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Iodomethane

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Iodomethane
Names
IUPAC name
Iodomethane
Other names
Monoiodomethane, Methyl iodine, MeI, Halon 10001, UN 2644
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.000.745 Edit this at Wikidata
EC Number
  • 200-819-5
RTECS number
  • PA9450000
  • InChI=1S/CH3I/c1-2/h1H3 checkY
    Key: INQOMBQAUSQDDS-UHFFFAOYSA-N checkY
  • CI
Properties
CH3I
Molar mass 141.94 g/mol
Appearance Clear colourless liquid with acrid odor
Density 2.28 g/cm3 (20 °C)[1]
Melting point −66.45 °C (−87.61 °F; 206.70 K)
Boiling point 42.43 °C (108.37 °F; 315.58 K)
1.4 g/100 mL (20 °C)
log P 1.51
Vapor pressure 50 kPa at 20 °C
53.32 at 25.3 °C
166.1 kPa at 55 °C
1.531
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
3
1
1
Explosive limits 8.5 - 66%
2 ppm (TWA), 5 ppm (STEL)
Lethal dose or concentration (LD, LC):
0.78 mmol/kg (mouse, s.c.)[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Methyl iodide, also called iodomethane, 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. Methyl iodide is miscible with organic solvents and is colourless. It is naturally emitted by rice plantations in small amounts.[2] Methyl iodide is used in organic synthesis to deliver a methyl group, and was approved for use as a pesticide in a controversial decision by the United States Environmental Protection Agency in 2007.[3]

Properties

Methyl iodide is a colorless liquid; it is relatively volatile, with a boiling point of 41-43 °C. It is soluble in many organic solvents. Like many organoiodide compounds, it needs to be kept in dark bottles as it decomposes upon exposure to light to give iodine, giving degraded samples a purplish tinge. Commercial samples may be stabilized by copper or silver wire.[4]

Preparation

Methyl iodide is formed via the exothermic reaction that occurs when iodine is added to a mixture of methanol with red phosphorus.[5] 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:[5]

(CH3O)2SO2 + KI → CH3I + CH3OSO2OK

The CH3I can be purified by distillation followed by washing with Na2S2O3 to remove iodine.

Methyl iodide can also be prepared by the reaction of methanol with potassium iodide, catalyzed by acid:

CH3OH + KI + H2SO4 → CH3I + K2SO4 + H2O

The reaction is carried out at low temperature and the water generated in the reaction is trapped by excess sulfuric acid so the reaction is not reversible. The generated methyl iodide can be distilled from the reaction mixture.

Reactions

Methylation reagent

Methyl iodide 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.[4] Unfortunately, it has a high equivalent weight: one mole of methyl iodide weighs almost three times as much as one mole of methyl chloride. On the other hand, methyl chloride and methyl bromide are gaseous, thus harder to handle; they are also poorer alkylating agents.

Iodides are generally expensive relative to the more common chlorides and bromides, though methyl iodide 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 methyl iodide may cause side reactions, as it is a powerful nucleophile. Finally, being highly reactive, methyl iodide is more dangerous for laboratory workers than related chlorides and bromides.

For example, it can be used for the methylation of phenols or carboxylic acids:[6]

Methylation of a carboxylic acid or phenol with MeI

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 Template:Sulfur rather than "hard" Template: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.

Other reactions

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 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).

Use as a pesticide

Methyl Iodide has also been proposed for use as a fungicide, herbicide, insecticide, nematicide, and as a soil disinfectant, replacing bromomethane (banned under the Montreal Protocol). Manufactured by Arysta LifeScience and sold under the brand name MIDAS, methyl iodide is registered as a pesticide in the U.S., Mexico, Morocco, Japan, Turkey, and New Zealand and registration is pending in Australia, Guatemala, Costa Rica, Chile, Egypt, Israel, South Africa and other countries.[7] In a controversial October 2007 decision, the United States Environmental Protection Agency approved the use of methyl iodide as a soil fumigant in some cases, although it cannot yet be used in Washington and New York due to lack of state approval.[8][9]

In February 2010, a scientific committee for the California Department of Pesticide Regulation (DPR) concluded a review for methyl iodide. The committee determined that methyl iodide is "highly toxic," and that "any anticipated scenario for the agricultural or structural fumigation use of this agent would result in exposures to a large number of the public and thus would have a significant adverse impact on the public health." It also concluded that adequate control of the chemical in these circumstances would be "difficult, if not impossible."[10]

On December 1, 2010, methyl iodide use was approved as a pesticide in the State of California.[11] Following the approval, a coalition of environmentalists, researchers and farmers—including California State Assemblyman Bill Monning, D-Carmel—gathered at locations in Santa Cruz County and six other sites around California to protest the impending use of methyl iodide and to ask governor-elect Jerry Brown to block it. Critics of methyl iodide cite both the cancer-causing aspects of the pesticide and the potential for it to leach out of fields, contaminating water supplies.[12] Objections were also raised by two dozen California legislators and 54 scientists, including five Nobel laureates.[3] The PBS show Marketplace covered the debate about pesticide-use approval, including interviews with John Froines, a professor at UCLA and opponent of the approval; Glen Hasagawa, a farmer in Oxnard, California; and Mary-Ann Warmerdam, head of the California DPR.[13]

