... intermediates in the preparation of dyes, drugs, and synthetic polymers. More than 1,500 organohalogen compounds have been identified as naturally occurring materials and are produced by various plants, fungi, bacteria, and marine organisms. A variety of synthetic methods to introduce halogen substituents are available, and organic halogen compounds may be converted to other functional-group classes by reliable methods. Examples illustrating the preparation, applications, and natural occurrence of individual organohalogen compounds are described in the separate subsections dealing with alkyl, vinylic, and aryl halides. Herbicides an agent, usually chemical, for killing or inhibiting the growth of unwanted plants--i.e., weeds. (See weed.) In the past, sea salt, by-products of chemical industries, and various oils were used as weed killers. Late in the 19th century the selective control of broad-leaved weeds among cereal crops came into practice. The central development in modern weed control was the introduction in 1945 of so-called organic herbicides; they ...

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