Dibromomethane

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Dibromomethane

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Template:Longitem	969143
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EC Number	Template:Unbulleted list
Template:Longitem	25649
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MeSH	methylene+bromide
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UN number	2664
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Molar mass	Template:Chem molar mass
Appearance	Colorless to yellow liquid
Density	2.477 g⋅mL−1
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Vapor pressure	4.65 kPa (at 20.0 °C)
Template:Longitem	9.3 μmol⋅Pa−1⋅kg−1
Template:Longitem	−65.10·10−6⋅cm3/mol
Template:Longitem	1.541
Template:Longitem	104.1 J⋅K−1⋅mol−1
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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.

Natural occurrence

It is naturally produced by marine algae and liberated to the oceans. Releasing on soil causes it to evaporate and leach into the ground. Releasing in water causes it to be lost mainly by volatilisation with a half life of 5.2 hours. It has no significant degradation biological or abiological effects. In the atmosphere it will be lost because of reaction with photochemically produced hydroxyl radicals. The estimated half life of this reaction is 213 days.

External links

• International Chemical Safety Card 0354