BIOCHEMISTRY - Human Essential Nutrients - 2016

3. CARBOHYDRATES

3.7. Practical Course. Carbohydrate Chemistry

3.7.2. Properties of Carbohydrates

Monosaccharides and certain Disaccharides containing a free aldehyde group can reduce metal oxides to lower oxidation states or to free metals in an alkaline environment (reducing sugars).

Experiment 4. Trommer's Test

In an alkaline medium, glucose acts as a reducing sugar, reducing copper (II) hydroxide - Сu(ОН)₂ to copper (I) oxide - Сu₂О while being oxidized to gluconic acid:

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Add 10 drops of a 1% glucose solution to the first numbered test tube, 10 drops of maltose to the second, sucrose to the third, and starch to the fourth. Add 10 drops of a 10% sodium hydroxide (NаОН) solution and 2 drops of a 5% copper (II) sulfate (СUSO₄) solution to each tube, then heat. Observe The formation of a brick-red precipitate in the tubes containing reducing sugars. In the tubes with non-reducing sugars, a black precipitate is formed.

The report should include a Conclusion stating which CARBOHYDRATES are reducing sugars and which are not.

A reducing sugar undergoes oxidation while reducing bismuth to its lower (hydrated) oxide or to the free state.

To prepare Nylander's reagent, mix 2 g of bismuth nitrate - Вi(NO₃)₃ and 4 g of Rochelle salt (potassium sodium tartrate) in a porcelain mortar. Grind thoroughly and gradually add 10 ml of a 10% sodium hydroxide (NаОН) solution in small portions. Filter and store in a dark Glass bottle in a cold place.

Add 10 drops of a carbohydrate solution (glucose, maltose, sucrose, and starch) into each of four numbered test tubes. Add 10 drops of Nylander's reagent to each tube and gently boil for 2 minutes. Observe the appearance of a dark brown color in the solutions containing reducing sugars.

The report should provide a conclusion on which carbohydrates are reducing sugars and which are not.

Experiment 6. Seliwanoff's Test for Ketohexoses

This reaction makes it possible to distinguish between aldoses (glucose) and ketoses (fructose). Seliwanoff's reagent (resorcinol mixed with a few drops of concentrated Hydrochloric acid) is used for this experiment.

Pour 2 ml of Seliwanoff's reagent into each of two test tubes. Add 3–4 drops of a 2% fructose solution to one, and an equal amount of a 2% glucose solution to the other. Heat both test tubes in a Water bath (80 °C) for 5–10 minutes. The fructose solution turns orange, then pinkish-red. The glucose solution remains colorless. The report should explain why.

The experimental results should be presented in the report as a table:

Experiment 7. Acid Hydrolysis of Starch

Place 2 ml of starch solution into a test tube, add 4 ml of water and 10 drops of concentrated sulfuric acid (Н₂SO₄). Place the test tube in a boiling water bath for 20 minutes. This causes the starch to hydrolyze, and the cloudy starch paste turns clear.

Next, cool the Contents of the test tube and perform two additional reactions using the resulting starch hydrolysate.

Iodine Starch Test. Pour 1 ml of the obtained solution (hydrolysate) into a clean test tube and add 1 drop of iodine solution (see Experiment 3). Note any color change.

If the test is negative for the iodine-starch reaction, take 1 ml of the hydrolysate into another test tube and perform Trommer's test.

Trommer's Test. To the previously set-aside 1 ml of hydrolysate, add 4 ml of a 10% sodium hydroxide (NаОН) solution to create an alkaline environment. Then add 1 ml of a 5% copper (II) sulfate (СuSO₄) solution and heat in a boiling water bath. Will Trommer's test be positive? (See Experiment 4.) Is a color change observed?

Determine whether the hydrolysis of starch has occurred or not. What are the products of this hydrolysis? Write the reaction Scheme for the sequential hydrolysis of starch.