Alkyl Halide Occurrence

Halogen containing organic compounds are relatively rare in terrestrial plants and animals. The thyroid hormones T₃ and T₄ are exceptions; as is fluoroacetate, the toxic agent in the South African shrub Dichapetalum cymosum, known as "gifblaar". However, the halogen rich environment of the ocean has produced many interesting natural products incorporating large amounts of halogen. Some examples are shown below.
The ocean is the largest known source for atmospheric methyl bromide and methyl iodide. Furthermore, the ocean is also estimated to supply 10-20% of atmospheric methyl chloride, with other significant contributions coming from biomass burning, salt marshes and wood-rotting fungi. Many subsequent chemical and biological processes produce poly-halogenated methanes.

Synthetic organic halogen compounds are readily available by direct halogenation of hydrocarbons and by addition reactions to alkenes and alkynes. Many of these have proven useful as intermediates in traditional synthetic processes. Some halogen compounds, shown in the box. have been used as pesticides, but their persistence in the environment, once applied, has led to restrictions, including banning, of their use in developed countries. Because DDT is a cheap and effective mosquito control agent, underdeveloped countries in Africa and Latin America have experienced a dramatic increase in malaria deaths following its removal, and arguments are made for returning it to limited use. 2,4,5-T and 2,4-D are common herbicides that are sold by most garden stores. Other organic halogen compounds that have been implicated in environmental damage include the polychloro- and polybromo-biphenyls (PCBs and PBBs), used as heat transfer fluids and fire retardants; and freons (e.g. CCl₂F₂ and other chlorofluorocarbons) used as refrigeration gases and fire extinguishing agents.

مواضيع ذات صلة

? What makes benzene special

Aromaticity

Conjugation and reactivity: looking forward to Chapter 10

The amide group

Delocalization over three atoms is a common structural feature The carboxylate anion

The allyl system, The allyl anion

The colour of pigments depends on conjugation

UV and visible spectra

The molecular orbitals of butadiene

Conjugation

Multiple double bonds not in a ring

Molecules with more than one C=C double bond Benzene has three strongly interacting double bonds