Isomerism

ISOMERISM

• Isomerism in Carbohydrates– Different 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 Rule– Relation 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. 

• Anomerism– Formation 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. 

• Mutarotation– is 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.

Don’t Forget to Solve all the previous Year Question asked on Isomerism