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Carbohydrates and Nucleic Acids
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
Both α-hydroxy aldehydes [CHOH-(C=O)H] and ketones as well as “normal” aldehydes react with Fehling’s solution, When an aldose or ketose derivative with an α-hydroxy unit is treated with Fehling’s solution, oxidation to the acid is accompanied by disappearance of the bluish color of the cupric solution and precipitation of a reddish-copper precipitate of cuprous oxide. This precipitation of a reddish-copper precipitate is taken as diagnostic of the presence of an aldehyde moiety (or an α-hydroxy aldehyde or ketone) in the carbohydrate. Sugars that react with Fehling’s solution are called reducing sugars. What is Benedict’s reagent?
Chemical pretreatment of corncob for the selective dissolution of hemicellulose and lignin: influence of pretreatment on the chemical, morphological and thermal features
Published in Biofuels, 2023
Alejandro Ramírez-Estrada, Violeta Y. Mena-Cervantes, Ignacio Elizalde-Martínez, Gabriel Pineda-Flores, Raúl Hernández-Altamirano
Fehling test is one of the most common tests used for the identification of aldehyde functional groups and water-soluble carbohydrates. The Fehling’s solution contains copper (II) ions complexed with tartrate ions in a sodium hydroxide solution. When the heat is applied to the mixture of sample and Fehling’s solution, the bistartarocuprate (II) complex oxidizes the sugars to corresponding acid. At the same time, the copper (II) ions are reduced to cuprous (I) oxide (Cu2O) giving a reddish-brown precipitate. In the absence of reducing sugars, a black cupric oxide precipitate forms. Therefore, the dark-green color of soluble-ALK denotes the absence or low concentration of reducing sugars. Moreover, the lignin dissolved in soluble-ALK fraction is composed of various phenylpropanoid units that are linked together by various types of chemical bonds, such as ether, carbon-carbon, and carbon-oxygen bonds. The phenylpropanoid units contain hydroxyl (−OH) and methoxyl (−OCH3) functional groups, but they do not contain aldehyde or ketone functional groups that are necessary for a compound to give a positive Fehling’s test.