Classification of Carbohydrates and Glucose
Carbohydrates are called saccharides.
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.
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
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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