7.2
Nomenclature of Alkenes
The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix…
The IUPAC names of alkenes are derived by replacing the suffix -ane of the parent alkane with -ene. The chain is numbered from the end nearest the double bond while assigning the lowest possible consecutive numbers to the double-bond carbon atoms.
In branched alkenes, the longest carbon chain containing the double bond is identified as the principal or parent chain. For instance, the parent alkene is pentene and not butene. The lowest number is allocated to the double-bond carbon atoms over the substituents. However, the name and position of the substituent group are cited before the position of the double bond, and the name is 2-ethyl-4-methyl-1-pentene.
Non-identical substituents are listed alphabetically. Thus, an ethyl substituent is cited before methyl, as in 4-ethyl-2-methyl-1-octene.
If more than one double bond is present in the parent chain, the position of each one is specified while the infix -en- is replaced by -adien- for two, -atrien- for three double bonds, and so on.
The principal chain is the one with the maximum number of double bonds, even if it is not the longest chain. Hence, the correct name of this molecule is 4-butyl-5-methyl-1,4-hexadiene.
However, if more than one chain has an equal number of double bonds, the longest chain is the parent, as illustrated by 5-methylene-1,9-decadiene.
In cycloalkenes, the lowest numbers are assigned to the double-bond carbon atoms, while the direction of numbering is such that the lowest possible number is allocated to the first point of difference.
When substituents containing double bonds, like vinyl, allyl, and isopropenyl groups, are present, the ring is a principal chain.
IUPAC has recognized that smaller alkenes are usually referred to by their common names. For example, ethene is called ethylene, propene as propylene, and 2-methylpropene as isobutylene.
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Q1: How do you name an alkene using IUPAC nomenclature?
IUPAC alkene names replace the -ane suffix of the parent alkane with -ene. The carbon chain is numbered from the end nearest the double bond, assigning the lowest possible consecutive numbers to the double-bond carbon atoms. The position of the double bond is indicated by the number of its first carbon atom. Understanding alkene nomenclature is foundational before studying introduction to electrophilic addition reactions of alkenes.
Q2: What is the priority when numbering a branched alkene chain?
The double bond receives priority over substituents when numbering branched alkenes. The longest carbon chain containing the double bond is identified as the parent chain and numbered to give the double-bond carbon atoms the lowest possible numbers. Substituent names and positions are cited before the double bond position. For instance, 2-ethyl-4-methyl-1-pentene places the double bond at position one.
Q3: How are multiple double bonds named in polyenes?
When more than one double bond is present, the position of each is specified and the infix -en- is replaced by -adien- for two double bonds, -atrien- for three, and so on. The principal chain is the one with the maximum number of double bonds, even if it is not the longest chain. For example, 4-butyl-5-methyl-1,4-hexadiene contains two double bonds at positions one and four.
Q4: How are substituents ordered in an alkene name?
Non-identical substituents are listed alphabetically in the alkene name. For example, an ethyl substituent is cited before methyl, as in 4-ethyl-2-methyl-1-octene. The substituent names and their positions appear before the double bond position in the complete IUPAC name, maintaining alphabetical order regardless of their location on the chain.
Q5: What rules apply to numbering cycloalkenes?
In cycloalkenes, the ring is the principal chain and the lowest numbers are assigned to the double-bond carbon atoms. The direction of numbering is chosen such that the lowest possible number is allocated to the first point of difference. When substituents containing double bonds, like vinyl, allyl, and isopropenyl groups, are present, the ring remains the principal chain for naming purposes.
Q6: What are common names for small alkenes?
IUPAC has recognized that smaller alkenes are commonly referred to by their common names rather than systematic names. Ethene is called ethylene, propene is called propylene, and 2-methylpropene is called isobutylene. These common names are widely used in practice and are acknowledged by IUPAC alongside their systematic nomenclature.
Q7: How do you identify the parent chain in a branched alkene?
The parent chain is the longest carbon chain containing the double bond. In branched alkenes, even if a longer chain exists without the double bond, the chain with the double bond is selected as the parent. For example, if a five-carbon chain contains the double bond and a four-carbon chain does not, the five-carbon chain is the parent alkene, making it pentene rather than butene.