Benedict’s test

What can be learned about sugars by performing the Benedict s test ... 

Data Table 3 Benedict s Test of Hydrolyzed Solutions ... 

Thinking Critically Were reducing sugars detected in the hydrolyzed starch solution using the Benedict s test Was this expected ... 

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.)... 

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

Aldehydes Alkanes Benedict s test Dichromate test 2,4-Dinitrophenylhydrazine (2,4-DNP) Fehling test Iodoform test Oxime Permanganate test Semicarbazone Tollen s test No test Positive for all aldehydes Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes Positive only for acetaldehyde Positive for all aldehydes (and ketones) Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes... 

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

The urine of dystrophic patients was reported in 1949 by Minot et al. (Mil) to contain quantities of organic ribose phosphate esters, which could be detected for diagnostic purposes by a modified qualitative Benedict s test (03). Walton and Latner (W5), however, found this proposed test to have no diagnostic value. Determined and thorough biochemical investigations in large numbers of patients with all types of muscular dystrophy (Pll, RIO) have since shown that any difference... 

Another test that is used to distinguish between aldehydes and ketones is Benedict s test. Here, a buffered aqueous solution of copper(II) hydroxide and sodium citrate reacts to oxidize aldehydes but does not generally react with ketones. Cu + is reduced to Cu+ in the process. Cu + is soluble and gives a blue solution, whereas the Cu+ precipitates as the red solid copper(I) oxide, CU2O. 

We should also note that when the carbonyl group of a ketone is bonded to a —CH2OH group, the molecule will give a positive Benedict s test. This occurs because such ketones are converted to aldehydes under basic conditions. In Chapter 17 we will see that this applies to the ketone sugars, as well. They are converted to aldehyde sugars and react with Benedict s reagent. 

The amount of precipitate formed, and thus the color change observed, in the Benedict s test are directly proportional to the amount of reducing sugar in the sample. 

Which of the following would give a positive Benedict s Test ... 

Benedict s test (14.4) a test used to determine the presence of reducing sugars or to distinguish between aldehydes and ketones it requires a buffered solution of Cu + ions that are reduced to Cu+, which precipitates as brick-red CU2O... 

Maltose and a-lactose would give positive Benedict s tests. Glycogen would give only a weak reaction because there are fewer reducing ends for a given mass of the carbohydrate. 

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