Hydrocarbons are organic compounds that contain only carbon and hydrogen. Alkenes are a specific subset of hydrocarbons that contain one or more carbon-carbon double bonds. Each carbon atom in the double bond has a trigonal planar arrangement with bond angles of 120°. If we consider the simplest alkene, ethene, it is a molecule that is flat, since both the carbons in the double bonds and the hydrogen atoms bonded to them all lie in the same plane. In any alkene, the double bond is rigid, which means there is no rotation possible around the double bond. This rigid structure means that there might be the possibility of different structures being drawn for different isomers, or molecules with the same chemical formulas but different arrangements of the atoms in the bonding in the molecule. If groups other than hydrogen are added to both carbons in the double bond, the alkene can be drawn as two different structures. We can consider the simplest example of an alkene that can create these different isomers, 2-butene. In this example, we have a methyl group added to each carbon of the double bond. The groups attached to the carbon atoms in the double bond can both be on one side of the double bond or on opposite sides. This gives two different compounds called geometric isomers or cis–trans isomers. Let’s look more closely at the naming of the cis-trans isomers for 2-butene. These compounds are named 2-butene because there are four carbons in the straight chain and the double bond is between carbons 2 and 3. The ending -ene is added since this is an alkene. 2-Butene can be drawn as two different molecules, depending on where the methyl and hydrogen groups are placed. We add the prefix cis or trans to denote whether the atoms bonded to the double bond are on the same side or opposite sides of the C-C double bond. In the cis isomer, the CH3 groups are on the same side of the double bond, and the name of this structure is cis-2-butene. In the trans isomer, the CH3 groups are on opposite sides of the double bond. This structure is named trans-2-butene. All cis-trans isomers in acyclic compounds must contain…? a, b, c or d? The correct answer is (b). Cis-trans isomers are geometric isomers of alkenes which all contain a double bond. You can model cis-trans isomers using gum drops and toothpicks. This is the cis isomer. Cis means “on this side.” The green gumdrops are on the same side. If we remove one green and one yellow gumdrop and have them trade places, we create a new structure. The green gumdrops are on opposite sides. This is now the trans isomer. Trans means “across.” Here is an example of where the two green gumdrops are attached to the same black gumdrop and the two yellow gumdrops are attached to the other black gumdrop. In this case, cis-trans isomerism does not exist. Here the models are side-by-side showing the cis and trans isomers. Which of the following pairs of compounds are cis-trans isomers? Is it a, b, c or d? The correct answer is (c). The pair of structures have the same formula C₅H₁₀ and position of the double bond on the chain. They differ from one another solely based on the positions of the methyl and ethyl groups. The alkyl groups are one the same side (cis) in the first structure and across from each other (trans) in the second. Let’s try naming a compound as a cis or trans isomer. Give the IUPAC name for the following compound including cis or trans if needed. Step 1 is to name the longest carbon chain that contains the double bond. There are seven carbon atoms in the longest carbon chain containing the double bond. Replace the -ane in the corresponding alkane name with -ene to give heptene. Step 2 is to number the carbon chain starting from the end nearer the double bond. Place the number of the first carbon in the double bond in front of the alkene name to give 3-heptene. Step 3 is to determine if there are cis-trans isomers for the compound. Sometimes it is easier to look at a skeletal structure if we add the hydrogen atoms to the carbons in the double bond. We can then ask if the alkyl groups are on the same or opposite sides. The alkyl groups are on opposite sides thus it is the trans isomer. Step 4 is to add the prefix cis- or trans- to the name. Name the following compound, a structure with 5 carbons, the double bond is between the 2nd and 3rd carbons, an ethyl group is above the double bond on the 3rd carbon and a methyl group is above the double bond on the 4th carbon. Is it a, b, c or d? The correct answer is (a). This compound has five carbons, the double bond is between carbons 2 and 3, and the alkyl groups are on the same side. Thus, it is the cis isomer and the name is cis-2-pentene.