Question

Are the following pairs identical, enantiomers, diastereomers, or constitutional isomers?

Short answer

1. They are diastereomers.
2. They are enantiomers.
3. They are Constitutional isomers.
4. They are identical.

Step-by-step solution

Definition of enantiomer, diastereomer, constitutional isomer

1. Identical: If a compound and its mirror image has exactly same structure, the compound is called identical.
2. Enantiomer: If a compound has a mirror image which is exactly opposite of the original compound, but don’t superimpose with each other is called enantiomers. Enantiomers also posses a plane of symmetry in a molecule.
3. Diastereomers: The compounds that has no mirror image, are non-identical and don’t superimpose with each other is known as diastereomers.
4. Constitutional isomer: The compounds that has same molecular formula but the connectivity through atoms in a bond are different and has no chiral centre are called constitutional isomers. They are also called structural isomers.

explanation of the answers

1. These two compounds don’t have a mirror plane as well as a plane of symmetry. They are non-identical and don’t superimpose with each other. So, they are diastereomers.
   
2. To understand the nature of the compound, we have to know the R, S name of the chiral centre.
   
   From the Chiral centre and their name, we can see that the R, S nomenclature of these two centres are totally opposite in this two isomers. So, they are exactly opposite to each other for which, they are enantiomers.
3. The R, S nomenclature of the groups are given below,
   
   Here, both the Cl centre and the CH₃ centres have same R, S nomenclature. So, they are identical compounds.

4. In case of the 4^th one, the position of the Cl changes in the cyclopentane group, that means in one structure, Cl is in 2 position and in other one, Cl is in 3 position.

   So, they are constitutional isomers.