• Isomerism in CarbohydratesDifferent compounds or structures having same molecular formula is called isomers and the phenomenon existing is called isomerism. 
  • Carbon to which four different substituent groups are attached called as Chiral or asymmetric carbon atom. 
  • Lebervon’t Hoff RuleRelation between asymmetric carbon atom and number of stereoisomers possible. (No. Of isomers= 2n) 
  • Types of Isomers In Carbohydrates- 1) Stereoisomerism        2) Optical isomerism

1)  Stereoisomers- compounds which have same structural formula but differ in spatial configuration of H and OH group around asymmetric carbon atoms.

  • Enantiomers (D and L isomerism) – *D sugars are naturally occurring. Examples areOptical Activity- The presence of asymmetric carbon atom causes optical activity.
    • D Glucose and L Glucose
    • D Fructose and L Fructose
    • D Mannose and L Mannose
    • D Glyceraldehyde and L Glyceraldehyde.    
  • When a beam of plane-polarized light is passed through a solution of carbohydrates, it will rotate the light either to right or to left. 
  • Depending on rotation it is called as

a.     Levorotatory (l or -) anticlockwise                              b.  Dexarotatory (d or +) clockwise

        (Rotate plane polarised light to left)                                  (Rotate plane polarised light to right)

        D Fructose                                                                      D glucose ( Dextrose) 

  • Racemic Mixture Equimolar mixture of optical isomers which has no net reaction of plane polarized light. 
  • Epimerism (Diastereoisomerism)When sugars are different from one another, only in configuration with regard to a single carbon atom, other than the Glucose reference carbon atom, they are called epimers. 

Epimers of glucose

  1. 2nd Epimer of Glucose – Mannose
  2. 3rd Epimer of Glucose – Allose
  3. 4th Epimer of Glucose – Galactose. 
  • AnomerismFormation of ring structure in monosaccharides results in creation of an additional asymmetric carbon called Anomeric Carbon atom.
    • Difference in orientation of H and OH group around the anomeric carbon atom results in Anomerism and the resulting isomers are called α and β anomers.
    • Examples of Anomerism:
  1. αDGlucose and  β DGlucose
  2. α D Fructose and β D Fructose. 
  • Mutarotationis the change in the specific optical rotation of plane polarised light with time.
    • The optical rotation of α glucose is +112.20
    • The optical rotation of β glucose is 18.70  ( 190)
    • The optical rotation of Fructose is -920
    • An equilibrium with a constant value is +52.70 
  • Benedict’s Reaction- Standard lab test to diagnose Diabetes mellitus. 

Shapes of Osazones-

  • Needle-shaped/Broomstick/sheaves of corn – Glucose, Fructose, Mannose
  • Pincushion with pins/Hedgehog/ Flower of Touch –me-not – Lactose
  • Sunflower Petal-shaped –  Maltose 
  • Sucrose is also called as invert sugar.

Exam Important

  • Monosaccharide with no Asymmetric Carbon atom- Dihydroxyacetone.
  • Aldose sugar is converted to ketose and vice versa by enzyme isomerase.
  • Glucose and mannose are C2 epimers and glucose & galactose are C4 epimers.
  • Maltose is a reducing disacharide.
  • Lactose is a reducing dissacharide.
  • Sucrose is a non-reducing sugar.
  • Benedict’s quantitative reagent detects any reducing sugar.
  • Fehling solution contains CuSO4 and Ruchelle salt .
  • Glucose oxidase method is used for glucose.
  • Perioxidase and oxidase enzymes are used for estimation of glucose. 
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