Organic Bromine and Iodine Compounds

An overview is given of the pathways for the degradation and transformation of selected brominated and iodinated aliphatic and aromatic compounds. Although greater emphasis is placed on reactions mediated by microorganisms, examples of important abiotic reactions are also given. A mechanistic outline of the enzymology is provided when possible and comparisons are made with the chlorinated analogues which have been more extensively studied.

On the basis of halogenated metabolites assembled by Gribble, an attempt is made to provide a hypothesis for the biosynthesis of representative groups. Examples of brominated and iodinated metabolites are drawn from the major classes of terpenoids and acetogenins, and related to the biosynthesis of their putative precursors. Brominated and iodinated carbocyclic and heterocyclic aromatic compounds are discussed as well as mechanisms for the oxidative coupling of phenolic substrates. A few chlorinated metabolites are introduced for additional illustration. Halogenation by cationoid (Hal⁺) reactions catalyzed by haloperoxidases is discussed, and attention drawn to the alternative role of anionoid (Hal^-) halogenation. The presentation is extended to the biosynthesis of isonitriles and dichloroimines by anionoid reaction of precursors with cyanide. The range of biota that produce brominated substrates is briefly indicated, and comments made on the ecological significance of metabolites and associations among biota.

Human beings are exposed to an increasing number and increasing amounts of organo-bromine and organochlorine compounds, both of man-made and natural origin. The commercial products of this nature present at highest levels in the general environment include alkyl halides (in particular, methyl bromide), aryl halides and polybrominated diphenyl ethers, all of which exert well-documented toxic effects on mammalian cells. The major systems affected include the points of contact with the environment (i.e., the pulmonary and gastrointestinal tracts and the skin), the central nervous system and the liver. In addition, some of these substances disrupt the hormonal status with respect to thyroid hormones and estrogens. If not easily predictable at present, many of these forms of toxicity are not surprising in light of the reactivity, hydrophobicity, and/or structural properties of the compounds in question.

In this chapter, the atmospheric sources, sinks, distributions, trends, and impacts of organic bromine and iodine compounds are reviewed. Most studies of bromine in the atmosphere have been driven by its well-characterized contribution to stratospheric ozone depletion. Most organic bromine can be grouped into three classes-methyl bromide, the man-made Halons, and a group of shorter-lived, naturally occurring species (e.g., CH₂Br₂, CHBr₃, etc.). Methyl bromide, which originates from an array of natural and anthropogenic sources, constitutes the major source of bromine to the stratosphere, contributing about half of the 20 ppt Br believed to be present there. The Halons, a group of long-lived compounds of strictly anthropogenic origin, are believed to contribute currently about 35% to this present-day stratospheric bromine burden, while the shorter-lived species (which emanate primarily from the oceans) contribute about 15%. Due to their link to ozone depletion, regulations are now in place (in the case of the Halons) or are soon to be in place (in the case of methyl bromide) to eliminate the production and sales of these species. Thus, the ensuing decades should see a reduction in the stratospheric burden of organic bromine. Most organic iodine in the atmosphere appears to originate from the ocean, though anthropogenic sources (rice paddies, biomass burning) also appear to contribute. Methyl iodide appears to be the largest contributor to the overall budget, though other iodinated methanes and higher alkanes (e.g., CH₂I₂, CH₂ICl, CH₂IBr, CH₃CH₂I, CH3CHICH3, CH₃CH₂CH₂I) also play a role. The lifetimes of iodinated species are short (of the order of a few days or less) due to their rapid photolysis in the troposphere. Thus, the impact of these species is largely restricted to the boundary layer, though a contribution to ozone depletion in the lower stratosphere cannot be entirely ruled out.

This chapter reviews the environmentally relevant physical-chemical properties, partitioning and reactivity properties of a selection of organobromine and organoiodine compounds. Substitution of hydrogens with bromine or iodine is shown to cause significant changes in vapor pressure, solubility in water, and hydrophobicity (octanol-water partition coefficient). These property changes are similar to those caused by substitution of hydrogen with chlorine, although there are quantitative differences attributable to the size and mass of the halogen atoms and to the effects on the strength of intermolecular interactions. The environmental implications of these changes are illustrated using simple multimedia partitioning models for volatile brominated and iodinated compounds, brominated flame retardants, and an iodinated X-ray contrast agent. The organobromine and organoiodine compounds are predicted to be less mobile in the environment than their organochlorine counterparts due to their generally higher reactivity and hydrophobicity, and lower volatility and solubility in water.