Naming Alkenes - Online Tutor, Practice Problems & Exam Prep

Alkenes are hydrocarbons characterized by a carbon-carbon double bond, with names ending in -ene. Geometric isomers, such as cis and trans, arise from different spatial orientations around the double bond, which restricts rotation. Cis indicates substituents on the same side, while trans indicates them on opposite sides. This distinction is crucial for understanding molecular behavior. In cyclic alkenes, cis-trans notation is not applicable. Proper naming and understanding of these isomers are essential for grasping the properties and reactions of alkenes in organic chemistry.

Rules for Naming Alkenes Concept 1

Now before we get into naming alkenes, recall that alkenes possess a carbon-carbon double bond. And the set of rules for naming alkenes is unique. Here we're going to say that the modification is, we're going to modify the ending from -ane to -ene. So we're thinking of an alkane, but because there's a presence of a double bond, it becomes an alkene. This naming convention will include terms like cis and trans, which we'll go into. We still have to give the location of our substituents. We have to give the location of our parent carbon chain in terms of the double bond, and then we have our modifier where we change the ending from -ane to -ene. Now here recall that geometric isomers have a different spatial orientation around a double bond. And remember that rotation is not possible around a double bond, which is a pi bond. Now, we need to indicate which side of the double bond both groups are on, where each group lies. And we do this by saying cis or trans. This only applies when there are two groups around a pi bond. So if we take a look here, we have our two double-bonded carbons here, and we have our two groups, which happen to be these methyl groups. We create a border; this border dissects right through the double-bonded carbons. And you're either here on the bottom side, or you're here on the top side, so you're on different sides of the pi bond. Both methyl groups happen to be on the same side. When they're on the same side, we say that this is cis.

Now, here if we take a look at this alkene, this is a geometric isomer with the same molecular formula, same connections, just a slight difference in spatial orientation. Again, here are our two double-bonded carbons. We create a border by cutting straight through them. Now we have one methyl group up here, and one methyl group down here. They're on different sides of this border that we've created. So if they're on different sides, that means that they are trans. So here we have a cisalkene on the left, and a transalkene on the right.

Now, if we're dealing with a cyclic alkene, we're going to say there are no cis or trans within the ring. So we don't have to worry about this type of notation. So again, for us to be cis or trans, we need to have two groups that are connected to our double-bonded carbons. If they're on the same side it's cis, if they're on opposite sides it's trans.

Naming Alkenes Example 1

Here it says, determine the systematic name of the following alkene. To name this alkene, we're going to follow the given steps. It instructs us to find the longest carbon chain, which represents our parent chain, and assign a name according to the prefix and the modifier. The parent chain should include a double bond, and have a greater number of carbons. If it's cyclic, we add the prefix 'cyclo' to the name of the alkene chain. If there is a tie between the longest chains, choose the chain with more substituents. So, if we take a look here, our longest chain appears to be this one. This carbon up here, which is a methyl, is our substituent. So, step 2 says assign names to all substituents, so this would be methyl. Start numbering the chain from the end closest to the double bond. The double bond is here on the left, so we'll start numbering on this side: 1, 2, 3, 4, 5, 6. Assign a location to the first double-bonded carbon. If the double bond is in a ring, no location is necessary. Repeat from previous naming topics. So, go back and take a look at my videos on naming alkanes with substituents, because it talks about how we give the numerical location for each of the substituents, how we name them alphabetically. And here we have step 7, where we add 'cis' or 'trans' designation to the front of the name, if applicable. First, let's just name this before we worry about cis and trans. We're going to say here our substituent is methyl, and it's on carbon number 4, so it'd be 4-methyl. We then have to talk about the first double-bonded carbon. The double bond starts on carbon number 2. So we'd say the location is 2; it's a 6-carbon chain which is hexane as an alkane, but here it's an alkene so we change the '-ane' ending to '-ene'. So, this would be 2-hexene. Now, we said that if we have two groups attached to our pi bond, then we use 'cis' and 'trans'. Here is a group attached to our double-bonded carbon, which contains the pi bond, and here is another group attached. So, we do have two groups attached to our pi bonds, in terms of those two alkene carbons. We create a boundary here between the double-bonded carbons, and we can see that the two groups that I've basically put in within those blue boxes, they're on different sides of this boundary. Because they're on different sides of this boundary, that means that they are trans to one another. So we put 'trans' at the beginning of the name. So the name of this particular alkene would be, 'trans-4-methyl-2-hexene'.

Provide systematic name for given alkene.

• A. trans-2,7-dimethyl-3-octene
• B. 2,7-dimethyl-3-octane
• C. trans-2,7-dimethyl-5-octene
• D. 2,7-dimethyl-5-octane

Provide systematic name for given alkene.

• A. 3-ethyl-5-hexene
• B. 4-isopropyl-1-hexene
• C. 4-ethyl-1-hexene
• D. 4-isopropyl-2-hexene

Draw a structure for cis-5-methyl-2-hexene.

• A.
• B.
• C.
• D.

Determine structure of the following alkene: 3-isopropylcyclohexene.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What are the rules for naming alkenes?

When naming alkenes, the following rules apply: 1) Identify the longest carbon chain containing the double bond and use the corresponding alkane name, changing the suffix from -ane to -ene. 2) Number the carbon chain starting from the end nearest the double bond. 3) Indicate the position of the double bond by the number of the first carbon involved in the double bond. 4) Name and number substituents as prefixes, listing them in alphabetical order. 5) For geometric isomers, use the prefixes cis- or trans- to indicate the relative positions of substituents around the double bond.

What is the difference between cis and trans isomers in alkenes?

Cis and trans isomers are types of geometric isomers found in alkenes. In cis isomers, the substituents attached to the double-bonded carbons are on the same side of the double bond. In trans isomers, the substituents are on opposite sides of the double bond. This difference in spatial arrangement affects the physical and chemical properties of the molecules, such as boiling points and reactivity.

Why can't rotation occur around a double bond in alkenes?

Rotation cannot occur around a double bond in alkenes due to the presence of a pi bond. A double bond consists of one sigma bond and one pi bond. The pi bond is formed by the sideways overlap of p orbitals, creating an electron cloud above and below the plane of the atoms. This electron cloud restricts rotation, locking the atoms in place and giving rise to geometric isomers like cis and trans.

How do you name cyclic alkenes?

To name cyclic alkenes, follow these steps: 1) Identify the ring structure and use the corresponding cycloalkane name, changing the suffix from -ane to -ene. 2) Number the ring starting from the double bond and proceed in the direction that gives the substituents the lowest possible numbers. 3) Indicate the position of the double bond by the number of the first carbon involved in the double bond. 4) Name and number substituents as prefixes, listing them in alphabetical order. Note that cis-trans notation is not applicable for cyclic alkenes.

What are geometric isomers and why are they important in alkenes?

Geometric isomers are molecules with the same molecular formula and connectivity but different spatial arrangements around a double bond. In alkenes, these isomers are classified as cis (substituents on the same side) or trans (substituents on opposite sides). Geometric isomers are important because they have different physical and chemical properties, such as boiling points, solubility, and reactivity, which can significantly impact their behavior in chemical reactions and biological systems.