a useful handle for cross‑coupling reactions like Suzuki–Miyaura or Sonogashira. Iodination of alkenes adds across the double bond to give vicinal iodo derivatives and can be driven under relatively mild conditions with I2 and appropriate promoters or reagents such as N‐iodosuccinimide (NIS).

Radical iodination targets C–H bonds in alkanes, using I2 with radical initiators or light to replace hydrogen with iodine. This method favors positions with weaker C–H bonds and can be less selective than electrophilic methods.

Biologically, iodination refers to the incorporation of iodine into tyrosine residues during thyroid hormone synthesis. Thyroid peroxidase catalyzes the oxidation and organification of iodide to form mono- and diiodotyrosines, which couple to produce hormones T3 and T4. Radiolabeled isotopes of iodine (such as I-123 or I-131) are used in diagnostics and therapy, providing tools for medical imaging and treatment.

Safety and handling considerations are important, as iodine and iodinating reagents are corrosive and reactive. Proper ventilation, protective equipment, and waste controls are essential in laboratory and clinical contexts.