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 while iodomethane is formed in situ.
Iodides are generally expensive relative to the more common chlorides and bromides, though iodomethane is reasonably affordable; on a commercial scale, the more toxic dimethyl sulfate is preferred, since it is cheap and has a higher boiling point. The iodide leaving group in iodomethane may cause unwanted side reactions. Finally, being highly reactive, iodomethane is more dangerous for laboratory workers than related chlorides and bromides.
For example, it can be used for the methylation of carboxylic acids or phenols:
In these examples, the base (K2CO3 or Li2CO3) removes the acidic proton to form the carboxylate or phenoxide anion, which serves as the nucleophile in the SN2 substitution.
Iodide is a "soft" anion which means that methylation with MeI tends to occur at the "softer" end of an ambidentate nucleophile. For example, reaction with thiocyanate ion favours attack at sulfur rather than "hard" nitrogen, leading mainly to methyl thiocyanate (CH3SCN) rather than methyl isothiocyanate CH3NCS. This behavior is relevant to the methylation of stabilized enolates such as those derived from 1,3-dicarbonyl compounds. Methylation of these and related enolates can occur on the harder oxygen atom or the (usually desired) carbon atom. With iodomethane, C-alkylation nearly always predominates.
Other reactions
In the Monsanto process and the Cativa process, MeI forms in situ from the reaction of methanol and hydrogen iodide. The CH3I then reacts with carbon monoxide in the presence of a rhodium or iridium complex to form acetyl iodide, the precursor to acetic acid after hydrolysis. The Cativa process is usually preferred because less water is required to use and there are less byproducts.
MeI is used to prepare the Grignard reagent, methylmagnesium iodide ("MeMgI"), a common source of "Me−". The use of MeMgI has been somewhat superseded by the commercially available methyllithium. MeI can also be used to prepare dimethylmercury, by reacting 2 moles of MeI with a 2/1-molar sodium amalgam (2 moles of sodium, 1 mol of mercury).