Glucose on prolonged heating with HI forms

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Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds. Some common examples of carbohydrates are cane sugar, glucose, starch, etc. Most of them have a general formula, C_x(H₂O)_y, and were considered hydrates of carbon from where the name carbohydrate was derived. For example, molecular formula of glucose (C₆H₁₂O₆) fits into this general formula, C₆(H₂O)₆. But all compounds which fit into this formula may not be classified as carbohydrates. For example, acetic acid (CH₃COOH) fits into this general formula, C₂(H₂O)₂ but is not a carbohydrate. Some of carbohydrates, which are sweet in taste, are also called sugars. Carbohydrates are classified on the basis of their behavior on hydrolysis. They have been broadly divided into following three groups: (i) Monosaccharides, (ii) Oligosaccharides, and (iii) Polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides, whether aldose or ketose, are reducing sugars. Glucose is an aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch and cellulose. Glucose is found to exist in two different crystalline forms which are named α-D-Glucose and β-D-Glucose. The two monosaccharides are joined together by an oxide linkage formed by loss of a water molecule. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage. Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages. Vitamins are classified into two groups depending upon their solubility in water or fat: (i) Fat-soluble vitamins, and (ii) Water-soluble vitamins.