Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one hydrogen atom by an atom of iodine. It is naturally emitted by rice plantations in small amounts. It is also produced in vast quantities estimated to be greater than 214,000 tons annually by algae and kelp in the world's temperate oceans, and in lesser amounts on land by terrestrial fungi and bacteria. It is used in organic synthesis as a source of methyl groups. Preparation and handling Iodomethane is formed via the exothermic reaction that occurs when iodine is added to a mixture of methanol with red phosphorus. The iodinating reagent is phosphorus triiodide that is formed in situ: 3 CH3OH + PI3 → 3 CH3I + H2PO3H Alternatively, it is prepared from the reaction of dimethyl sulfate with potassium iodide in the presence of calcium carbonate: (CH3O)2SO2 + KI → CH3I + CH3OSO2OK Iodomethane can also be prepared by the reaction of methanol with aqueous hydrogen iodide: CH3OH + HI → CH3I + H2O The generated iodomethane can be distilled from the reaction mixture. Iodomethane may also be prepared by treating iodoform with potassium hydroxide and dimethyl sulfate under 95% ethanol. Storage and purification Like many organoiodide compounds, iodomethane is typically stored in dark bottles to inhibit degradation caused by light to give iodine, giving degraded samples a purplish tinge. Commercial samples may be stabilized by copper or silver wire. It can be purified by washing with Na2S2O3 to remove iodine followed by distillation. Biogenic iodomethane Most iodomethane is produced by microbial methylation of iodide. Oceans are the major source, but rice paddies are also significant. Reactions Methylation reagent Iodomethane is an excellent substrate for SN2 substitution reactions. It is sterically open for attack by nucleophiles, and iodide is a good leaving group. It is used for alkylating carbon, oxygen, sulfur, nitrogen, and phosphorus nucleophiles. Unfortunately, it has a high equivalent weight: one mole of iodomethane weighs almost three times as much as one mole of chloromethane and nearly 1.5 times as much as one mole of bromomethane. On the other hand, chloromethane and bromomethane are gaseous, thus harder to handle, and are also weaker alkylating agents. Iodide can act as a catalyst when reacting chloromethane or bromomethane with a nucleophile