Classify each pair of compounds as constitutional isomers or stereoisomers.

Compounds that have the same molecular formula but exist as different structures (i.e., adopts different structures) are termed as constitutional isomers.

The structures/compounds that differ only in the three-dimensional (3D) arrangement of atoms are referred to as stereoisomers.

a. Compounds 1 and 2 (from the left) differ only in the arrangement of groups i.e., they have the same molecular formula and different structural formula.

Therefore, the compounds in a are constitutional isomers.

b. In this case, the compounds differ only by their structure and hence they are constitutional isomers.

c. The two compounds differ in the position of the methyl groups i.e., there is a change in their structural formula.

Therefore, the given compounds are constitutional isomers.

d. The given compounds have the same molecular and structural formula, and they differ only by their arrangement in space i.e., the methyl groups in the given structures adopt different orientations.

Therefore, they are stereoisomers.