Question 3.12: Recognizing Isomers. Are the following pairs of molecules is......
Recognizing Isomers
Are the following pairs of molecules isomers or not?
(a) CH_{3}CH(CH_{3})(CH_{2})_{3}CH_{3} and CH_{3}CH_{2}CH(CH_{3})(CH_{2})_{3}CH_{3}
(b) \begin{matrix} \\\\ CH_{3}— CH—CH_{2}—CH_{3} \\ | \ \ \ \ \ \ \ \ \ \ \ \ \\ CH_{2}—CH_{3}&&&\end{matrix} and CH_{3}—\underset{\begin{matrix} \mid \\ CH_{3} \end{matrix} }{CH}—CH_{2}—CH_{2}—CH_{3}
Analyze
Isomers have the same molecular formula but different structures. We first check to see if the molecular formulas are the same. If the formulas are not the same, they represent different compounds. If the formulas are the same, the compounds may be isomers, but only if their structures are different.
Solve
(a) The molecular formula of the first compound is C_{7}H_{16}, while that of the second compound is C_{8}H_{18}.
The molecules are not isomers.
(b) These molecules have the same formula, C_{6}H_{14}, but they differ in structure. They are isomers. The difference in structure is that in the first structure, a methyl side chain is on the middle carbon atom of a five-carbon chain, and in the second structure it is on the second carbon atom from the end of the chain.
Assess
The compounds shown in part (a) have different molecular formulas and, thus, are clearly different compounds. We expect these compounds to have markedly different properties. The compounds shown in part (b) have the same molecular formula but different molecular structures. They are different compounds and have different properties. For example, the compound shown on the left in part (b) has a slightly higher boiling point than the compound shown on the right (63 °C versus 60 °C).