Question

lab report on qualitative test for carbohydrates using seliwanoff's, Benedict's, and barfoed's test.
purpose, results, discussion and conclusion. sources of error

Answer 1

ANS: Objective:[PURPOSE]To characterize carbohydrates present in an unknown solution on the basis of various chemical assays.

Theory: Carbohydrates are polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones. Aldehydes (–CHO) and ketones ( = CO) constitute the major groups in carbohydrates.Carbohydrates are mainly divided into monosaccharides, disaccharides and polysaccharides. The commonly occurring monosaccharides includes glucose, fructose, galactose, ribose, etc. The two monosaccharides combine together to form disaccharides which include sucrose, lactose and maltose. Starch and cellulose fall into the category of polysaccharides, which consist of many monosaccharide resdues

[LAB REPORT]

[*******************Benedict’s Test:[DISCUSSION AND CONCLUSION +RESULT]

As in Fehling’s test, free aldehyde or keto group in the reducing sugars reduce cupric hydroxide in alkaline medium to red colored cuprous oxide. Depending on the concentration of sugars, yellow to green color is developed . All monosaccharides are reducing sugars as they all have a free reactive carbonyl group. Some disaccharides, like maltose, have exposed carbonyl groups and are also reducing sugars, but less reactive than monosaccharides]

[****************Barfoed’s Test:[DISCUSSION AND CONCLUSION +RESULT]

Barfoed's test is used to detect the presence of monosaccharide (reducing) sugars in solution. Barfoed's reagent, a mixture of ethanoic (acetic) acid and copper(II) acetate, is combined with the test solution and boiled. A red copper(II) oxide precipitate is formed will indicates the presence of reducing sugar. The reaction will be negative in the presence of disaccharide sugars because they are weaker reducing agents. This test is specific for monosaccharides . Due to the weakly acidic nature of Barfoed's reagent, it is reduced only by monosaccharides.]

[********************Seliwanoff’s Test:[DISCUSSION AND CONCLUSION +RESULT]

It is a color reaction specific for ketoses. When conce: HCl is added. ketoses undergo dehydration to yield furfural derivatives more rapidly than aldoses. These derivatives form complexes with resorcinol to yield deep red color. The test reagent causes the dehydration of ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural reacts with resorcinol present in the test reagent to produce a red product within two minutes (reaction not shown). Aldohexoses reacts so more slowly to form the same product.]

**************[EXTRA POINT: PURPOSE These three test is used for the detection of carbohydrate , whether it is Aldose or Ketose and MONOSACCHARIDES .NOW These test is useful to identify the properties of carbohydrate] ********************************** THANK YOU VERY MUCH***********