Diiodomethane (Methylene Iodide) Simmons Smith Reaction

Diiodomethane

Other Names: Methylene iodide, Methylene diiodide, Diiodomethane, Diiodomethan
Chemical Formula: CH₂I₂
Molar Mass: 267.84 g/mol
CAS Number: 75-11-6
SMILES: ICI
Appearance: Colorless to pale yellow liquid; may darken on exposure to light
Odor: Characteristic odor

Diiodomethane is an organohalogen compound consisting of a methylene group bonded to two iodine atoms. It is the iodine analogue of dichloromethane and dibromomethane and is distinguished by its exceptionally high molecular mass and density for a small organic molecule. The highly polarizable iodine atoms strongly influence its optical and interfacial properties. Diiodomethane is important in synthetic organic chemistry as a methylene-transfer reagent and is also widely employed as a probe liquid in surface-science measurements. Its combination of high density, high refractive index and well-characterized surface properties makes it useful in physicochemical and materials research.

Chemical and physical properties

Diiodomethane is a liquid under ambient conditions. Experimental data report a melting point of approximately 5–6 °C and a boiling point close to 181 °C at atmospheric pressure.

The compound has a particularly high density, approximately 3.3 g/cm³ near room temperature. This unusually high value results primarily from the presence of two heavy iodine atoms in the relatively small CH₂I₂ molecule.

Diiodomethane is only slightly soluble in water but is miscible or readily soluble in many common organic solvents. The large, highly polarizable iodine atoms also give the compound a high refractive index, making its optical properties significantly different from those of lighter halogenated methanes.

The liquid has a surface tension of approximately 50 mN/m near room temperature and is frequently treated as a predominantly dispersive probe liquid in surface-energy measurements. Scientific studies have shown, however, that its small but non-zero polar contribution can become significant when highly polar solid surfaces are analyzed using certain surface-energy models.

Diiodomethane is sensitive to light and can gradually undergo decomposition, producing iodine-containing species that cause discoloration. Samples intended for precise physicochemical or synthetic applications are therefore generally protected from prolonged exposure to light.

Applications

Diiodomethane is an important reagent in organic synthesis, particularly in the Simmons–Smith cyclopropanation. In the classical reaction, diiodomethane reacts with a zinc–copper couple to generate an iodomethylzinc carbenoid, commonly represented as ICH₂ZnI. This reactive intermediate transfers a methylene unit to an alkene, converting the carbon–carbon double bond into a cyclopropane ring.

The Simmons–Smith reaction is valued for its stereospecificity. The geometry of substituents on the starting alkene can be retained during formation of the cyclopropane ring. Modified procedures use reagents such as diethylzinc together with diiodomethane to generate reactive zinc carbenoids under comparatively mild conditions. Such methods are extensively investigated in stereoselective synthesis and the preparation of structurally complex organic compounds.

Diiodomethane is also widely used as a probe liquid for determining the surface free energy of solid materials through contact-angle measurements. It is commonly combined with a polar probe liquid such as water to estimate dispersive and polar contributions to the surface energy of polymers, coatings, composites and other solid surfaces.

In polymer and materials research, contact angles measured with diiodomethane provide information about changes in surface chemistry caused by functionalization, plasma treatment, chemical modification, coating or incorporation of different functional groups. The compound is particularly useful because its surface-tension characteristics are well documented and its interactions with many surfaces are dominated by dispersion forces.

Diiodomethane is additionally employed in mechanistic studies of organozinc chemistry and metal carbenoids. Reactions involving CH₂I₂ have contributed to understanding the formation, structure and reactivity of zinc carbenoids and their role in carbon–carbon bond formation.

Scientific references

Charette, A. B.; Beauchemin, A. Simmons–Smith Cyclopropanation Reaction. Organic Reactions, 2001, 58, 1–415. Comprehensive scientific review of zinc-carbenoid cyclopropanation chemistry, including reactions employing diiodomethane.

Simmons, H. E.; Smith, R. D. A New Synthesis of Cyclopropanes from Olefins. Journal of the American Chemical Society, 1958, 80, 5323–5324. DOI: 10.1021/ja01552a080.

Simmons, H. E.; Smith, R. D. A New Synthesis of Cyclopropanes. Journal of the American Chemical Society, 1959, 81, 4256–4264. DOI: 10.1021/ja01525a036.

Beletskaya, I. P.; Sviridov, S. I.; et al. Simmons–Smith Cyclopropanation: A Multifaceted Synthetic Protocol toward the Synthesis of Natural Products and Drugs: A Review. Molecules, 2023, 28, 5651. DOI: 10.3390/molecules28155651.

Tretinnikov, O. N. On Neglecting the Polar Nature of Halogenated Hydrocarbons in the Surface Energy Determination of Polar Solids from Contact Angle Measurements. Journal of Colloid and Interface Science, 2000, 229(2), 644–647. DOI: 10.1006/jcis.2000.7024.

National Institute of Standards and Technology (NIST). NIST Chemistry WebBook: Diiodomethane, CAS 75-11-6. Experimental physicochemical and thermodynamic data for diiodomethane.

Safety

Signal Word: Danger

First Aid Measures

Eye Contact: Rinse thoroughly with water for several minutes. Seek immediate medical attention.
Skin Contact: Wash with soap and water. Remove contaminated clothing. Seek medical help if irritation occurs.
Inhalation: Move to fresh air. Seek medical attention if breathing becomes difficult.
Ingestion: Rinse mouth. Do not induce vomiting. Seek immediate medical attention.

Handling and Storage

Storage Conditions: Store in a tightly closed amber glass container in a cool, dry, and well-ventilated place. Protect from light and moisture.
Handling Precautions: Avoid inhalation and contact with skin or eyes. Use proper ventilation and protective equipment when handling.