Sunday, October 11, 2009

The reaction between Benedict's solution and reducing sugars?

The reaction between Benedict's solution and reducing sugars?
Benedict's Test For Reducing Sugars
Fehling's solution versus Benedict's. They are chemically very similar both being alkaline solutions of copper(II) sulphate solution. They are made by having copper(II) sulphate solution, sodium hydroxide solution and a complexing agent (a ligand). The latter is tartrate ions (2,3-dihydroxybutanedioate ions) in Fehling's but there is no information what it is for Benedict's. It is different but acts the same way. It will be the anion of a carboxylic acid and will act as a ligand for copper(II) ions like tartrate. The ligand forms a complex with the copper(II) ions and stops the copper(II) ion from precipitating with the hydroxide ions as copper(II) hydroxide. This would not be a very good oxidising agent. The trick is that the copper(II) ions have to be alkaline to be oxidising but that they must be in solution not in a precipitate. The tartrate ions complex to the copper(II) ions and protect it from precipitating with the hydroxide ions.

Now the redox reaction can be carried out. The reaction works as aldehydes and reducing sugars are easily oxidised. i.e they like to lose electrons. Aldehydes are oxidised to carboxylic acids. Fehling's and Benedict's are weak oxidising agents and can do the job. The copper(II) ion gains an electron to form copper(I) in the form of copper(I) oxide, Cu2O.

2Cu2+ + H2O + 2e- = Cu2O + 2H+

The hydroxide ions remove the H+ ions and help the forward reaction to go better and become more electron accepting.

The reducing sugar e.g glucose, gets oxidised because it exists in aqueous solution in three different forms (two cyclic and one non-cyclic). The non-cyclic is the least common but it is present in small amounts. This form has an aldehydic group at the end of the chain and is the group that is oxidised. As the aldehyde form gets oxidised, more of the non-cyclic form is produced due to Le Chatelier's principle.
Benedict's Test for Reducing Sugar
benedict's test for reducing sugars-how-is-brick-red-coloration-formed

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