Question 12.7: The following structures all have the formula C7H16. Which o...
The following structures all have the formula C_7H_{16}. Which of them represent the same molecule?
(a) \begin{array}{r c} \underset{|}{\quad \; CH_3}\quad \quad \quad \quad \quad \quad \quad \enspace \; \\ CH_3CHCH_2CH_2CH_2CH_3 \end{array} (b) \begin{array}{r c} \underset{|}{\quad \; CH_3} \quad \enspace \> \\ CH_3CH_2CH_2CH_2CHCH_3 \end{array}
(c) \begin{array}{r c} \underset{|}{\quad \; CH_3} \quad \quad \quad \enspace \> \\ CH_3CH_2CH_2CHCH_2CH_3 \end{array}
ANALYSIS Pay attention to the connections between atoms. Do not get confused by the apparent differences caused by writing a structure right to left versus left to right. Begin by identifying the longest chain of carbon atoms in the molecule.
Molecule (a) has a straight chain of 6 carbons with a – CH_3 branch on the second carbon from the end. Molecule (b) also has a straight chain of 6 carbons with a – CH_3 branch on the second carbon from the end and is therefore identical to (a). That is, (a) and (b) are conformers of the same molecule. The only difference between (a) and (b) is that one is written “forward” and one is written “backward.” Molecule (c), by contrast, has a straight chain of 6 carbons with a – CH_3 branch on the third carbon from the end and is therefore an isomer of (a) and (b).