Iodomethane

Chembox new
ImageFileL1 = Iodomethane.png ImageSizeL1 = 100px
ImageFileR1 = Iodomethane-3D-vdW.png ImageSizeR1 = 100px
IUPACName = Iodomethane
OtherNames = Methyl iodide, Monoiodomethane, Methyl iodine, MeI, Halon 10001, UN 2644
Section1 = Chembox Identifiers
CASNo = 74-88-4
EINECS = 200-819-5
PubChem = 6328
SMILES = CI
InChI = 1/CH3I/c1-2/h1H3
RTECS = PA9450000
Section2 = Chembox Properties
Formula = CH₃I
MolarMass = 141.94 g/mol
Appearance = Clear colourless liquid with acrid odor
Density = 2.2789 g/cm³ at 20 °C
MeltingPt = -66.45 °C (206.70 K)
BoilingPt = 42.43 °C (315.58 K)
Solubility = 14 g/l at 20 °C
LogP = 1.51
VaporPressure = 50 kPa at 20 °C53.32 at 25.3 °C166.1 kPa at 55 °C
Section3 = Chembox Structure
MolShape = Tetrahedral
Section3 = Chembox Hazards
EUClass = Toxic (T), Carc. Cat. 3
ExternalMSDS = [http://ptcl.chem.ox.ac.uk/MSDS/IO/iodomethane.html MSDS at Oxford University]
FlashPt = -28 °C
Autoignition = 352 °C
NFPA-H = 3
NFPA-F = 0
NFPA-R = 1
NFPA-O =
RPhrases = R21, R23/25, R37/38, R40
SPhrases = S1/2, S36/37, S38, S45
ExploLimits = 8.5 - 66%

Iodomethane, commonly called methyl iodide and commonly abbreviated "MeI", is the chemical compound with the formula CH₃I. This dense volatile liquid is related to methane by replacement of one hydrogen atom by an atom of iodine and its dipole moment is 1.59 D. Refractive index is 1.5304 (20 °C, D), 1.5293 (21 °C, D). It is miscible with common organic solvents. It is colourless, although upon exposure to light, samples develop a purplish tinge caused by the presence of I₂. Storage over copper metal absorbs the iodine. Methyl iodide is widely used in organic synthesis to deliver a methyl group, via the transformation called methylation. It is naturally emitted by rice plantations in small amounts. [cite journal
title = Emissions of Methyl Halides and Methane from Rice Paddies
author = K. R. Redeker, N.-Y. Wang, J. C. Low, A. McMillan, S. C. Tyler, and R. J. Cicerone
journal = Science
volume = 290
pages = 966–969
year = 2000
doi = 10.1126/science.290.5493.966
pmid = 11062125]

Chemical properties

Methyl iodide is an excellent substrate for S_N2 substitution reactions. It is sterically open for attack by nucleophiles, and iodide is a good leaving group. For example, it can be used for the methylation of phenols or carboxylic acids: [cite journal
title = Efficient methylation of carboxylic acids with potassium hydroxide/methyl sulfoxide and iodomethane
author = Avila-Zárraga, J. G., Martínez, R.
journal = Synthetic Communications
volume = 31
issue = 14
pages = 2177–2183
month = January | year = 2001
doi = 10.1081/SCC-100104469]

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 S_N2 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 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 methyl iodide, C-alkylation nearly always predominates.

MeI is also an important precursor to methylmagnesium iodide or "MeMgI", which is a common reagent. Because MeMgI forms readily, it is often prepared in instructional laboratories as an illustration of Grignard reagents. The use of MeMgI has been somewhat superseded by the commercially available methyl lithium.

In the Monsanto 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 complex to form acetyl iodide, the precursor to acetic acid after hydrolysis. Most acetic acid is prepared by this method.

MeI hydrolyzes at 270 °C forming hydrogen iodide, carbon monoxide and carbon dioxide.

