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Biomolecules

Classification of Carbohydrates and Glucose

  • Carbohydrates are called saccharides.

  • Classification

Classification of Monosaccharides

  • Monosaccharides are classified based on the number of carbon atoms and the functional group present in them.

  • Different types of monosaccharides arelisted in the given table.

Carbon atoms

General term

Aldehyde

Ketone

3

Triose

Aldotriose

Ketotriose

4

Tetrose

Aldotetrose

Ketotetrose

5

Pentose

Aldopentose

Ketopentose

6

Hexose

Aldohexose

Ketohexose

7

Heptose

Aldoheptose

Ketoheptose

Glucose

  • Preparation of glucose

  • By boiling sucrose with dilute HCl or H2SO4 in alcoholic solution

  • By boiling starch with dilute H2SO4, at 393 K, under pressure

  • Structure

  • Glucose has been assigned the above structure based on the following evidences.

(i) Molecular formula − C6H12O6

(ii) Suggestion of straight chain

(iii) Confirmation of carbonyl (> C = O) group

(iv) Confirmation of the presence of carbonyl group as aldehydic group

(v) Confirmation of the presence of five −OH groups

(vi) Indication of the presence of a primary alcohol

  • The correct configuration of glucose is given by

  • Glucose is correctly named as D (+) − Glucose

  • To understand the concept of configuration further, let us go through the following puzzle.

Cyclic Structure of Glucose

  • The following reactions of glucose cannot be explained by its open-chain structure.

  • Aldehydes give 2, 4-DNP test, Schiff’s test, and react with NaHSO3 to form the hydrogen sulphite addition product. However, glucose does not undergo these reactions.

  • The penta-acetate of glucose does not react with hydroxylamine. This indicates that a free −CHO group is absent from glucose.

  • Glucose exists in two crystalline forms, α and β.

The α-form (m.p = 419 K) crystallises from a concentrated solution of glucose at 303 K and the β-form (m.p = 423 K) crystallises from a hot and saturated aqueous solution at 371 K. This behaviour cannot be explained by the open-chain structure of glucose.

  • Glucose exists in two cyclic forms, which exist in equilibrium with the open- chain …

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