The carbonyl group is the carbon–oxygen double bond (C=O) functional group. It matters because the more electronegative oxygen pulls electron density from the carbon, leaving that carbon electron-deficient (δ+, partially positive) and therefore open to attack by nucleophiles. The carbonyl group lies at the heart of a whole family of compounds, including carboxylic acids, esters, amides, aldehydes and ketones, whose reactions all begin with this polarised double bond.

羰基是碳氧双键(C=O)官能团。它之所以重要,是因为电负性较大的氧把电子云从碳上拉走,使该碳缺电子(δ+,带部分正电荷),因而易受亲核试剂进攻。羰基处于一大类化合物的核心,包括羧酸、酯、酰胺、醛和酮,它们的反应都从这个极化的双键开始。