Atomic groups left after hydroxyl groups are removed from inorganic or organic oxyacids. It's called acyl. For example, acetyl, benzenesulfonamide and acyl can combine with —X,—OR ′, —NH2 and—HNR ′— NR ′ 2 ′ to generate carboxylic acid derivatives: acid halides, anhydrides, esters, amides, (n-substituted amides) and (n-disubstituted amides). The carbonyl groups in acyl groups in carboxylic acid derivatives are not as active as those in aldehydes and ketones, but a series of addition-elimination reactions can still occur. When an acyl group is attached to the benzene ring, the benzene ring can be passivated, and the reentrant group mainly enters the intermediate position. In organic chemistry, acyl mainly refers to groups with structure. Almost all aldehydes, ketones, carboxylic acids and carboxylic acid derivatives have acyl groups. Acyl is not a group that distinguishes organic compounds. The reaction of introducing acyl groups into nitrogen, oxygen, carbon and other atoms in organic compounds is collectively called acylation reaction, but the traditional reaction of introducing nitro groups, sulfonic groups and carboxyl groups into carbon atoms (carboxyl groups can be used as acyl groups of carbonic acid) is called nitration, sulfonation and carboxylation respectively. HO-NO2 nitric acid -NO2 nitroxyl HO-SO3 H sulfuric acid -SO3 H sulfonyl HO-CO-OH carbonic acid C=O carbonyl (carbonyl) Cl-SO-Cl thionyl chloride (thionyl chloride) S=O thionyl chloride.