CBSE Class 12 Chemistry Notes: Biomolecules – Classification of carbohydrates

The carbohydrates are divided into three major classes depending upon whether or not they undergo hydrolysis, and if they do, on the number of products formed.

(i)   Monosaccharides:The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones which cannot be decomposed by hydrolysis to give simpler carbohydrates. Examples are glucose and fructose, both of which have molecular formula, C₆H₁₂O₆.

ii)    Oligosaccharides:The oligosaccharides (Greek, oligo, few) are carbohydrates which yield a definite number (2-9) of monosaccharide molecules on hydrolysis. They include,

(a)  Disaccharides, which yield two monosaccharide molecules on hydrolysis. Examples       are sucrose and maltose, both of which have molecular formula, C₁₂H₂₂O₁₁.

(b)  Trisaccharides, which yield three monosaccharide molecules on hydrolysis. Example             is raffinose, which has molecular formula, C₁₈H₃₂O₁₆.

(c)  Tetrasaccharides, etc.

(iii)  Polysaccharides:The polysaccahrides are carbohydrates of high molecular weight which yield many monosaccharide molecules on hydrolysis. Examples are starch and cellulose, both of which have molecular formula, (C₆H₁₀O₅)_n.

In general, the monosaccharides and oligosaccharides are crystalline solids, soluble in water and sweet to taste. They are collectively known as sugars. The polysaccharides, on the other hand, are amorphous, insoluble in water and tasteless. They are called non-sugars. The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which have the ability to reduce Fehling’s solution and Tollen’s reagent are referred to as reducing sugars, while others are non-reducing sugars. All monosaccharides and the disaccharides other than sucrose are reducing sugars.

Illustration 1.    The reaction of glucose with acetic anhydride & tollen’s reagent suggest that it is

                        (A)  a penta hydroxyl aldehyde                (B)  hydrate of carbon

                        (C)  a polyhydroxy ketone                       (D)  an alcohols

Solution:          (A)

This confirm the presence of -OH group in glucose on separate carbons.

Confirm the presence of CHO group.

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