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Laboratory work regarding Fehling's Test and Benedict's Test

David Dionisio

on 15 January 2013

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Transcript of Carbohydrates

Comparative reactions: Carbohydrates Fehling's Test Results of the Benedict's test: Comparative reactions: It is a test that will let us be able to identify the different reducing sugars.
A positive result can be indicated when a brick-red or reddish-brown precipitate appears due to the precipitation of cuprous oxide.
The Principle involved with this test is Oxidation.
The Fehling's reagent is made up of alpha (CuSO4 = copper sulfate) and beta (NaOH and NaKC4H4O6 = sodium hydroxide and potassium sodium tartrate). Benedict's Test The Benedict's Test is a simple test for reducing sugars.
It is also a test to identify the presense of aldehyde groups by oxidizing it into carboxylic acids.
It obtains a positive result if a brick-red or reddish-brown solution is observed after mixing.
This test is always positive when Monosaccharides are tested.
Disaccharides result to a positive test sometimes only if they contain a hemiacetal on their terminal sugars.
This test always results negative when starches are tested because they are polysaccharides, and we must remember that this test is only for identifying reducing sugars. Benedict's Test Fehling's test Results of the test: Given sugars: 1). glucose - C6H12O6

2). sucrose - C12H22O11

3). fructose - C6H12O6

4). lactose - C12H22O11 Discussion: All monosaccharides are reducing sugars, which resulted to the positive results of the glucose and fructose. Lactose on the other hand is a disaccharide which may also gives a positive result because of the free reducing end. Sucrose on the otherhand does not, and will not react to the Fehling's reagent because it has the anomeric carbon of glucose and the anomeric carbon of fructose which are in a glycocidic bond; hence they cannot open to produce an aldehyde As you can see, the copper sulfate and the potassium sodium tartrate are needed to form the final Fehling's reagent together with the sodium hydroxide. These make a deep blue colored solution. For the preparation of the Fehling's reagent, it is required to prepare this fresh because, as the two reagents dissolve and produce a final reagent, their particles float around the aqueous solution like the copper. The tartrate ions from the beta reagent chelate the copper ions and prevent them from forming Cu(OH)2 from the CuSO4 + 2H2O with the help of the NaOH. SUGARS REACTIONS DISCUSSION Benedict's reagent: Fehling's reagent: Alpha: Beta: Final: - Benedict’s reagent is composed of copper sulfate (CuSO4), sodium carbonate (Na2CO3) and sodium citrate (Na3C6H5O7). REACTIONS Discussion: General Equation:
Na2CO3 + 2H2O -> 2NaOH + H2CO3
2NaOH + CuSO4 -> Cu(OH)2 + NaSO4
Cu(OH)2 -> CuO + H2O
R-CHO + Cu2+ ◊ R-COOH + Cu2O(s) Understanding: Fructose is able to undergo Fehling’s test because it has a free anomeric carbon or carbonyl group, which allows it to react with mild reducing agents such as the Fehling’s test. - The reason for why the Benedict's Test has the same color reaction as the Fehling's test is because of the presense of CuSO4 (copper II sulfate). They both react to give off a rusty, reddish-brown, or brick-red color because Cu2+ reduces into copper I. Results on urine samples: In the clinical field, the Benedict’s test is used to detect a reducing sugar in the urine of a person also called glucosuria. This test is usually done to diabetic patients to see if they have too much sugar in their blood. It is an indication that a patient may have diabetes mellitus. This test only obtains a positive result if there is too much sugar/glucose in the blood, which can no longer be absorbed by the kidneys, and is secreted through the urine. All monosaccharide molecules are reducing sugars because they all have a free reactive carbonyl group, which is why glucose and fructose react to the Benedict’s reagent. Disaccharides, such as lactose, have exposed carbonyl groups, which is why it also reacts with the Benedict’s reagent. Sucrose, another disaccharide, does not react to the reagent because it does not contain a free carbonyl group in its composition. The copper sulfate (CuSO4), present in Benedict's solution, reacts with electrons from the aldehyde or ketone group of the reducing sugar to form cuprous oxide (Cu2O), a red-brown precipitate. Notes: Fehling's and Benedict's tests are oxidation tests because the aldehydes are actually the ones reducing the reagents. The copper (II) ion complex reduces to the copper (I) because the solutions are alkaline. The aldehydes are then oxidized into salts of corresponding carboxylic acids.
For the Benedict's test the hydroxide ions are not seen in the equations. These ions come from the sodium carbonate which is made alkaline because the carbonate ions act reversaibly with water to produce the hydroxide and hydrogen carbonate ions. R-CHO + 2OH -> R-C(OH)=O + H2O + 2e-
2Cu2+ + 2e- -> 2Cu+
2Cu+ +2OH -> Cu2O + H2O
R-CHO + 2Cu2+ + 4OH -> R-C(OH)=O + Cu2O + 2H2O

- Where Cu2O is the red precipitate
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