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derivatives of acids in which the hydroxyl groups are replaced by halogen atoms. Examples are sulfuryl chloride, S02C12 [an acid halide of sulfuric acid, H2S04—that is, SO2(OH)2]; thionyl chloride, SOCl2 [an acid halide of sulfurous acid, H2S03—that is, SO(OH)2]; phosphorus trichloride, PCI3 [an acid halide of phosphorous acid, H3P03—that is, P(OH)3]; and acetyl chloride, CH3COCl (an acid halide of acetic acid, CH3COOH).
Acid halides are very reactive: their halogen atoms can be easily replaced by other groups, such as —OH, —OR, —NH2, —SH, and —CN. They hydrolyze in moist air, giving volatile hydrogen halide (they “smoke”)—for example, S02C12 + 2H2O = H2SO4 + 2HCl ↑. The acid fluoride of sulfuric acid, sulfuryl fluoride, resists hydrolysis. In organic synthesis, organic acid halides are used to introduce the RCO (acyl) group into compounds (the acylation reaction):
RCOCl + HOR′ → RCOOR′ + HCl
Organic acid chlorides, which are made by treating carboxylic acids with acid chlorides of inorganic acids (PCl3, PCl5, and SOCl2), are most frequently used for this type of reaction:
The acid halides of most inorganic acids, as well as the acid chlorides of the lower acyclic carboxylic acids, are liquids with very pungent odors.