Monosaccharides: Glucose and Fructose

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Preparation from Sucrose (Cane sugar): (Laboratory Method):

  • When powdered cane sugar is heated with a concentrated alcoholic solution of HCl on a water bath for about 2 hours at about 323 K, glucose is formed.

Glucose 01

  • It is insoluble in alcohol while fructose is soluble in alcohol. Hence glucose crystallises out first leaving fructose in the solution.  A few crystal of glucose may be added to the solution for quicker crystallisation. This is known as seeding.
  • Purification is done by recrystallizing it from methanol.

Preparation From Starch: (Commercial Method):

  • Starch on hydrolysis with dilute sulphuric acid by heating under 3 to 5 atmospheric pressure gives glucose.

Glucose 02

  • When hydrolysis is complete the excess of unreacted sulphuric acid is neutralised with calcium carbonate and filtered to remove the precipitate of calcium sulphate.

CaCO3 + H2SO4   →   CaSO4   + H2O + CO2 ­

  • The filtrate which contains glucose and one molecule of water is decolourised using animal charcoal. The clear solution is then evaporated in vacuum to get a thick syrup on cooling crystalises to give glucose-                                                                                                                                     monohydrate. It is recrystallized from methanol.

Open Chain Structure:

Glucose 03



  • Following chemical reactions of glucose confirm its open chain structure
  • On prolonged heating with HI it gives n-hexane suggesting that all the six carbons are linked in a straight chain.

Glucose 04

  • Hydroxylamine condenses with an aldehydic group to form glucose-oxime.

Glucose 05

  • Hydrogen cyanide adds to an aldehydic group to form cyanohydrin.

Glucose 06

  • On oxidation by a mild oxidising agent like bromine water it gives gluconic acid, which shows that the carbonyl group is aldehyde group.

Glucose 07



  • Glucose, as well as gluconic acid on oxidation by dilute nitric acid, gives dicarboxylic acid, saccharic acid, which shows the presence of the alcoholic group.

Glucose 10



 

  • On acetylation by acetic anhydride it gives glucose-pentaacetate., which confirms the presence of five hydroxyl group. As glucose is a stable compound these five hydroxyl group must be on five different carbon atoms.

Glucose 12

  • Challenges to open chain structure or the need of cyclic structure:
  • Following points indicate the absence of free aldehyde group in glucose.
    • In spite of having aldehyde group it does not give condensation reaction with                   2,4 dinitro-phenyl hydrazine
    • Glucose-pentaacetate does not condense with hydroxylamine
    • It is found to exist in two different crystalline forms α and β called anomers.

Haworth Projection of Glucose:

  • The cyclic structure in Haworth projection depicts the ring as being flat. The substituents that are to the right in a Fischer projection formula are down and those to the left are up in the corresponding Haworth projection formula. Orient the Haworth projection formula with the ring oxygen at the back and the anomeric carbon at the right.
  • For carbohydrates of D series a) If hydroxyl is down, the configuration of anomeric carbon is α and b) If hydroxyl is up, the configuration of anomeric carbon is β.
  • For carbohydrates of L series a) If hydroxyl is up, the configuration of anomeric carbon is α and b) If hydroxyl is down, the configuration of anomeric carbon is β.

Glucose 13

Note: A ring containing five carbons and one oxygen is referred pyran



Physical Properties:

  • It is white crystalline solid.
  • It is soluble in water but sparingly soluble in alcohol.
  • As it contains 4 asymmetric carbon atoms, it is optically active compound.
  • It is dextrorotatory. It has a specific rotation of  + 52.5°.

Fructose:

  • Fructose also has the molecular formula C6H12O6 and on the basis of its reactions, it was found to contain a ketonic functional group at carbon number 2 and six carbons in the straight chain as in the case of glucose.
  • It belongs to D-series and is a laevorotatory compound. It is appropriately written as                D-(–)-fructose. Its open chain structure is as shown.

Glucose 14

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