SACCHARIDES I1 (1)

8/9/2019 SACCHARIDES I1 (1)

1/4

1

SACCHARIDES I

Key words: monosaccharides, disaccharides, polysaccharides, addition reaction ofaldehydes, hemiacetals, acetals, glycosides, oxidation, reduction, complex

compounds, hydrolysis, acid hydrolysis of starch, composition and structure of starch.

Reagents:

1. Glucose 0.05 mol/l

2. Fructose 0.05 mol/l

3. Ribose 0.05 mol/l

4. Glucuronic acid 0.05 mol/l

5. Maltose 0.03 mol/l

6. Sucrose 0.02 mol/l

7. Starch 10 g/l

8. Formaldehyde 4%

9. Schiffs reagent (fuchsine solution decolourised with sodium hydrogensulphite)

10. Fehlings solution I CuSO4 . 5 H2O 70 g/l

11. Fehlings solution II sodium potassium tatrate 350 g/l

NaOH 250 g/l

12. Benedicts reagent CuSO4 . 5 H2O 17.3 g/l

Na2CO3 100 g/l

sodium citrate 173 g/l)

13. Sodium hydroxide (caustic) 2 mol/l14. Lugols solution diluted

15. Hydrochloric acid concentrated (caustic)


2/4

2

1. Reaction with Schiffs reagent

Principle:

The possession of aldehyde group endows sugars with reaction properties of

aldehydes (e.g. addition, reducing properties, see Reactions of functional groups of

organic compounds of biochemical importance, task 1.); although due to itsinvolvement in the formation of hemiacetal form (ring form), the reactivity of aldoses

is different from that of aldehydes.

Procedure:

a. Add 2 drops of Schiffs reagent (use a dropper) to 1 ml of glucose solution.

b. Do the same reaction with formaldehyde solution.

c. Compare and explain the differences in the colouring!

2. Reducing properties of saccharides

2.1. Reducing properties of glucose (Glc)

Principle:

Alkaline copper reagents - with complex bonded Cu2+

- such as Benedicts

reagent, Trommers reagent or Fehlings reagent are sensitive to reducing agents

including monosaccharides and reducing disaccharides. By heating in alkaline

medium, glucose is cleaved into reducing products. The presence of the free

hemiacetal hydroxyl group is necessary to start the cleavage.

2.1.1. Fehlings test (see Reactions of functional groups of organic compounds ofbiochemical importance, task 2. and 4.3.).

Procedure:

a. Mix 0.5 ml of Fehlings I solution with 0.5 ml of Fehlings II solution in a test

tube.

b. Add 1 ml of glucose solution, mix and heat the mixture in the boiling water bath;

the red precipitate of copper (I) oxide appears.

c. Indicate the ligand in the table 1.

2.1.2. Benedicts test

Procedure:

a. Add 3 drops of glucose solution to 1 ml of Benedicts reagent, mix.

b. Heat the mixture in the boiling water bath, red coloured copper (I) oxide

precipitates.

c. Indicate the ligand in the table 1.


3/4

3

2.1.3. Trommers reaction (see Reactions of functional groups of organiccompounds of biochemical importance, task 4.3.).

Procedure:

a. Mix 2 ml of sodium hydroxide solution with 3 drops of copper (II) sulphate

solution. Pale blue precipitate should be formed.b. While shaking the test tube, add glucose solution until the precipitate disappears.

c. Heat the contents of the tube in the boiling water bath - copper (I) oxide

precipitate is formed.

d. Try to perform the same reaction without glucose.

e. Write the reactions a. and d. in the chemical equations:

a. NaOH + CuSO4

heating

d.

f. Indicate the ligand in the table 1.

Table 1.

Reduced reagent Fehlings Benedicts Trommers

Ligand

2.2. Reducing properties of saccharides

Principle:

Saccharides have reducing properties only if they possess a free hemiacetal

hydroxyl group like glucose. In this case, they are able to reduce the complex copper

(II) to copper (I).

Procedure:

a. Repeat the Fehlings test mentioned above (2.1.1.) with the solution of fructose

(Fru), ribose (Rib), glucuronic acid (GlcUA), maltose, sucrose and starch.

b. Indicate the ligand in the table 2.

Table 2.

Saccharide Glc Fru Rib GlcUA Maltose Sucrose Starch

Reduction


4/4

4

3. Acid hydrolysis of starch

Principle:

Starch can be broken down into its monosaccharide constituents by hydrolysis

under acidic conditions. When boiled in an acid solution, starch is hydrolyzed

successively to dextrins, maltose and D-glucose finally. During starch hydrolysis, thecolouring of starch-iodine complex should change with time from blue (starch) to red

(erythrodextrin), till no reaction (when dextrin contains linear arrays of fewer than six

1-4 linked residues they fail to give colours with iodine).

Procedure:

a. Put 1 ml of diluted Lugols solution in each of 8 test tubes.

b. Bring 10 ml of starch water suspension (in a Erlenmeyer flask) to boil. Cover the

flask by aluminium film.

c. Into another test tube, put 0.5 ml of conc. hydrochloric acid (use a doser) and add

it to the reaction mixture (caution!). Keep the content of the flask to boil gently.

d. Immediately take off a small sample of reaction mixture (use a plastic pipette) andadd it into the first tube containing Lugols solution.

e. In 1 min intervals take aliquots from the flask using a plastic pipette and repeat the

same procedure (use the next tubes containing Lugols solution).

f. When negative reaction with iodine is observed, continue to boil the mixture for

another 5 min and perform the reducing tests afterwards (Fehlings test, see task

2.1.1.).

g. Indicate your observation in the table 3.

Table 3.

Tube No 1 2 3 4 5 6 7 8

Colour

Documents

SACCHARIDES I1 (1)