Naming Alkenes: Videos & Practice Problems

Alkenes are hydrocarbons characterized by a carbon-carbon double bond, which alters their naming from -ane to -ene. Geometric isomers, such as cis and trans, arise from different spatial orientations around the double bond, with cis indicating groups on the same side and trans on opposite sides. This distinction is crucial when two substituents are present on the double-bonded carbons. In cyclic alkenes, cis-trans notation is not applicable. Understanding these concepts is essential for grasping the behavior and properties of alkenes in organic chemistry.

Rules for Naming Alkenes

Alkenes are hydrocarbons characterized by the presence of a carbon-carbon double bond (C=C). When naming alkenes, the suffix changes from -ane (used for alkanes) to -ene to reflect this modification. This naming convention also requires specifying the location of the double bond and any substituents attached to the carbon chain.

Geometric isomerism is a key concept in understanding alkenes, as it arises from the restricted rotation around the double bond due to the presence of a pi bond. This restriction leads to different spatial arrangements of substituents, which can be classified as either cis or trans. The terms cis and trans are used when there are two different groups attached to the double-bonded carbons. If the two groups are on the same side of the double bond, the configuration is termed cis. Conversely, if the groups are on opposite sides, it is referred to as trans.

For example, consider two double-bonded carbon atoms with two methyl groups attached. If both methyl groups are positioned on the same side of the double bond, the compound is classified as a cis isomer. If one methyl group is above and the other is below the double bond, the compound is classified as a trans isomer. It is important to note that in cyclic alkenes, the cis/trans nomenclature does not apply, as the ring structure does not allow for the same type of spatial orientation.

In summary, when naming alkenes, remember to modify the suffix to -ene, indicate the position of the double bond, and use the terms cis or trans to describe the arrangement of substituents around the double bond, provided there are two different groups present.

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 is the general rule for naming alkenes?

The general rule for naming alkenes involves modifying the ending of the corresponding alkane name from -ane to -ene to indicate the presence of a carbon-carbon double bond. Additionally, the position of the double bond must be specified by numbering the carbon atoms in the longest chain that contains the double bond, starting from the end nearest to the double bond. Substituents are also named and their positions indicated by numbers. For example, 2-butene indicates a four-carbon chain with a double bond between the second and third carbons.

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

To determine if an alkene is cis or trans, you need to look at the spatial orientation of the groups attached to the double-bonded carbons. Draw a line through the double bond to create a border. If the two substituent groups are on the same side of the border, the alkene is cis. If the substituent groups are on opposite sides of the border, the alkene is trans. This notation only applies when there are two different groups attached to each of the double-bonded carbons.

Why can't cyclic alkenes have cis or trans notation?

Cyclic alkenes cannot have cis or trans notation because the ring structure restricts the spatial orientation of the substituents. In a ring, the substituents are inherently on the same side of the double bond due to the cyclic nature of the molecule. Therefore, the concepts of cis and trans do not apply to cyclic alkenes.

What is the significance of geometric isomers in alkenes?

Geometric isomers in alkenes are significant because they have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their substituents around the double bond. This difference in spatial orientation can lead to different physical and chemical properties. For example, cis and trans isomers of the same alkene can have different boiling points, melting points, and reactivity, making the distinction important in chemical synthesis and applications.

How do you name an alkene with multiple double bonds?

When naming an alkene with multiple double bonds, you need to indicate the positions of all double bonds and modify the suffix accordingly. For example, if there are two double bonds, the suffix changes from -ene to -diene. Number the carbon chain to give the lowest possible numbers to the double bonds. For instance, 1,3-butadiene indicates a four-carbon chain with double bonds between the first and second carbons and between the third and fourth carbons.