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How many constitutional isomers are possible for octane with the formula C8H18?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

For the hydrocarbon octane, which has the molecular formula $C_{8} H_{18}$ , there are 18 possible constitutional isomers.21

These isomers are chemical compounds that share the same molecular formula but differ in the connectivity of their atoms. As the number of carbon atoms in an alkane chain increases, the number of possible constitutional isomers grows significantly, as shown in the table below:23

| Molecular Formula | Number of Constitutional Isomers |
| :--- | :--- |
| $C_{6} H_{14}$ | 5 |
| $C_{7} H_{16}$ | 9 |
| $C_{8} H_{18}$ | 18 |
| $C_{9} H_{20}$ | 35 |

Because these isomers possess different structures, they exhibit distinct physical and chemical properties.21