Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) is a polychlorinated aromatic mainly used as a broad spectrum, nonsystemic fungicide, with other uses as a wood protectant, pesticide, acaricide, and to control mold, mildew, bacteria, algae. It was first registered for use in the US in 1966. In 1997, the most recent year for which data are available, it was the third most used fungicide in the US, behind only sulfur and copper, with some 12 million lb used in agriculture alone that year. Chlorothalonil reduces fungal intracellular glutathione molecules to alternate forms which cannot participate in essential enzymatic reactions, ultimately leading to cell death, similar to the mechanism of trichloromethyl sulfenyl. Chlorothalonil can be produced by the direct chlorination of isophthalonitrile or by treating tetrachloroisophthaloyl amide with phosphoryl chloride.
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