Define Reaction of Fehling Soxhlet method?

The reducing sugars formed are acted upon by the alkali of Fehling's solution to form enediols. These enediols are very unstable and reactive and they reduce Cu²⁺ ions to Cu⁺ ions ions and in turn sugars are oxidized to mixture of sugar acids. These Cu⁺ ions combine with hydroxyl groups to from cuprous hydroxide which on heating gives red precipitate of cuprous oxide. To get sharp end point, methylene blue is added which is an indicator. It is a redox indicator. The end point is indicated by reduction of blue coloured methylene blue to colourless leuco compound by glucose after all the Cu²⁺ ions have been reduced by glucose. This restores the red colour of the solution. The Fehling Soxhlet solution, of above composition is reduced by about 50 mg of glucose.

The increment method is used when the approximate volume of sugar solution required for reduction of Fehling's solution is not known (as for sample titration). Sucrose is also called cane sugar. Reducing sugars like glucose reduce Fehling's solution. However, since surcose is a non reducing sugar, it does not have the free sugar groups and cannot reduce Fehling's solution. Thus sucrose has to be hydrolyzed. When a solution of sucrose is heated with acid, it is hydrolyzed into its constituent monosaccharides glucose and fructose because the glycoside linkage between glucose and fructose is broken. Glucose and fructose have a free sugar group and can reduce Fehling's solution.

The alkali in the Fehling's solution acts on these monosaccharides to form enediols, which are very unstable and reactive. They reduce Cu²⁺ ions to Cu⁺ ions and themselves get converted to a variety of sugar acids. The Cu⁺ ions combine with hydroxyl groups to form cuprous hydroxide which on heating forms a red precipitate of cuprous oxide. Methylene blue is added as an indicator. It is a redox indicator. The end point is indicated by the reduction of blue coloured methylene blue to a leuco compound by the excess drop of hydrolyzed sugar solution.