BIOL I 100 – 2
PRACTICAL 2: Investigating the Chemical Properties of Sugars.
The aim of this practical was to test the property of sugars to act as reducing agents. Benedict’s reagent was used to test this property. In this solution Cu2+ ions are reduced to Cu+ by certain sugars changing its colour.
Two monosaccharides sugars, fructose and glucose were used, and to disaccharides lactose and sucrose. Solutions as 0.25% and 1% solutions in water were mixed witn Benedict’s reagent and left in warm water for five minutes. The following table of results was obtained.
As it can be seen the two monosaccharides sugars, fructose and glucose, are clearly reducing agents, this is due to the fact that it is the aldehyde group which gives sugars its reducing properties and in the case of monosaccharides the aldehyde group is free to react as a reducing agent, especially at 1%, this reducing capacity is decreased in a lower concentration. On the other hand the two disaccharides, lactose and fructose, have very little reducing properties in the case of lactose and none in the case of sucrose.This is due to the fact that it is the aldehyde group which gives sugars its reducing properties.In the case of lactose we find two glucose rings joined by glycosidic bond forming an acetal (one carbon joined to two oxygens), the ring with the acetal is non reducing but the other ring still has a hemiacetal (carbon joined to two oxygens one of the oxygens being attached to hydrogen) and is capable of acting as a reducing agent though the reaction is weak.
In the case of sucrose we observe that it is non reducing in both concentrations, this is due to the fact that the glycosidic bond is formed between the reducing ends of both glucose and fructose, therefore both rings are locked and unable to act as reducing agents.
Therefore, Benedict’s reagent could be used as a mean of testing the amount of reducing sugar in a solution as it would result in a different colour depending on the concentration of sugar, and whether it is monosaccharide or disaccharide.
PRACTICAL 2: Investigating the Chemical Properties of Sugars.
The aim of this practical was to test the property of sugars to act as reducing agents. Benedict’s reagent was used to test this property. In this solution Cu2+ ions are reduced to Cu+ by certain sugars changing its colour.
Two monosaccharides sugars, fructose and glucose were used, and to disaccharides lactose and sucrose. Solutions as 0.25% and 1% solutions in water were mixed witn Benedict’s reagent and left in warm water for five minutes. The following table of results was obtained.
As it can be seen the two monosaccharides sugars, fructose and glucose, are clearly reducing agents, this is due to the fact that it is the aldehyde group which gives sugars its reducing properties and in the case of monosaccharides the aldehyde group is free to react as a reducing agent, especially at 1%, this reducing capacity is decreased in a lower concentration. On the other hand the two disaccharides, lactose and fructose, have very little reducing properties in the case of lactose and none in the case of sucrose.This is due to the fact that it is the aldehyde group which gives sugars its reducing properties.In the case of lactose we find two glucose rings joined by glycosidic bond forming an acetal (one carbon joined to two oxygens), the ring with the acetal is non reducing but the other ring still has a hemiacetal (carbon joined to two oxygens one of the oxygens being attached to hydrogen) and is capable of acting as a reducing agent though the reaction is weak.
In the case of sucrose we observe that it is non reducing in both concentrations, this is due to the fact that the glycosidic bond is formed between the reducing ends of both glucose and fructose, therefore both rings are locked and unable to act as reducing agents.
Therefore, Benedict’s reagent could be used as a mean of testing the amount of reducing sugar in a solution as it would result in a different colour depending on the concentration of sugar, and whether it is monosaccharide or disaccharide.
Good disucssion of the difference between sucrose and lactose, esp. with regards to the anomeric-to-anomieric position of the glycosidic bond in sucrose which prevents the ring from opening.
ReplyDeleteI think you should reconsider the comments about fructose though. Only aldoses are reducing sugars, and fructose is a ketose. However, you have (correctly) found that fructose works are a reducing sugar. This is due to an isomerisation reaction which converts a ketose (i.e. fructose) into an aldose.
/Steve