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Introduction to the Chemistry of Alkyl Halides

Haloalkanes (Alkyl Halides) in Nature

A large number of halogen-containing compounds have been found in nature and many of these are utilized in medicine and technology. These compounds include, for example, the following:

  • Ethyl chloride (chloroethane), used as a skin coolant in tropical areas
  • Halothan (2-bromo-2-chloro-1,1,1-trifluoroethane), applied today as an anaesthetic - similarly to chloroform which was used previously

Forest fires, volcanic eruptions and maritime metabolic processes produce about five million tons of methyl chloride each year while industrial emissions of methyl chloride amount to just about 26,000 tons per year.

Bromotrifluoromethane is a favorite fire-extinguishing agent which is used in airplanes and for electrical components because it evaporates without leaving any residue. In the human organism, the iodine-containing thyroxine is an important thyroid hormone. Thyroxine deficiency may lead to goiter. This symptom is particularly concentrated in regions far off coastlines. Small amounts of potassium iodide, therefore, must be added to table salt in order to enable the organism to produce thyroxine.

Fig.1
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Fig.2

Halogen-containing organic compounds are especially frequently found in marine organisms (e.g. in corals). They generally serve as toxic repellents against natural enemies. Red algae, for example, produces a compound (a) with a putrid smell meant to ward off attackers. However, sea hares are not repulsed by this repellent, on the contrary even seem to enjoy it, since they live on red algae. After consuming red algae, sea hares convert the initial haloalkane into a structurally related compound (b) which aids in their self-defence. In 2000, only a couple of hundred secondary metabolites produced by marine organisms from an estimated number of approximately 500,000 had been thoroughly investigated.

Fig.3
Haloalkanes as repellents of red algae (a) and sea hares (b)

Exercise: Haloalkanes (alkyl halides) in nature

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