How can I draw cis-2-octene according to IUPAC?

1 Answer
Feb 14, 2015

When trying to draw compounds according to IUPAC nomenclature, you have to break down the name into its individual components. The six factors that determine the name of a compound are

• Functional group
• Unsaturation;
• Parent chain;
• Substituents;
• Stereoisomerism;
• Numbering.

I won't go into the rules for each one of these factors because it would make for a very long answer. So, let's analyze the name of the compound.

Compound names that end in $\text{-e}$ have no functional groups attached, so we've cleared that up quickly. IUPAC nomenclature uses $\text{-en-}$ to designate double bonds and $\text{-yn-}$ to designate triple bonds. As you can see, the name ends in $\text{-ene}$,,which tells you it has one double bond and no functional groups.

The $\text{oct-}$ part of the name designates the parent chain; in this case, $\text{oct-}$ is short for octane, which means that the parent chain will have 8 carbon atoms.

Now for the first part of the name, cis-2. The 2 is used to designate the first atom that forms the aforementioned double bond. This implies that the double bond will be placed between carbon 2 and carbon 3 on the parent chain.

Cis just means that you have two identical groups on the same side of the double bond. In this case, cis will refer to the two hydrogen atoms attached to the carbons that form the double bond.

Now to put all this together. Start by drawing the parent chain, octane.

Remember that the double bond is placed between carbon 2 and carbon 3, so you'll have

Now for the tricky part. The compound drawn above is actually trans-2-octene, because the two hydrogens are attached to different sides of the double bond. This is how it would look with these hydrogens attached

Since the cis stereoisomer must have the two hydrogen atoms on the same side of the bond, the actual bond line notation fir cis-2-octene will be

or