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neutral compound that reacts with water to produce an acid and a hydrogen halide. Acid halides are ordinarily derived from acids or their salts by replacement of hydroxyl groups by halogen atoms. The most important organic acid halides are the chlorides derived from carboxylic acids and from sulfonic acids. The carboxylic acid chlorides, called acyl halides, are generally more reactive than the sulfonic acid chlorides, called sulfonyl chlorides.