Reactions of carbohydrates with strong acid

Reactions of Carbohydrates with Strong Acid, and the Qualitative and Quantitative Determination of Carbohydrates... 

The degradation of carbohydrates by strong acids has been recognized for many years. It was a common elementary school demonstration to add sulfuric acid to sugar (sucrose) to produce carbon. Under more controlled conditions, the reaction of all carbohydrates in aqueous solution with strong acid has been shown to produce the formation of a derivative of furfural as the major product [7]. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

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