Alkenes are named according to the International Union of Pure and Applied Chemistry (IUPAC) rules, which involve identifying the longest carbon chain, determining the position of the double bond, and noting any substituents present.
To elaborate, the first step in naming alkenes is to locate the longest continuous chain of carbon atoms that includes the double bond. This chain is referred to as the parent chain. The name of the parent chain is derived from the number of carbon atoms it contains, following the same nomenclature as for alkanes: “meth-” for one carbon, “eth-” for two, “prop-” for three, and so forth. However, the suffix “-ene” is used to signify the presence of a double bond.
The next step is to determine the position of the double bond within the parent chain. The carbon atoms are numbered starting from the end that is closest to the double bond. The position of the double bond is denoted by the lowest numbered carbon atom that is part of the bond. For instance, if the double bond is located between the second and third carbon atoms, the compound is referred to as a “2-ene”.
When substituents—groups of atoms attached to the parent chain—are present, they must also be included in the name. Substituents are prefixed to the name of the parent chain, accompanied by a number that indicates their position within the chain. If there are multiple substituents, they should be listed in alphabetical order, regardless of their numerical position.
For example, consider a molecule with a three-carbon chain that has a double bond between the first and second carbons, along with a methyl group attached to the second carbon. This compound would be named “2-methylprop-1-ene”. It is important to note that the double bond takes precedence over substituents when numbering the carbon chain.
In instances where there are multiple double bonds, the positions of all double bonds should be indicated, and the suffix of the name must be adjusted to reflect the total number of double bonds. For example, the suffix “-diene” is used for compounds with two double bonds, while “-triene” is used for those with three double bonds.
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