Describe the manner in which ketohexose acts as a reducing sugar in the Benedict test.
The simplified reaction is:
C5H11O5-CHO + 2Cu^2+(aq) —> C5H11O5-COOH + Cu2O(s) + H2O
The aldehyde group,-CHO, in the ketohexose (such as glucose, galactose, etc) oxidizes to a carboxyl group, -COOH. That releases electrons which reduce Cu+2 to Cu^+1 (blue cupric citrate to red cuprous oxide).
A more detailed explanation would involve the cyclic structure of glucose and other ketoxexoses, and drawings we can't show here.
To understand how ketohexose acts as a reducing sugar in the Benedict test, let's break it down step-by-step:
1. The Benedict test is used to detect the presence of reducing sugars, which includes both aldoses and ketoses. Ketohexose is a type of ketose sugar, meaning it has a ketone group in its molecular structure.
2. In the case of ketohexose, its ketone group can undergo a redox reaction with the copper (II) ions present in the Benedict reagent. This reaction is known as the Fehling's reaction or the Benedict reaction.
3. The Benedict reagent contains copper (II) sulfate, sodium citrate, and sodium carbonate. When heated, the copper (II) sulfate is reduced to copper (I) oxide, which precipitates as a brick-red solid.
4. The presence of a ketohexose sugar in the solution leads to the oxidation of the ketone group. The ketone group is oxidized to a carboxylic acid group (COOH) and simultaneously reduces the copper (II) ions to copper (I) ions.
5. During the reduction process, the blue copper (II) ions (Cu2+) in the Benedict reagent are converted to red copper (I) oxide (Cu2O). The extent of the reduction is proportional to the amount of reducing sugar present in the solution.
6. This reduction reaction causes a color change from blue to brick-red or green, indicating a positive result for the presence of a reducing sugar such as ketohexose.
In summary, ketohexose acts as a reducing sugar in the Benedict test by undergoing a redox reaction with copper (II) ions, resulting in the reduction of the copper ions and the formation of a precipitate of copper (I) oxide.
To describe how ketohexose acts as a reducing sugar in the Benedict test, we need to understand the principles of the Benedict test and the structure of ketohexose.
The Benedict test is used to detect the presence of reducing sugars in a solution. Reducing sugars are carbohydrates that have a free carbonyl group and can reduce certain oxidizing agents, such as the copper(II) ions in the Benedict solution.
Ketohexose refers to a hexose sugar that contains a ketone functional group. It is important to note that most hexose sugars are aldehyde sugars, like glucose and fructose, which can readily act as reducing sugars. However, ketohexoses, like fructose, are also capable of reducing copper(II) ions due to their ability to tautomerize to an enol form.
In the Benedict test, the copper(II) ions in the Benedict solution get reduced to copper(I) ions by the reducing sugars present in the solution. This reduces the blue copper(II) ions to a reddish-brown precipitate of copper(I) oxide.
When a ketohexose, such as fructose, is present in the solution, it undergoes tautomerization where the carbonyl carbon forms a double bond with its neighboring carbon. This forms an enol form of the sugar. This enol form, just like the aldehyde form of other reducing sugars, can donate electrons to the copper(II) ions in the Benedict solution, leading to their reduction.
In summary, ketohexoses, like fructose, act as reducing sugars in the Benedict test by tautomerizing to an enol form and donating electrons to the copper(II) ions, resulting in the reduction of copper(II) ions to copper(I) oxide precipitate.