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答:2,4-二硝基苯肼是一个极为灵敏的羰基试剂,能与醛或酮类发生脱水缩合产生黄色的腙;若羰基与双键或苯环共轭,则为红棕色。有些不饱和醇类如维甲醇(即维生素 A)、桂皮醇等,都具有还原性,能被2,4-二硝基苯肼氧化成相应的醛基,后者与试剂能定量的产生2,4-二硝基苯腙。实验证明:维甲醇与2,4-二硝基苯肼反应只能得到10～25%腙,说明维甲醇只能部分地被2,4-二硝基苯肼氧化。
A: 2,4-Dinitrophenylhydrazine is an extremely sensitive carbonyl reagent that undergoes dehydration condensation with aldehydes or ketones to give the yellow hydrazone; if the carbonyl group is conjugated with a double bond or a benzene ring, it is reddish brown. Some unsaturated alcohols such as retinol (ie, vitamin A), cinnamyl alcohol, etc., are reductive, can be 2,4-dinitrophenylhydrazine oxidation to the corresponding aldehyde group, the latter with the reagent can produce 2 , 4-dinitrophenylhydrazone. The experiment proves that the reaction of Dimethanol with 2,4-dinitrophenylhydrazine can only get 10 ~ 25% hydrazone, indicating that Dimethyl alcohol can only be partly oxidized by 2,4-dinitrophenylhydrazine.