Explain why a change in the position of a substituent group (i. e., which carbon the group is attached to) can, but does not always, result in a different compound

Changing the position of the substituents in an organic molecule will often result in a different chemical compound. If the structure of the molecule changes but it still has the same molecular formula, i.e., the same number of each atom, then this is called a structural isomer. However, it is possible to change the position of a substituent and the structure of the molecule remain unchanged. This would most likely occur in symmetric molecules.

CH₃ - CH₂ - CH₂ - CH₂ - OH    1-butanol

CH₃ - CH₂ - CH - CH₃    2 - butanol
                    |
                   OH

Looking at the two structures above, the position of the -OH was moved from the 1-carbon to the 2-carbon, resulting in two different molecules. These are structural isomers. However, we can also move the -OH group without changing the molecule.

CH₃ - CH₂ - CH - CH₃     2-butanol
                    |
                   OH
CH₃ - CH - CH₂ - CH₃     2-butanol
           |
          OH
In the above structure, it appears that the -OH group was moved from the 2-carbon to the 3-carbon, but since this molecule is symmetrical, the 2- and 3-carbons are equivalent. Therefore, swapping substituents has no effect on the structure of the molecule and these are equivalent.

Density and pressure