CARBOHYDRATES 31 



CHO COOH 



! I 



(CHOH)„ + 2CuO *► (CHOH)„ + Cu^O 



CH.OH CHjOH 



An aldose Fehling's An aldonic Cuprous 



reagent acid oxide 



The copi)cr of Fehling's solution is actually present as a copper com- 

 pound of sodium potassium tartrate, NaKCuC4H20r„ but CuO more 

 clearly indicates the oxidizing character of the solution. 



The above reaction accounts for less than half of the cuprous oxide 

 actually produced during the Fehling's reaction. The rest is produced 

 indirectly by the oxidation of simpler substances into which the reducing 

 sugars are converted by the strong alkali (sodium hydroxide) in the 

 reagent. Decomposition by alkali is a characteristic property of reducing 

 sugars generally. Nef isolated 93 substances from the decomposition 

 of sugars in alkaline solution. 



Benedict's solution is a less sensitive reagent for reducing sugars than 

 Fehling's solution because it contains a weaker alkali, sodium carbonate. 

 This is an advantage, since it is used to test for sugar in urine, which 

 commonly contains small amounts of nonsugar reducing substances. 

 These materials are less apt to give a false result with Benedict's than 

 with Fehling's solution. Barfoed's reagent is not alkaline at all, but 

 rather is acidic; hence it requires a very strong reducing agent to pro- 

 duce a positive result. It is for this reason that the Barfoed's reagent 

 can be used to distinguish simple sugars from other carbohydrates (see 

 p. 42). 



Any sugar capable of reducing Fehling's solution is called a reducing 

 sugar. All monosaccharides are reducing sugars, as are also the common 

 disaccharides, maltose, lactose, and cellobiose (but not sucrose). Fehl- 

 ing's reaction forms the basis of a useful method of analyzing foods for 

 their sugar content. A weighed sample is extracted with hot water, 

 and a portion of the solution so obtained, after treatment to remove 

 interfering substances, is hydrolyzed and allowed to react with Fehling's 

 solution under carefully standardized conditions. The precipitate of 

 cuprous oxide is collected and weighed, and from the weight found the 

 percentage of sugar in the sample may be calculated. 



In the analysis of fruits, vegetables, sirups, candies, blood, urine, and 

 so on, total reducing sugar is generally expressed as glucose. No attempt 

 is made to distinguish between glucose or fructose as both have the 

 same nutritive value and approximately the same reducing power. 



Glucose reacts with an excess of phenylhydrazine to form an insoluble 

 precipitate known as glucosazone according to the following equation: 



