Naming Alkenes: Videos & Practice Problems

Alkenes are hydrocarbons characterized by the presence of a carbon-carbon double bond (C=C). When naming alkenes, the suffix of the parent alkane is modified from -ane to -ene, reflecting the double bond's presence. 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 alkenes, arising from the restricted rotation around the double bond due to the formation 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 describe the relative positions of substituents around the double bond. For a compound to exhibit cis or trans isomerism, it must have two different groups attached to the double-bonded carbons.

In a cis configuration, both substituents are located on the same side of the double bond, while in a trans configuration, they are positioned on opposite sides. For example, if two methyl groups are attached to the double-bonded carbons, and both are on the same side, the compound is classified as cis. Conversely, if one methyl group is above and the other is below the double bond, the compound is classified as trans.

It is important to note that when dealing with cyclic alkenes, the cis/trans nomenclature does not apply, as the ring structure does not allow for the same spatial orientation considerations. In summary, understanding the naming conventions and geometric isomerism of alkenes is crucial for accurately describing their structure and properties.

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 the naming process, if there is a tie between chains of equal length, the chain with more substituents is preferred. For example, if we identify the longest chain and find a methyl substituent, we proceed to assign names to all substituents. The numbering of the carbon chain begins from the end closest to the double bond. This ensures that the double bond is assigned the lowest possible number.

When naming the alkene, the location of the first double-bonded carbon is crucial. If the double bond is part of a ring, a location number is not necessary. The substituents are named alphabetically, and their positions are indicated by their respective carbon numbers. In this case, if the substituent is a methyl group located on carbon number 4, it is denoted as "4-methyl." 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 alkanes is replaced with "-ene" for alkenes, resulting in "2-hexene."

Furthermore, if there are two groups attached to the double bond, the designation of "cis" or "trans" is applied based on their spatial arrangement. If the groups are on opposite sides of the double bond, they are classified as "trans." Therefore, the complete systematic name for this alkene is "trans-4-methyl-2-hexene."