Carboxylic acid derivatives also contain a carbonyl unit, but they have a leaving group attached to the acyl carbon that facilitates acyl substitution reactions. Carboxylic acids contain a carbonyl that is connected to an OH unit in a COOH unit called the carboxyl group. The IUPAC nomenclature for carboxylic acids identifies the longest continuous chain for the acid that contains the CO2H unit. The nomenclature of each acid derivative is based on the carboxylic acid precursor, the “parent” acid. Esters are a combination of a carboxylic acid and an alcohol. Sulfonic acids form acid halides, anhydrides, esters, and amides. A detergent is a surfactant is usually an alkylbenzene sulfonate but more soluble in hard water than a soap.