How do you draw #C_8H_18# (octane)? How many isomers are there? What are the names of the isomers?

You work systematically. First, draw the straight-chain octane.

• Octane

Next, draw all the heptanes. They will have a methyl group.

• 2-Methylheptane
• 3-Methylheptane (2 enantiomers)
• 4-Methylheptane

Next, come the hexane isomers. They will have either two methyl groups or an ethyl group.

All the isomers with a methyl group on carbon 2.

• 2,2-Dimethylhexane
• 2,3-Dimethylhexane (2 enantiomers)
• 2,4-Dimethylhexane (2 enantiomers)
• 2,5-Dimethylhexane

Then, all the isomers with a methyl group on carbon 3.

• 3,3-Dimethylhexane
• 3,4-Dimethylhexane (2 enantiomers + 1 meso compound)

Then, the isomers with an ethyl group

• 3-Ethylhexane

Now we draw the pentanes. They will have either three methyl groups or an ethyl and a methyl.

Isomers with three methyl groups. The possible patterns are 2,2,3, 2,2,4, 2,3,3 and 2,3,4.

• 2,2,3-Trimethylpentane (2 enantiomers)
• 2,2,4-Trimethylpentane (isooctane)
• 2,3,3-Trimethylpentane
• 2,3,4-Trimethylpentane

Then the ethylmethylpentanes.

• 3-Ethyl-2-methylpentane
• 3-Ethyl-3-methylpentane

Finally, the butanes.

There is only one possibility.

• 2,2,3,3-Tetramethylbutane