Make sure you are familiar with the information from the lesson nomenclature of alkanes.
Alkenes (CnH2n), sometimes called olefins, are carbohydrates just like alkanes, but with at least one double bond between two carbon atoms. Alkenes are common in nature, for example as plant hormones (such as ethene) or as smell or taste molecules (such as limonene). Alkynes (CnH2n-2) have at least one triple bond between two carbon atoms. Alkynes also appear in nature (such as tararic acid) and in some medicinal drugs. Analogous to alkanes there are also cycloalkenes and cycloalkynes.
Nomenclature of alkenes
The general nomenclature of alkenes is analogous to that of alkanes. Find the longest carbon chain to determine the root and then number the substituents. Because the chain now contains a double bond, the name of the molecule will end with -ene instead of -ane, like with ethene and propene. The position of the double bond also needs to be given when talking about longer alkenes, where the lowest possible number is the correct one. Compare for example but-1-ene (NOT but-3-ene) and but-2-ene.
Be careful, there are older nomenclature rules that put the position of the double bond before the root name: 1-butene is an older name for but-1-ene.
When there are multiple double bonds the prefixes di-, tri-, and so on, are used as is to be expected. See for example buta-1,3-diene.
As a substituent
If the double bond is not part of the longest carbon chain it will change the name of the substituent. If the substituent is connected to the main chain through the double bond then the substituent becomes an alkylidene, otherwise it becomes an alkenyl. Compare ethylidineheptane to ethenylheptane as an example.
Nomenclature of alkynes
The only difference compared to alkenes is that the name of the alkyne ends with -yne instead of -ene or -ane, like with ethyne and propyne. Here too the position of the triple bond must be given when there are multiple possibilities, like with but-1-yne and but-2-yne.
A molecule with two triple bonds is, analogous to the dienes, a -diyne (and so on with tri-, etc.). See for example butadiyne.
If the triple bond is not part of the longest carbon chain the substituent will become an alkynyl. See for example 4-ethynylheptane.
Nomenclature of alkenynes
A molecule can also contain double and triple bonds ate the same time. In this case the numbering preferentially starts at the double or triple bond with the lowest number. When the lowest double and triple bond are on the same position, the double bond is preferred (the alkene). The general name for these molecules is root-X-en-Y-yne (always alphabetically, so the -ene is always before -yne). Notice that the final -e is dropped because the next part (-yne) starts with a vowel. See for example the compound hexa-1,3-dien-5-yne.
Nomenclature of cycloalkenes and -alkynes
Analogous to the alkanes there are also cycloalkenes and cycloalkynes. The nomenclature is exactly the same except for the suffix. See for example cyclohexene and cyclohexyne.
Practice