# Which of these molecules are isomers of each other and explain why that is?

Jul 10, 2017

$\text{Molecule 2}$ and $\text{molecule 4}$ are isomeric.......i.e. structural isomers. $\text{Molecule 1}$ and $\text{molecule 3}$ are different molecules.
Molecules that have the same molecular formula, but different CONNECTIVITY are said to be $\text{structural isomers}$. Structural isomers have distinct physical and chemical properties.
And thus $\text{molecule 2}$ and $\text{molecule 4}$ have a chemical formula of ${C}_{4} {H}_{10}$; however, clearly, they are structurally different in that their carbon-carbon skeletons are manifestly different. They should thus express different physical and chemical properties. You should look up their boiling points; straight chain $\text{n-butane (4)}$ will be more involatile than $\text{isobutane (3)}$.
On the other hand, for $\text{molecule 1}$ and $\text{molecule 3}$, we have different chemical formula, ${C}_{3} {H}_{5} O H$ versus ${C}_{3} {H}_{6} {O}_{2}$ in $\text{propanoic acid}$.