Fehling s test

Method m. p. crude m. p. recryst. Fehling s Test on recryst. 

This is the basis of the very sensitive Fehling s test for sugars and other reducing agents. A solution of a copper(H) salt dissolved in alkaline tartrate solution is added to the substance in question. If this is a reducing agent then a characteristic red precipitate is produced. 

Copper(II) also forms stable complexes with O-donor ligands. In addition to the hexaaquo ion, the square planar /3-diketonates such as [Cu(acac)2l (which can be precipitated from aqueous solution and recrystallized from non-aqueous solvents) are well known, and tartrate complexes are used in Fehling s test (p. 1181). 

In several instances an induction period has been observed but this has been shortened by elimination of dissolved oxygen . Deposition of cuprous oxide (Fehling s test) does not appear to influence the zero-order disappearance of Cu(II), but the induction period can be eliminated by adding copper powder (but not Cu20) ° . 

When the reduction is complete and all of the sodium is dissolved, (Fehling s test can be bypassed by adding an approximate total of 45 to 50 gms. of sodium and dissolving completely ... 

Carbohydrates such as aldoses that undergo oxidation with metal ions are referred to as reducing sugars. Both copper(II) ions and silver ions are capable of oxidizing aldoses. Oxidation by copper(II) ions is the basis for Fehling s test and Benedict s test, whereas oxidation by silver ions is the key to Tollen s test. (Note These tests work for any sugar with a hemiacetal, but they don t work on acetals or ketals.)... 

Problem 22.7 (a) Explain how in basic solution an equilibrium is established between an aldose, its C epimer (a diastereomer with a different configuration at one chiral C) and a 2-ketose. (b) Will fructose give a positive Fehling s test which is done in a basic solution <4... 

Fehling s test The test is similar to the Benedict s test (see above). 

Fehling s test (Section 25.6) a chemical test for aldehydes, involving treatment with cupric ion in aqueous sodium tartrate. 

One milliliter of mixed solution will react with 5 mg of glucose the empirically determined ratio is the basis for quantitative determination of the sugar. The Fehling s test is not specific to reducing sugars, because ordinary aldehydes reduce the reagent although by a different mechanism and at a different rate. 

Since Fehling s test gives a positive result for any aldose or 2-ketose, it is evidence that there could be a sugar in urine but would not be a definitive test for glucose per se. 

Fehling s test A chemical test for aldehydes by oxidation to a carboxylic acid and Ibrmaiion of cuprous oxide, a red precipitate. 

PROBLEM 16.14 Will lactose give a positive Fehling s test Will it mutarotate ... 

Thus, the chromic acid reagent gives a clear-cut distinction between primary and secondary alcohols and aldehydes on the one hand and tertiary alcohols and ketones on the other. Aldehydes may be distinguished from primary and secondary alcohols by means of Schiff s, Tollens s, Benedict s (Sec. 23.4), and Fehling s tests, and primary and secondary alcohols of lower molar mass may be differentiated on the basis of their rates of reaction with concentrated hydrochloric acid containing zinc chloride—the Lucas reagent (Sec. 25.11B). 

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