What are the properties of glucose?

Glucose is a solid that is well soluble in water.

• glucose,

• ethanol,

• water,

• multi‑purpose indicator paper,

• watch glass,

• glass rod,

• test tubes.

1. Determine the properties of glucose: state of aggregation, colour and odour.

2. Fill one test tube with water, and another one with ethanol. Test the solubility of glucose in these solvents.

3. Investigate the reaction of the glucose solution.

The following results were obtained in the experiment:

Does glucose react with freshly precipitated copper(II) hydroxide?

The reaction of glucose with copper(II) hydroxide produces a characteristic colour.

• aqueous solution of glucose,

• copper(II) sulphate,

• sodium hydroxide solution,

• beaker,

• water,

• burner,

• tripod,

• test tube.

1. Add a few cubic centimeters of the sodium hydroxide solution to the solution of copper(II) sulphate.

2. The resulting blue gelatinous sediment is copper(II) hydroxide.

3. Heat the mixture in a beaker with hot water.

4. Observe the changes that occur.

What reactions occurred during the experiment?
The reaction of copper(II) sulphate with the sodium hydroxide solution produces copper(II) hydroxide.

$\text{CuS}{\text{O}}_{\text{4 }}\text{+ 2NaOH → Cu(OH}{\text{)}}_{\text{2}}\text{+ }{\text{Na}}_{\text{2 }}\text{S}{\text{O}}_{\text{4}}$

As a result of heating, copper(II) hydroxide is decomposed into copper(II) oxide and water.

$\text{Cu(OH}{\text{)}}_{\text{2 }}\stackrel{\text{temp}}{\to }\text{ CuO + }{\text{H}}_{\text{2}}\text{O}$

$\text{copper(II) hydroxide }\stackrel{\text{temp}}{\to }\text{copper(II) oxide + water}$

Glucose reacts with copper(II) oxide.

${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}\text{ + 2CuO}\stackrel{\text{temp}}{\to }\text{ C}{\text{u}}_{\text{2}}\text{O + }{\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{7}}$

$\text{glucose + copper(II) oxide}\stackrel{\text{temp}}{\to }\text{ copper(I) oxide + gluconic acid}$

This reaction is called Trommer's testTrommer's testTrommer's test. Glucose has reducing properties.

What are the properties of fructose?

Fructose is a solid that is well soluble in water.

• fructose,

• ethanol,

• water,

• multi‑purpose indicator paper,

• watch glass,

• glass rod,

• test tubes.

1. Determine the properties of fructose: state of aggregation, colour and odour.

2. Fill one test tube with water, and another one with ethanol. Test the solubility of fructose in these solvents.

3. Investigate the reaction of the fructose solution.

The following results were obtained in the experiment:

Does fructose give a positive result to the Tollens test?

During the positive test of Tollens, metallic silver is formed, depositing in the form of a mirror coating on the test tube.

• aqueous solution of fructose,

• silver nitrate ${\mathrm{AgNO}}_3$,

• potassium hydroxide solution,

• 5% ammonia solution ${\mathrm{NH}}_3·{\mathrm{H}}_2\mathrm{O}$,

• beaker with hot water,

• pipette,

• test tube rack,

• test tubes.

1. Add a few cubic centimeters of the sodium silver nitrate to the test tube and then add solution of potassium hydroxide.

2. Add a few drops of ammonia water to the resulting sediment until the sediment is dissolved.

3. Add a few drops of the fructose solution.

4. Place the tube in a beaker with hot water.

5. Observe the changes that occur.

The Tollens test or the silver mirror test is a chemical reaction used to detect aldehydes. Ketones give a negative test result, with the exception of ketoses, such as fructose. During the positive Tollens test, a metallic silver is formed, depositing in the form of a mirror coating on the surface of the tube.

For the Tollens test, a Tollens reagent is used, i.e. a solution containing diamminesilver(I) ions ${\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}_{+}$. The Tollens reagent is obtained by adding ammonia water to the silver nitrate solution. In the first stage, a brown silver oxide precipitate:

$2{\mathrm{AgNO}}_3+2{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ag}}_2\mathrm{O}\downarrow +2{\mathrm{NH}}_4{\mathrm{NO}}_3$

The precipitate is reconstituted with an excess of ammonia:

${\mathrm{Ag}}_2\mathrm{O}+4{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\to 2{\left[\mathrm{Ag}\left({\mathrm{NH}}_3\right)2\right]}_{+}+2{\mathrm{OH}}_{-}$

$\mathrm{R}-\mathrm{CHO}+2{\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}_{+}+3{\mathrm{OH}}_{-}\to \mathrm{R}-\mathrm{COOH}+2\mathrm{Ag}\downarrow +4{\mathrm{NH}}_3+2{\mathrm{H}}_2\mathrm{O}$

Both glucose and fructose belong to the reducing sugars that give a positive test of Tollens and Trommer:

• in the case of glucose, the aldehyde group is oxidized to the carboxyl group and gluconic acid is formed,

• in the case of fructose, which contains a ketone group, a positive result indicates that the fructose undergoes transformation (isomerization) in the alkaline environment to form epimers: D‑glucose and D‑mannose, these molecules contain reducing aldehyde groups, hence the positive tests .