Iodomethane
From Wikipedia, the free encyclopedia
 |
This page in a nutshell: Iodomethane is a very powerful reagent primarily used for adding methyl groups to other compounds. |
| Iodomethane | |
|---|---|
  |
|
| General | |
| Systematic name | Iodomethane |
| Other names | Methyl iodide |
| Molecular formula | CH3I |
| SMILES | CI |
| Molar mass | 141.94 g/mol |
| Appearance | colourless liquid |
| CAS number | [74-88-4] |
| Properties | |
| Density and phase | 2.28 g/mL, liquid |
| Solubility in water | Slightly soluble |
| Solubility in organic solvents |
Fully miscible |
| Melting point | -66.5 °C (206.7 K) |
| Boiling point | 42.4 °C (315.6 K) |
| Viscosity | ? cP at ? °C |
| Structure | |
| Molecular shape | Tetrahedral |
| Dipole moment | 1.59 D (gas) |
| Hazards | |
| MSDS | External MSDS |
| EU classification | Toxic (T') Carc. Cat. 3 |
| NFPA 704 | |
| R-phrases | R21, R23/25 R37/38, R40 |
| S-phrases | S1/2, S36/37 S38, S45 |
| RTECS number | PA9450000 |
| Related compounds | |
| Other methyl halides | Fluoromethane Chloromethane Bromomethane |
| Other iodomethanes | Diiodomethane Iodoform Tetraiodomethane |
| Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references |
|
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. It is colourless, although upon exposure to light, samples develop a purplish tinge caused by the formation of I2. Storage over copper metal inhibits this process. 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.[1]
Contents |
[edit] Chemical properties
Methyl iodide is an excellent reagent for SN2 substitution reactions – it is sterically open for attack by nucleophiles, and it has an excellent leaving group (iodide), making it a reactive substrate for such reactions. For example, it can be used for the methylation of phenols or carboxylic acids:
In both of these examples, the base (K2CO3 or Li2CO3) removes the acidic proton to form an anion, which serves as the nucleophile in the SN2 substitution.
Iodide is a "soft" leaving group, which means that methylation with MeI tends to occur at the "softer" end of an ambident nucleophile. For example, reaction with thiocyanate ion favours attack at "soft" S rather than "hard" N, leading mainly to methyl thiocyanate (CH3SCN) rather than CH3NCS. This behavior is relevant to the methylation of stabilized enolates such as those 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 preparated 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.
[edit] Preparation
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.
The CH3I can easily be distilled from the mixture and purified by washing with Na2S2O3 (to remove iodine) and then water, aq. Na2CO3 and water. It is then dried over CaCl2 and distilled. Another purification method involves percolation of the product through silica gel or activated alumina.
An alternative preparation involves the addition of dimethyl sulfate to a stirred suspension of calcium carbonate in aqueous potassium iodide:
(CH3O)2SO2 + KI → CH3I + CH3OSO2OK
Both methods of preparation give high chemical yields of methyl iodide.
Methyl iodide can be formed during nuclear accidents by the reaction of organic matter with the "fission iodine."
[edit] 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.
[edit] See also
- Methylating reagents
[edit] References
- ^ 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: 966-969. DOI:10.1126/science.290.5493.966.
- March, J. (1992). Advanced Organic Chemistry (4th Edn.), 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.