Toxicity and biological effects

Methyl Iodide has been shown to induce cancer in rats and mice,[14] and is considered a potential occupational carcinogen by the National Institute for Occupational Safety and Health , the Occupational Safety and Health Administration and the Centers for Disease Control and Prevention.[15] Iodomethane is not ranked as a carcinogen by the International Agency for Research on Cancer or the National Toxicology Program—both the definitive agencies that determine carcinogenity. Iodomethane was listed on California’s Proposition 65 list on April 1, 1988 based on citations found in animal studies conducted in 1977 and reviewed in 1986. The most recent IARC study, conducted in 1999, found that “No epidemiological data relevant to the carcinogenicity of methyl iodide were available. There is limited evidence in experimental animals for the carcinogenicity of methyl iodide.” The IARC overall evaluation concluded that: “Methyl iodide is not classifiable as to its carcinogenicity to humans (Group 3).”

Methyl iodide is listed under California Proposition 65 (1986) as a chemical known by the state to cause cancer or reproductive toxicity.[16] The American Conference of Industrial Hygienists (ACGIH) and The National Institute for Occupational Safety and Health (NIOSH) classifies methyl iodide as a suspected human carcinogen.[17] The Environmental Protection Agency classifies it as "not likely to be carcinogenic to humans in the absence of altered thyroid hormone homeostatis," i.e. it is a human carcinogen but only at doses large enough to disrupt thyroid function (via excess iodide).[18]

Methyl iodide has an LD50 for oral administration to rats 76 mg/kg, and in the liver it undergoes rapid conversion to S-methylglutathione.[19] Breathing methyl iodide 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.

References

  1. ^ a b Merck Index, 11th Edition, 6002
  2. ^ K. R. Redeker, N.-Y. Wang, J. C. Low, A. McMillan, S. C. Tyler, and R. J. Cicerone (2000). "Emissions of Methyl Halides and Methane from Rice Paddies". Science. 290 (5493): 966–969. doi:10.1126/science.290.5493.966. PMID 11062125.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ a b Zitto, Kelly "Methyl iodide gains state OK for use on crops". San Francisco Chronicle. December 2, 2010.
  4. ^ a b Sulikowski, Gary A.; Sulikowski, Michelle M.; Haukaas, Michael H.; Moon, Bongjin (2005). "Iodomethane". e-EROS. doi:10.1002/047084289X.ri029m.pub2.
  5. ^ a b King, C. S.; Hartman, W. W. (1943). "Methyl Iodide". Organic Syntheses{{cite journal}}: CS1 maint: multiple names: authors list (link); Collected Volumes, vol. 2, p. 399.
  6. ^ Avila-Zárraga, J. G., Martínez, R. (2001). "Efficient methylation of carboxylic acids with potassium hydroxide/methyl sulfoxide and iodomethane". Synthetic Communications. 31 (14): 2177–2183. doi:10.1081/SCC-100104469. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  7. ^ "Iodomethane Approved in Mexico and Morocco". Business Wire. October 25, 2010.
  8. ^ "EPA approves new pesticide despite scientists' concerns". Los Angeles Times. October 6, 2007.
  9. ^ "California sun and spray". High Country News. August 4, 2009.
  10. ^ "Report of the Scientific Review Committee on Methyl Iodide to the Department of Pesticide Regulation" (PDF). special Scientific Review Committee of the California Department of Pesticide Regulation. February 5, 2010.
  11. ^ "Calif approves use of pesticide linked to cancer". San Francisco Chronicle. December 1, 2010.
  12. ^ "Statewide protest targets new strawberry pesticide". San Jose Mercury News. November 30, 2010.
  13. ^ Hill, Adrienne, Can methyl iodide be used as a pesticide?, Marketplace, December 27, 2010. Retrieved December 27, 2010.
  14. ^ Druckrey H, Kruse H, Preussman R, Ivankovic S, Landschutz C: (Cancerogenic (sic) alkylating substances--III. Alkyl-halides, -sulfates, -sulfonates and ring strained heterocyclic compounds.] Z Krebsforsch 74:241-70 (1970) (Ger.)
  15. ^ http://www.cdc.gov/niosh/84117_43.html
  16. ^ http://www.oehha.ca.gov/prop65/prop65_list/Newlist.html
  17. ^ OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq
  18. ^ http://www.regulations.gov/search/Regs/home.html#documentDetail?R=09000064802f9119
  19. ^ Johnson, M. K. (1966). "Metabolism of iodomethane in the rat". Biochem. J. 98: 38–43. PMID 5938661.

Additional sources

  • March, Jerry (1992), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (4th ed.), New York: Wiley, ISBN 0-471-60180-2
  • 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.
  • Bolt H. M., Gansewendt B. (1993). "Mechanisms of carcinogenicity of methyl halides". Crit Rev Toxicol. 23 (3): 237–53. doi:10.3109/10408449309105011. PMID 8260067.
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