Dibromomethane
chemical compound
Dibromomethane or methylene bromide, or methylene dibromide is a halomethane with the formula CH2Br2. It is slightly soluble in water but very soluble in organic solvents. It is a colorless liquid.
Physical properties
At ambient temperature, dibromomethane solidifies around 0.61 GPa. The crystal structure strongly suggests both interhalogen and hydrogen-halogen interactions.
Preparation
Dibromomethane is prepared commercially from dichloromethane via bromochloromethane:
6 CH2Cl2 + 3 Br2 + 2 Al → 6 CH2BrCl + 2 AlCl3
CH2Cl2 + HBr → CH2BrCl + HCl
The latter route requires aluminium trichloride as a catalyst.
The bromochloromethane product from either reaction can further react in a similar manner:
6 CH2BrCl + 3 Br2 + 2 Al → 6 CH2Br2 + 2 AlCl3
CH2BrCl + HBr → CH2Br2 + HCl
In the laboratory, it is prepared from bromoform using sodium arsenite and sodium hydroxide:
CHBr3 + Na3AsO3 + NaOH → CH2Br2 + Na3AsO4 + NaBr
Another way is to prepare it from diiodomethane and bromine.
Uses
Dibromomethane is used as a solvent, gauge fluid, and in organic synthesis (often as 1H-NMR internal standard). It conviently converts polyols (such as catechols) to their methylenedioxy derivatives, and bromomethylenates enolates. It is a much cheaper precursor to a Simmons-Smith-type reagent than diiodomethane.
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