JEE Main & Advanced Chemistry Biomolecules Fructose, fruit sugar \[({{C}_{6}}{{H}_{12}}{{O}_{6}})\] or ketohexose

Fructose, fruit sugar \[({{C}_{6}}{{H}_{12}}{{O}_{6}})\] or ketohexose

Category : JEE Main & Advanced

It is present in abundance in fruits and hence is called fruit sugar. It is also present in cane sugar and honey alongwith glucose in combined form. The polysaccharide inulin is a polymer of fructose an gives only fructose on hydrolysis. Since naturally occurring fructose is laevorotatory, it is also known as laevulose.

(1) Preparation :

(i) Hydrolysis of cane sugar                

\[\underset{\text{Cane sugar}}{\mathop{{{C}_{12}}{{H}_{22}}{{O}_{11}}}}\,+{{H}_{2}}O\underset{\text{Warm}}{\mathop{\xrightarrow{{{H}_{2}}S{{O}_{4}}\text{(dil}\text{.)}}}}\,\underset{\text{D-Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,+\underset{\text{D-Fructose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,\]

The solution having equal molecules of D-glucose and D-fructose is termed invert sugar and the process is known as inversion.

  •  The excess of sulphuric acid is neutralised by adding milk of lime. A little more of lime is added which converts both glucose and fructose into calcium glucosate and calcium fructose respectively.                

\[\underset{\text{Calcium fructose}}{\mathop{{{C}_{6}}{{H}_{11}}{{O}_{5}}-O-CaOH}}\,+C{{O}_{2}}\xrightarrow{{}}\underset{\text{Fructose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,+CaC{{O}_{3}}\]

(ii) Hydrolysis of Inulin with dilute sulphuric acid

\[\underset{\text{Inulin}}{\mathop{{{({{C}_{6}}{{H}_{10}}{{O}_{5}})}_{n}}}}\,+n{{H}_{2}}O\xrightarrow{{{H}_{2}}S{{O}_{4}}\text{(dil}\text{.)}}\underset{\text{Fructose}}{\mathop{n{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,\]

(2) Properties : The anhydrous fructose is a colourless crystalline compounds. It melts at \[{{102}^{o}}C.\] It is soluble in water but insoluble in benzene and ether. It is less soluble in water than glucose. It is the sweetest of all sugars and its solution is laevorotatory. Like glucose, it also shows mutarotation.

   

 

Comparison between glucose and fructose

Property Glucose Fructose
Molecular formula \[{{C}_{6}}{{H}_{12}}{{O}_{6}}\] \[{{C}_{6}}{{H}_{12}}{{O}_{6}}\]
Nature Polyhydroxy aldehyde. Polyhydroxy ketone
Melting point \[146{}^\circ C\] \[102{}^\circ C\]
Optical activity of natural form Dextrorotatory Laevorotatory
With ethyl alcohol Almost insoluble More soluble
Oxidation (a) With bromine water (b) With nitric acid   Gluconic acid   Saccharic acid (Glucaric acid)   No reaction   Mixture of glycollic acid, tartaric acid and trihydroxy glutaric acid
Reduction Sorbitol Mixture of sorbitol and mannitol
Calcium hydroxide Forms calcium glucosate, soluble in water Forms calcium fructosate, insoluble in water
Molisch's reagent Forms a violet ring Forms a violet ring
Fehling's solution Gives red precipitate Gives red precipitate
Tollen's reagent Forms silver mirror Forms silver mirror
Phenyl hydrazine Forms osazone Forms osazone
Resorcinol + HCl (dil.) (Selivanoff's test) No colouration Gives red or brown colour or precipitate
Freshly prepared ammonium molybdate sol. + few drops of acetic acid (Pinoff's test). Light blue colour Bluish green colour on heating
Alcoholic a-naphthol + HCl (conc.) (Furfural test) No colouration A purple colour (violet) on boiling

 

  • Fructose gives reactions similar to glucose. The difference in properties is due to the fact that it contains a ketonic group while glucose contains an aldehydic group.  

Interconversions :

(1) Chain Lengthening of Aldoses (Killiani-Fischer synthesis) : The conversion of an aldose to the next higher member involves the following steps :

(i) Formation of a cyanohydrin.

(ii) Hydrolysis of - CN to -COOH forming aldonic acid.

(iii) Conversion of aldonic acid into lactone by heating.

(iv) The lactone is finally reduced with sodium amalgam or sodium borohydride to give the higher aldose.

(2) Chain Shortening of aldoses

(i) An aldose can be converted to the next lower member by Ruff Degradation.            

It involves two steps.

 

(ii) By Wohl's method

 (3) Conversion of an aldose to the isomeric Ketose

 Three steps are involved,  

 

(4) Conversion of a ketose to the isomeric aldose



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