醛Aldehyde

系统命名用后缀 -al，醛基碳编为 1 号：乙醛 ethanal，丙醛 propanal。实验室中用酸化重铬酸钾氧化伯醇制醛，为防止醛被进一步氧化为羧酸，要在醛生成后立即蒸出（醛的沸点低于醇）。工业上甲醛由甲醇催化氧化制得。

Systematic names use the suffix -al, with the aldehyde carbon as C-1: ethanal, propanal. In the laboratory, aldehydes are made by oxidising primary alcohols with acidified dichromate, distilling the aldehyde off as it forms (it boils lower than the alcohol) to stop further oxidation to the acid. Methanal is made industrially by catalytic oxidation of methanol.

醛容易被氧化为羧酸，这是它与酮的主要区别：托伦试剂（氨性硝酸银）被醛还原生成银镜，费林试剂被脂肪醛还原生成砖红色 Cu₂O 沉淀，酸化重铬酸钾由橙变绿。醛被 NaBH₄ 还原为伯醇；与 HCN 发生亲核加成生成羟基腈；与 2,4-二硝基苯肼生成橙色沉淀。

Aldehydes are easily oxidised to carboxylic acids, the main difference from ketones: they reduce Tollens' reagent (ammoniacal silver nitrate) to a silver mirror, reduce Fehling's solution to a brick-red precipitate of Cu₂O (aliphatic aldehydes), and turn acidified dichromate from orange to green. NaBH₄ reduces them to primary alcohols; HCN adds to give hydroxynitriles; 2,4-dinitrophenylhydrazine gives an orange precipitate.

甲醛是无色有刺激性气味的气体，其水溶液（福尔马林）用于保存生物标本，也是脲醛树脂、酚醛树脂的原料；甲醛有毒，是室内空气污染物之一。苯甲醛有苦杏仁气味。肉桂醛、香草醛是常见的香料成分。葡萄糖等醛糖分子中也含醛基。

Methanal is a colourless, pungent gas; its aqueous solution, formalin, preserves biological specimens, and it is a raw material for urea–formaldehyde and phenol–formaldehyde resins. It is toxic and an indoor air pollutant. Benzaldehyde smells of bitter almonds; cinnamaldehyde and vanillin are common flavourings. Aldoses such as glucose also contain an aldehyde group.