Acylhalogenide

An acyl halide, commonly called an acyl chloride, is an organic compound containing the acyl group R–CO–X, where X is a halogen (Cl, Br, or I; fluorides are rare). The most familiar members are acid chlorides (RCOCl). In these compounds, the carbonyl carbon is highly electrophilic and the C–X bond is a good leaving group, making acyl halides highly reactive toward nucleophiles and versatile intermediates in organic synthesis.

Preparation: Most acyl chlorides are prepared by chlorinating carboxylic acids with reagents such as thionyl chloride (SOCl2) or oxalyl chloride (COCl)2, often in the presence of catalytic DMF. Acid bromides and iodides are prepared analogously from carboxylic acids using PBr3 or red phosphorus with bromine (to give bromides) and using PI3 or iodine with red phosphorus (to give iodides). Interconversion among acyl halides and from other carboxylic derivatives is also common.

Reactions and uses: Acyl halides are strong acylating agents. They readily react with alcohols to give esters, with amines to give amides, and with water to yield the parent carboxylic acids and hydrogen halide. They can form anhydrides from carboxylic acids, and they participate in further transformations with organometallic reagents. Because of their high reactivity and moisture sensitivity, they are typically handled under anhydrous conditions and used as intermediates to activate carboxylic acids for diverse synthesis.

Properties and safety: Acyl halides are typically liquids or low-melting solids with sharp, often pungent odors. They are moisture-sensitive and corrosive, reacting vigorously with water and air. They require storage under inert atmosphere or dry conditions and appropriate protective equipment in handling.