羰基Carbonyl group

羰基碳原子为 sp² 杂化，与相连的三个原子处于同一平面，键角约 120°。C=O 由一个 σ 键和一个 π 键组成。氧的电负性大，π 电子偏向氧，使碳带部分正电荷 δ+、氧带部分负电荷 δ−，羰基因此是强极性基团，醛酮之间有偶极–偶极作用。

The carbonyl carbon is sp² hybridised and coplanar with its three neighbours at about 120°. C=O is one σ plus one π bond. Oxygen is more electronegative, so the π electrons are pulled towards it, leaving carbon δ+ and oxygen δ−; the carbonyl group is therefore strongly polar, and aldehydes and ketones attract each other by dipole–dipole forces.

带部分正电荷的羰基碳易受亲核试剂进攻，平面结构使进攻可以从两侧进行。醛、酮与 HCN（在 CN⁻ 催化下）发生亲核加成生成羟基腈，碳链增加一个碳；与 NaBH₄ 反应被还原为醇。羧酸衍生物则多发生亲核取代（加成–消除）。

The partially positive carbonyl carbon is attacked by nucleophiles, and its planar shape allows attack from either face. Aldehydes and ketones add HCN (with CN⁻ as catalyst) by nucleophilic addition to give hydroxynitriles, lengthening the chain by one carbon, and are reduced to alcohols by NaBH₄. Carboxylic acid derivatives usually undergo nucleophilic substitution (addition–elimination) instead.

2,4-二硝基苯肼（布雷迪试剂）与醛、酮生成橙黄色沉淀，用于检验羰基；沉淀的熔点可用于鉴定具体的醛酮。区分醛和酮可用托伦试剂（醛产生银镜）或费林试剂（脂肪醛产生砖红色沉淀）。红外光谱中羰基在约 1680–1750 cm⁻¹ 有强吸收。

2,4-Dinitrophenylhydrazine (Brady's reagent) gives an orange-yellow precipitate with aldehydes and ketones, a test for the carbonyl group; the precipitate's melting point identifies the compound. Tollens' reagent (silver mirror) or Fehling's solution (brick-red precipitate with aliphatic aldehydes) distinguishes aldehydes from ketones. In infrared spectra the carbonyl group absorbs strongly at about 1680–1750 cm⁻¹.