Iodomethane is a halogenated hydrocarbon with molecular formulas CH3I and Mei. It is an iodine substitute of methane. Boiling point 42.4 ℃, relative density 2.279 (20 / 4 ℃), volatile liquid with high density at room temperature, dipole moment 1.59d, refractive index 1.5304 (20 ° C, d) and 1.5293 (21 ° C, d). Iodomethane is miscible with common organic solvents, slightly soluble in water and soluble in ethanol and ether. Pure products are colorless and smelly. They will decompose I2 and turn purple when exposed to sunlight. They can be removed by adding metal copper. Iodomethane exists in a small amount in rice fields. Iodomethane is a common methylation reagent in organic synthesis. Iodomethane is an ideal substrate for SN2 substitution reaction. On the one hand, it is easy for nucleophiles to attack because of its small steric effect; On the other hand, iodine ion is a good leaving group. Therefore, iodomethane can be used to methylate phenol and carboxylic acid. The alkali (K2CO3 or Li2CO3) removes the acidic protons of carboxylic acid or phenol to form carboxylate ions and phenolic salt ions, and then SN2 reaction occurs as a nucleophilic reagent.