Isomerism

Isomerism


ISOMERISM

  • 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 are
    • D Glucose and L Glucose
    • D Fructose and L Fructose
    • D Mannose and L Mannose
    • D Glyceraldehyde and L Glyceraldehyde.    
  • Optical Activity- The presence of asymmetric carbon atom causes optical activity.
  •  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

  1. Monosaccharide with no Asymmetric Carbon atom- Dihydroxyacetone.
  2. Aldose sugar is converted to ketose and vice versa by enzyme isomerase.
  3. Glucose and mannose are C2 epimers and glucose & galactose are C4 epimers.
  4. Maltose is a reducing disacharide.
  5. Lactose is a reducing dissacharide.
  6. Sucrose is a non-reducing sugar.
  7. Benedict’s quantitative reagent detects any reducing sugar.
  8. Fehling solution contains CuSO4 and Ruchelle salt .
  9. Glucose oxidase method is used for glucose.
  10. Perioxidase and oxidase enzymes are used for estimation of glucose. 
Don’t Forget to Solve all the previous Year Question asked on Isomerism

Module Below Start Quiz

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