Isomers and Stereoisomers MCQ - Practice Questions with Answers

Isomers and Stereoisomers

Isomers:

• These are the organic molecules that have identical molecular formula but different arrangement of atoms.
• For example, D-Glucose and D-fructose. Both have the same molecular formula as C6H12O6. But But D-fructose has a primary alcohol at C-1 and a ketone at C-2 and D-glucose has an aldehyde at C-1 and a secondary alcohol at C-2.

Enantiomers:

• Enantiomers are those isomers that differ in their absolute configuration at each chiral carbon such that they are non-superimposable mirror images to each other. 
• For example, D-Glucose and L-Glucose.

TIP: The D and L in the name of glucose refers to the direction of the -OH group of the chiral carbon which farthest from the functional group. D-Glucose has -OH placed on right side while L-Glucose has -OH placed on the left side.
Diastereomers and Epimers:

• These are the isomers that are non-superimposable on each other and are optical isomers of each other. 
• Epimers are the diastereoisomers that differ in the stereochemistry at only one stereocenter.
• For example, D-Glucose and D-Galactose differ in the configuration at C-4. They are non-superimposable, and they are not mirror images of each other.

TIP: The diastereoisomers are different compounds with different names and properties while enantiomers differ only in the direction in which they rotate polarized light. They are the same compounds.

Anomers:

• When a molecule such as glucose converts to a cyclic form, it generates a new chiral centre at C-1.
• The carbon atom that generates the new chiral centre (C-1) is called the anomeric carbon.
• Anomers are the epimers that differ in configuration only at the anomeric carbon.
• For example, α-D-glucose and β-D-glucose are anomers.
• The α form has the anomeric -OH group at C-1 on the opposite side of the ring from the CH2OH group at C-5. The β form has the anomeric -OH group on the same side as the CH2OH group.