Naming Alkenes: Videos & Practice Problems

Naming Alkenes Example

To determine the systematic name of an alkene, follow a structured approach that involves identifying the longest carbon chain, naming substituents, and considering the position of the double bond. The longest carbon chain, which serves as the parent chain, must include the double bond and have the maximum number of carbon atoms. If the chain is cyclic, the prefix "cyclo" is added to the name. In cases where there are ties in chain length, the chain with more substituents is preferred.

For example, when analyzing a specific alkene, the longest chain may be identified, and any substituents, such as a methyl group, should be noted. The numbering of the carbon chain begins from the end closest to the double bond to ensure the double bond is assigned the lowest possible number. If the double bond is part of a ring structure, a location number is not necessary.

In this case, the substituent is a methyl group located on carbon number 4. The double bond starts at carbon number 2, indicating that the parent chain is a six-carbon alkene, which is referred to as hexene. The "-ane" ending of the alkane is replaced with "-ene" to reflect the presence of the double bond. Thus, the base name becomes "2-hexene."

Furthermore, if there are two groups attached to the carbon atoms involved in the double bond, it is essential to determine their spatial arrangement. If the groups are on opposite sides of the double bond, the designation "trans" is used. In this example, since the two groups are positioned on different sides, the complete systematic name of the alkene is "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

Answer: (A)

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

Answer: (C)

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

• A.
• B.
• C.
• D.

Answer: (B)

Determine the structure of the following alkene: 3-isopropylcyclohexene.

• A.
• B.
• C.
• D.

Answer: (D)

Here's what students ask on this topic:

What are the basic rules for naming alkenes in organic chemistry?

Alkenes are named by modifying the name of the corresponding alkane. The key difference is changing the suffix from -ane to -ene to indicate the presence of a carbon-carbon double bond. When naming alkenes, you must identify the longest carbon chain that contains the double bond and number it so that the double bond gets the lowest possible number. Substituents attached to the chain are named and their positions indicated by numbers. Additionally, if there are two groups attached to the double-bonded carbons, the spatial arrangement is described using cis or trans prefixes, where cis means the groups are on the same side and trans means they are on opposite sides. This system helps clearly communicate the structure of the alkene.

How do you determine if an alkene is cis or trans?

To determine if an alkene is cis or trans, first identify the two carbon atoms involved in the double bond. Then, look at the groups attached to these carbons. If each carbon has two different groups, draw an imaginary border through the double bond. If the similar groups are on the same side of this border, the alkene is cis. If the similar groups are on opposite sides, it is trans. This distinction arises because rotation around the double bond is restricted due to the pi bond, so the spatial arrangement is fixed. Note that cis and trans notation only applies when there are two substituents on the double-bonded carbons.

Why is rotation not possible around the double bond in alkenes?

Rotation around the double bond in alkenes is not possible because the double bond consists of one sigma (σ) bond and one pi (π) bond. The sigma bond allows free rotation, but the pi bond is formed by the sideways overlap of p orbitals above and below the plane of the atoms. This pi bond creates an electron cloud that locks the two carbons in place, preventing rotation without breaking the bond. Breaking the pi bond requires significant energy, so the spatial arrangement of groups around the double bond remains fixed, leading to geometric isomers such as cis and trans.

Is cis-trans notation used for cyclic alkenes? Why or why not?

Cis-trans notation is generally not used for cyclic alkenes because the ring structure restricts the spatial arrangement of substituents. In cyclic alkenes, the substituents attached to the double-bonded carbons are constrained by the ring, so the typical cis and trans designations based on opposite or same side positions do not apply in the same way. Instead, the ring's geometry inherently fixes the relative positions of substituents, and other naming conventions or descriptors are used to describe their orientation.

How do you assign the location of the double bond when naming an alkene?

When naming an alkene, the carbon chain is numbered to give the double bond the lowest possible number. Start numbering from the end of the chain closest to the double bond. The position of the double bond is indicated by the number of the first carbon involved in the double bond. For example, if the double bond starts at the second carbon, the name will include "2-ene". This numbering ensures clarity about where the double bond is located within the molecule, which is essential for correct identification and naming.