脎（英语：Osazone，汉语拼音：sà），也称糖脎，是糖类的苯肼衍生物。糖脎为黄色结晶，由糖与苯肼反应生成，分解又得到原来的糖，因此可以用于糖的提纯。费歇尔在当年研究糖的构型时，采用了分析各种糖生成的糖脎的方法，因为不同的脎结晶形状不同，熔点不同，生成时间也不同。

以葡萄糖为例，生成糖脎的反应需用三分子的苯肼与一分子的糖进行反应。当苯肼用量为一摩尔时，得到苯腙。而后经互变异构发生1,4-消除，转化为亚氨基酮，再与两分子的苯肼成脎。

苯肼只和糖的C-1和C-2成脎后，分子内氢键使其形成较为稳定的六元环结构，从而糖的其他碳原子不再成脎。

Osazones are a class of carbohydrate derivatives found in organic chemistry formed when sugars are reacted with excess of phenylhydrazine. The famous German chemist Emil Fischer developed and used the reaction to identify sugars whose stereochemistry differed by only one chiral carbon. The reaction involves formation of a pair of phenylhydrazone functionalities, concomitant with the oxidation of the hydroxymethylene group adjacent to the formyl center. The reaction can be used to identify monosaccharides. It involves two reactions. Firstly glucose with phenylhydrazine gives glucosephenylhydrazone by elimination of a water molecule from the functional group. The next step involves reaction of one mole of glucosephenylhydrazone with two moles of phenylhydrazine (excess). First phenylhydrazine is involved in oxidizing the alpha carbon to a carbonyl group, and the second phenylhydrazine involves in removal of one water molecule with the formyl group of that oxidized carbon and forming the similar carbon nitrogen bond. The alpha carbon is attacked here because its more reactive than the others. Osazones are highly coloured and crystalline compounds and can be easily detected. Glucose gives broomstick or needle shaped crystals with this whereas maltose gives sunflower shaped crystals.