Halomethane
Halomethanes are both naturally occurring, especially in marine environments, and human-made, most notably as refrigerants, solvents, propellants, and fumigants.
Many, including the chlorofluorocarbons, have attracted wide attention because they become active when exposed to ultraviolet light found at high altitudes and destroy the Earth's protective ozone layer.
The halomethanes are produced on an industrial scale from abundant precursors such as natural gas or methanol, and from halogens or halides.
An idealized equation is: Halons are usually defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens.
[9] The higher global warming potential has two causes: HFCs remain in the atmosphere for long periods of time, and they have more chemical bonds than CO2, which means that they are able to absorb more solar energy per molecule than carbon dioxide.
[10] Most permutations of hydrogen, fluorine, chlorine, bromine, and iodine on one carbon atom have been evaluated experimentally.
Its volatility, low flammability, and ability to dissolve a wide range of organic compounds makes this colorless liquid a useful solvent.
In the food industry, it was previously used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavorings.
One major use of CFCs has been as propellants of aerosols, including metered-dose inhalers for drugs used to treat asthma.
As of 2023[update], due to ozone depletion problems, halon fire extinguishers are largely banned in some countries and alternatives are being deployed by the US military.
[12] Halon 1301 total flooding systems are typically used at concentrations no higher than 7% by volume in air, and can suppress many fires at 2.9% v/v.
By contrast, carbon dioxide fire suppression flood systems operate from 34% concentration by volume (surface-only combustion of liquid fuels) up to 75% (dust traps).
(Experimental animals have also been exposed to 2% concentrations of Halon 1301 for 30 hours per week for 4 months, with no discernible health effects.
In these systems, banks of halon cylinders are kept pressurised to about 4 MPa (600 psi) with compressed nitrogen, and a fixed piping network leads to the protected enclosure.
On triggering, the entire measured contents of one or more cylinders are discharged into the enclosure in a few seconds, through nozzles designed to ensure uniform mixing throughout the room.
This level is maintained for some time, typically with a minimum of ten minutes and sometimes up to a twenty-minute "soak" time, to ensure all items have cooled so reignition is unlikely to occur, then the air in the enclosure is purged, generally via a fixed purge system that is activated by the proper authorities.
(There exists a common myth that this is because halon is highly toxic; in fact, it is because it can cause giddiness and mildly impaired perception, and due to the risk of combustion byproducts.)
The stream evaporates under reduced pressure, producing strong local cooling, as well as a high concentration of halon in the immediate vicinity of the fire.