Preparation

Iodomethane is formed via the exothermic reaction that occurs when iodine is added to a mixture of methanol with red phosphorus.OrgSynth | author = King, C. S.; Hartman, W. W. | title = Methyl Iodide | collvol = 2 | collvolpages = 399 | year = 1943 | prep =CV2P0399] The iodinating reagent is phosphorus triiodide that is formed "in situ:":3 CH₃OH + PI₃ → 3 CH₃I + H₃PO₃

Alternatively, it is prepared from the reaction of dimethyl sulfate with potassium iodide in the presence of calcium carbonate::(CH₃O)₂SO₂ + KI → K₂SO₄ + 2 CH₃IThe CH₃I can be purified by distillation followed by washing with Na₂S₂O₃ to remove iodine.

Methyl iodide forms during nuclear accidents by the reaction of organic matter with the "fission iodine."

Choice of iodomethane as a methylating agent

Iodomethane is an excellent reagent for methylation, but there are some disadvantages to its use. It has a high equivalent weight: one mole of MeI weighs almost three times as much as one mole of methyl chloride. However, the chloride is a gas (as is methyl bromide), making it more awkward to work with than liquid MeI. Methyl chloride is a poorer methylating reagent than MeI, though it is often adequate.

Iodides are generally expensive relative to the more common chlorides and bromides, though iodomethane is reasonably affordable; on a commercial scale the toxic dimethyl sulfate is preferred, since it is both cheap and liquid. The iodide leaving group in MeI may cause side reactions, as it is a powerful nucleophile. Finally, being highly reactive, MeI is more dangerous for laboratory workers than related chlorides and bromides. When considering alternatives to MeI, it is necessary to consider cost, handling, risk, chemical selectivity, and ease of reaction work-up.

Uses

Besides use as a methylation agent, there have been proposals of its use as a fungicide, herbicide, insecticide or nematicide and as a fire extinguisher. Further it can be used as a soil disinfectant, replacing bromomethane (which was banned under the Montreal Protocol), and in microscopy due to properties related to refraction index. In a controversial October 2007 decision, the United States Environmental Protection Agency approved its use as a soil fumigant in some cases, although it cannot yet be used in California (a major potential market) due to lack of state approval. [cite news | url = http://www.latimes.com/news/printedition/california/la-me-pesticide6oct06,0,3454295.story | title = EPA approves new pesticide despite scientists' concerns | work = Los Angeles Times | date = October 6, 2007]

Toxicity and Biological effects

Iodomethane has an LD₅₀ for oral administration to rats 76 mg/kg, and in the liver it undergoes rapid conversion to S-methylglutathione. [cite journal | title = Metabolism of iodomethane in the rat | author = Johnson, M. K. | journal = Biochem. J. | volume = 98 | issue = | pages = 38–43 | year = 1966 | doi = ] Iodomethane is a possible carcinogen based on its IARC, ACGIH, NTP, or EPA classification. According to IARC it is classified as a group 3 substance (Group 3:The agent is not classifiable as to its carcinogenicity to humans).

Breathing iodomethane fumes can cause lung, liver, kidney and central nervous system damage. It causes nausea, dizziness, coughing and vomiting. Prolonged contact with skin causes burns. Massive inhalation causes pulmonary edema.

See also

* Haloalkane
* Halomethane
* Fluoromethane
* Chloromethane
* Bromomethane
* Diiodomethane
* Iodoform
* Tetraiodomethane;Methylating reagents
* Dimethyl carbonate
* Dimethyl sulfate

References

*
* Sulikowski, G. A.; Sulikowski, M. M. (1999). in Coates, R.M.; Denmark, S. E. (Eds.) "Handbook of Reagents for Organic Synthesis, Volume 1: Reagents, Auxiliaries and Catalysts for C-C Bond Formation" New York: Wiley, pp. 423–26.
* cite journal
title = Mechanisms of carcinogenicity of methyl halides.
author = Bolt H. M., Gansewendt B.
journal = Crit Rev Toxicol.
volume = 23
issue = 3
pages = 237–53
year = 1993
pmid = 8260067