For each pair of structures, determine whether they represent different compounds or a single compound.

a.

b.

c,

d.

Unsaturated compounds like alkenes that have restricted rotation due to C=C bond are geometrical isomers that is cis-trans or E/Z isomers.The compounds having opposite absolute configuration that is R and S are enantiomers (non-superimposable mirror images) while those having same absolute configuration are identical compounds.

a) The correct stereochemical descriptor for the compound is as follows:

Pair of Structure

The stereochemical descriptors are different and hence these are different compounds (geometrical isomers).

b) The correct stereochemical descriptor for the compound is as follows:

Pair of structures

These two compounds are identical as bond connectivity is same and chiral centers are absent.

c)

Pair of structures

The stereochemical descriptors are different and hence these are different compounds (geometrical isomers).

d) The correct stereochemical descriptor for the compound is as follows:

Pair of structures

The bond connectivity of all the atoms as well as the stereochemical descriptor remains the same. Hence, these compounds are same or identical.