作者:PAUL HAAS
DOI:10.1038/167325a0
日期:1951.2
MOST text-books of organic chemistry contain the statement that formic acid and formates owe their reducing character to the presence of an aldehyde group in their molecule, quoting in support of this their ability to reduce ammoniacal silver nitrate to the metal; while this reaction certainly is given by aldehydes, it is not specific for this class of compound since many other acids, such as tartaric or citric, similarly reduce silver nitrate. The reduction of mercuric chloride to mercurous chloride by formates likewise offers no support for the aldehyde theory, since aldehydes in general, including formaldehyde, cannot effect this change. Actually, formates give none of the reactions which characterize an aldehyde; thus they fail to restore the colour to Schiff's reagent, produce no reaction with 2.4 dinitrophenylhydrazine hydrochloride, and above all do not reduce Fehling's solution, which is probably the most specific of all aldehyde reagents.
大多数有机化学教科书都会提到,甲酸及其盐类之所以具有还原性,是因为它们的分子中含有一个醛基。为了证明这一点,书中会引用它们能够将氨合硝酸银还原为金属银的能力;虽然醛类确实能发生这种反应,但这并非它们独有的特性,因为许多其他酸,如酒石酸或柠檬酸,同样能还原硝酸银。甲酸盐将氯化汞还原为氯化亚汞,这一现象同样无法支持醛基理论,因为包括甲醛在内的多数醛类都无法实现这种转变。实际上,甲酸盐并不表现出任何特征性的醛反应;因此,它们不会使席夫试剂恢复颜色,与2,4-二硝基苯肼盐酸盐不发生反应,最重要的是,它们无法还原斐林试剂——这可能是所有醛试剂中最具特异性的一种。