Isomerism

Isomerism

Asymmmetric carbon atom :

 it is the carbon atom to which 4 different groups or atoms are attached . Any substance containing asymmetric carbon atom show 2 properties optical activity and optical isomerism .

Optical activity :

It is the ability of substance to rotate plane – polarized light either to the right or to the left .

• If the substance rotates plane polarized light to the right so it is called : dextrorotatory or d or (+) .
• If rotates it to the left so it is called: levorotatory or "I" or (-).

1. Glucose contains 4 asymmetric carbon atoms . It is dextrorotatory so it is sometimes named dextrose 
2. Fructose contains 3 asymmetric carbon atoms . It is levorotatory so it is  sometimes called : levulose .
3. Racemic mixture : It is the mixture containing equal number of molecules of 2 optically active sugars one is dextrorotatory and the other is levorotatory .Thus, it show no optical activity
4. Resolution : It is the separation of optically inactive racemic mixture in to it is optically active substances

Optical isomerism : 

It is the ability of substance to present in more than one form (isomer) .

• A substance containing one asymmetric carbon atom has 2 isomers
• A substance containing2 or more asymmetric carbon atoms can exist in a number of isomers = 2n where n is the number of symmetric carbon atoms .E.g glucose has 4 asymmetric carbon atoms so the number of its isomer equal 24 = 2x2x2x2 = 16 isomers

Configuration (Enantiomers):

• The simplest carbohydrate is glyceraldehydes that has one asymmetric carbon atom. So it has 2 optically active forms :L and its mirror image D forms .
• Reference sugar:It is the glyceraldehydes which may be presnt in( D) form in which – OH group attached to asymmetric carbon  atoms is on the right side. (L) form in which  – OH group attached to asymmetric carbon  atoms is on the left side.

Anomers:

Anomeric carbon : is the asymmetric carbon atom obtained from active carbonyl sugar group : carbon number 1 in aldoses and carbon number 2 in ketoses .

   Anomers : These are isomers obtained from the change of position of hydroxyl group attached to the anomeric carbon e.g. a and β glucose are 2 anomers .Also a and β fructose are 2 anomers .

Alose – ketose isomerism : 

Fructose has the same molecular formula  as glucose but differs in structure formula ) one contains kito  group (c=o) and the other contains aldehyde group (-CHo)both are isomers.

Epimeric carbon and epimers :

Epimeric carbon is the asymmetric carbon atom other the carbon of aldehyde or ketone group e.g. carbons number 2,3 and 4 of glucose .

epimers : are isomers resulting from the change of position of groups around the epimeric carbns i.e ( glucose galactose and mannose are epimers ) .

• glucose has 3 epimeric  carbons  3,4 and 5 .
• galactose : epimer of cabon 4 .
• mannose: epimer of carbon 2 .

Pyranose and furanose

• pyranose as it resembles an organic compound called pyran e.g a and β glucopyranose
• furanose as it resembles an organic compound called furan e.g a and β glucofuranose

Haworth and chair forms :

• Cyclic structure of sugars may be present in the form of Haworth or chair forms.In Haworth formula , the arrangement of  H and – OH groups around carbon atoms is as follows :
• All the – OH groups on the right side in old ring structure are written downwards in Haworth formula 
• All the – OH groups on the left  side in old ring structure are written updownwards in Haworth formula