ORGANOHALOGEN AND ORGANOMETALLIC COMPOUNDS

I he general description organohalogen refers to compounds with covalent carbon-halogen bonding. Substances such as bromomethane, CH3Br, and chlo-roethene, CH2=CHC1, are examples of organohalogen compounds, whereas 

There is wide diversity in the nature of organohalogen compounds but, of necessity, we have restricted this chapter to alkyl, cycloalkyl, alkenyl, alkynyl, and aryl halides. Some of the chemistry of the carbon-halogen bonds already will be familiar to you because it involves the addition, substitution, and elimination reactions discussed in previous chapters. To some extent, we will amplify these reactions and consider nucleophilic substitution by what are called the addition-elimination and elimination-addition mechanisms. Subsequently, we will discuss the formation of carbon-metal bonds from carbon-halogen bonds. The latter type of reaction is of special value because compounds that have carbon-metal bonds are potent reagents for the formation of carbon-carbon bonds, as we will show later in this chapter. 

Although large numbers of organohalogens are known, very few of them occur naturally. Thyroid hormones (e.g., thyroxine) that contain iodine 

Sometimes the long-term effects are quite unexpected and difficult to predict. For example, millions of kilograms of CF2C12, which is used as a propellant, have been released into the atmosphere from aerosol cans. This compound appears to be wholly free of direct adverse physiological effects. However, as the substance diffuses into the upper atmosphere, it is slowly decomposed by sunlight to produce chlorine atoms. Serious danger then is possible because chlorine atoms are known to catalyze the decomposition of ozone, and it is the ozone layer in the upper atmosphere that absorbs most of the sun s ultraviolet radiation that is strongly harmful to life. 

The boiling points of many halogen compounds are similar to hydrocarbons of the same molecular weight, but there are some conspicuous exceptions. lodomethane, for example, has about the same molecular weight as decane (MW 142), but the boiling point of iodomethane is 132° lower than that of decane. Likewise, fluorocarbons (e.g., tetrafluoromethane, CF4, MW 88, bp —129°) are far more volatile than hydrocarbons of similar weights (e.g., hexane, CBH14, MW 86, bp 69°). 

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