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Glucose
Glucose occurs in nature in Free State as well as in combined state. In Free State, it occurs in sweet fruits and honey. Ripe grapes contain – 20% glucose and hence the name grape sugar. In combined state, glucose is present in di- and polysaccharides. Maltose and starch on hydrolysis yield only glucose.

Preparation of glucose
   
By hydrolysis of Cane-sugar: in laboratory glucose can be prepared by hydrolysis of cane-sugar in the presence of alcohol using dilute hydrochloric acid. Glucose and fructose are formed in equal amounts. Glucose, being less soluble in ethyl alcohol than fructose, crystallizes out.
                                  

   
By hydrolysis of Starch: glucose is obtained, on commercially scale, by hydrolysis of starch by boiling it with dilute sulphuric acid at 393 K under a pressure of 2-3 bar.
                              


Structure of glucose

Glucose is an aldohexose. It is monomer of many of the larger carbohydrates such as starch, cellulose. The reactions of glucose indicate that its molecule contains one primary (-CH2OH) and four secondary (-CHOH) hydroxyl groups. Glucose was assigned the following structure on the basis of evidences that follow:



Evidences that support the structure of Glucose

The molecular formula of glucose is C6H12O6.
   
Reduction: glucose is reduced to sorbitol, a hexahydric alcohol, on reaction with hydrogen in the presence of nickel as catalyst or on reaction with sodium amalgam in aqueous solution.


   
Reaction with hydrogen iodide: Glucose on prolonged heating with HI gives n-hexane.



This reaction suggests that the six carbon atoms in glucose are linked linearly.
   
Oxidation: (i) glucose on oxidation with acid oxidizing agents, such as bromide water. Tollen’s reagent or Fehling’s solution gives Gluconic acid.



This confirms the presence of an aldehydic group in glucose.

(ii) stronger oxidizing agents such as concentrated nitric acid oxidize glucose to glucaric acid (saccharic acid).


   
Acetylation: when glucose is reacted with acetic anhydride in the presence of sulphuric acid, a pentaacetyl derivative of glucose is obtained.



Formation of pentaacetyl derivative indicates the presence of five –OH groups in glucose molecule.
   
Ether formation: Glucose reacts with dimethyl sulphate in the presence of alkali to form penta-O-methyl derivative.



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