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An acid halide has the general structure R–COX, with chlorine or bromine as the halide. The halogen’s inductive effect and the ability of halide to leave help make acid halides more reactive than other common carboxylic acid derivatives. They can be prepared from carboxylic acids using reagents such as thionyl chloride, phosphorus pentachloride, or oxalyl chloride. In nucleophilic acyl substitution, a nucleophile adds to the carbonyl, forming a tetrahedral intermediate, which then expels the halide. The nucleophile determines the product: hydrolysis gives a carboxylic acid, alcoholysis an ester, and aminolysis an amide. Other reactions include acylation, reductions, and carbon–carbon bond formation.