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
RPhrases = R21, R23/25, R37/38, R40
SPhrases = S1/2, S36/37, S38, S45
ExploLimits = 8.5 - 66%
Iodomethane, commonly called
methyliodide and commonly abbreviated "MeI", is the chemical compoundwith the formula CH3I. This dense volatile liquid is related to methaneby replacement of one hydrogenatom by an atom of iodineand its dipole moment is 1.59 D. Refractive indexis 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 coppermetal absorbs the iodine. Methyl iodide is widely used in organic synthesisto deliver a methyl group, via the transformation called methylation. It is naturally emitted by riceplantations 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]
Methyl iodide is an excellent substrate for SN2 substitution reactions. It is sterically open for attack by
nucleophiles, and iodideis 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.
volume = 31
issue = 14
pages = 2177–2183
month = January | year = 2001
doi = 10.1081/SCC-100104469]
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 thiocyanateion 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 oxygenatom or the (usually desired) carbon atom. With methyl iodide, C-alkylation nearly always predominates.
MeI is also an important precursor to
methylmagnesium iodideor "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.
Monsanto process, MeI forms "in situ" from the reaction of methanoland hydrogen iodide. The CH3I then reacts with carbon monoxidein the presence of a rhodiumcomplex to form acetyl iodide, the precursor to acetic acidafter hydrolysis. Most acetic acid is prepared by this method.
MeI hydrolyzes at 270 °C forming
hydrogen iodide, carbon monoxideand carbon dioxide.
Iodomethane is formed via the
exothermic reactionthat occurs when iodineis added to a mixture of methanolwith 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 triiodidethat is formed "in situ:":3 CH3OH + PI3 → 3 CH3I + H3PO3
Alternatively, it is prepared from the reaction of
dimethyl sulfatewith 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.
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 sulfateis 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.
Besides use as a methylation agent, there have been proposals of its use as a
fungicide, herbicide, insecticideor nematicideand 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 Agencyapproved 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
liverit undergoes rapid conversion to S-methyl glutathione. [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
* 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.
Crit Rev Toxicol.
volume = 23
issue = 3
pages = 237–53
year = 1993
pmid = 8260067
* IARC Summaries & Evaluations: [http://www.inchem.org/documents/iarc/vol15/methyliodide.html Vol. 15 (1977)] , [http://www.inchem.org/documents/iarc/vol41/methyliodide.html Vol. 41 (1986)] , [http://www.inchem.org/documents/iarc/vol71/100-methiodi.html Vol. 71 (1999)]
* [http://www.biochemj.org/bj/098/0038/bj0980038_browse.htm Metabolism of iodomethane in the rat]
* [http://www.muhlenberg.edu/depts/chemistry/chem201woh/1Hiodomethane.htm Iodomethane NMR spectra]
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