- Last updated
- Save as PDF
- Page ID
- 872
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)
The molecular formula of a hydrocarbon provides information about the possible structural types it may represent. For example, consider compounds having the formula \(\ce{C5H8}\). The formula of the five-carbon alkane pentane is \(\ce{C5H12}\) so the difference in hydrogen content is 4. This difference suggests such compounds may have a triple bond, two double bonds, a ring plus a double bond, or two rings. Some examples are shown here, and there are at least fourteen others!
Hence, as with alkanes, a consistent nomenclature system needs to be adopted that can separate the nature of these unsaturated chemicals. The simplest are the alkenes, which are hydrocarbons which have carbon-carbon double bond functional groups and are unsaturated hydrocarbons with the molecular formula is \(\ce{CnH2n}\), which is also the same molecular formula as cycloalkanes.
IUPAC Rules for Alkene Nomenclature
- The ene suffix (ending) indicates an alkene or cycloalkene.
- The longest chain chosen for the root name must include both carbon atoms of the double bond.
- The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
- The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator.
- If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
- Substituent groups containing double bonds are:
- H2C=CH– Vinyl group
- H2C=CH–CH2– Allyl group
Rule 1
Alkenes are named using the same general naming rules for alkanes, except that the suffix is now -ene. Here is a chart containing the systemic name for the first twenty straight chain alkenes.
| Name | Molecular formula | Name | Molecular formula | |
|---|---|---|---|---|
| Ethene | C2H4 | Undecene | C11H22 | |
| Propene | C3H6 | Dodecene | C12H24 | |
| Butene | C4H8 | Tridecene | C13H26 | |
| Pentene | C5H10 | Tetradecene | C14H28 | |
| Hexene | C6H12 | Pentadecene | C15H30 | |
| Heptene | C7H14 | Hexadecene | C16H32 | |
| Octene | C8H16 | Heptadecene | C17H34 | |
| Nonene | C9H18 | Octadecene | C18H36 | |
| Decene | C10H20 | Nonadecene | C19H38 |
Did you notice how there is no methene? Because it is impossible for a carbon to have a double bond with nothing.
Rule 2
The parent structure is the longest chain containing both carbon atoms of the double bond. If the alkene contains only one double bond and that double bond is terminal (the double bond is at one end of the molecule or another) then it is not necessary to place any number in front of the name.
- butane: C4H10 (\(\ce{CH3CH2CH2CH3}\))
- butene: C4H8 (\(\ce{CH2=CHCH2CH3}\))
If the double bond is not terminal (if it is on a carbon somewhere in the center of the chain) then the carbons should be numbered in such a way as to give the first of the two double-bonded carbons the lowest possible number, and that number should precede the "ene" suffix with a dash, as shown below.
- correct: pent-2-ene (\(\ce{CH3CH=CHCH2CH3}\))
- incorrect: pent-3-ene (\(\ce{CH3CH2CH=CHCH3}\))
The second one is incorrect because flipping the formula horizontally results in a lower number for the alkene.
Exercise \(\PageIndex{1}\)
Name the following compounds:
a:
b.
- Answer a
-
1-pentene or pent-1-ene
- Answer b
-
2-ethyl-1-hexene or 2-ethylhex-1-ene
Exercise \(\PageIndex{2}\)
a: Name the following compound (hint: give the double bond the lowest possible numbers regardless of substituent placement).
b. Draw a structure for 4-methyl-2-pentene.
- Answer a
-
4-methylpent-1-ene
- Answer b
-
Rule 4
If there is more than one double bond in an alkene, all of the bonds should be numbered in the name of the molecule - even terminal double bonds. The numbers should go from lowest to highest, and be separated from one another by a comma. The IUPAC numerical prefixes are used to indicate the number of double bonds.
- octa-2,4-diene: \(\ce{CH3CH=CHCH=CHCH2CH2CH3}\)
- deca-1,5-diene: \(\ce{CH2=CHCH2CH2CH=CHCH2CH2CH2CH3}\)
Note that the numbering of "2-4" above yields a molecule with two double bonds separated by just one single bond. Double bonds in such a condition are called "conjugated", and they represent an enhanced stability of conformation, so they are energetically favored as reactants in many situations and combinations.
Rule 4: Geometric Isomers
Double bonds can exist as geometric isomers and these isomers are designated by using either the cis / trans designation or the more flexible E / Z designation.
cis Isomers have the two largest groups are on the same side of the double bond (left structure above) and trans Isomers have the two largest groups are on opposite sides of the double bond (right structure above).E/Z nomenclature
If there are 3 or 4 non-hydrogen different atoms attached to the alkene then use the E, Z system.
- E (entgegen) means the higher priority groups are opposite one another relative to the double bond.
- Z (zusammen) means the higher priority groups are on the same side relative to the double bond.
E = entgegan ("trans") Z = zusamen ("cis")
Priority of groups is based on the atomic mass of attached atoms (not the size of the group). An atom attached by a multiple bond is counted once for each bond.
fluorine atom > isopropyl group > n-hexyl group
deuterium atom > hydrogen atom
-CH2-CH=CH2 > -CH2CH2CH3
Example \(\PageIndex{5}\)
Try to name the following compounds using both cis-trans and E/Z conventions:
a:
b:
- Answer a
-
4-methylpent-1-ene
- Answer b
-
Example \(\PageIndex{3}\)
What is the name of this molecule?
Solution
In this diagram this is a cis conformation. It has both the substituents going upward. This molecule would be called (cis) 5-chloro-3-heptene.)
Trans would look like this
Example \(\PageIndex{4}\)
What is the name of this molecule?
Solution
In this example it is E-4-chloro-3-heptene. It is E because the Chlorine and the CH2CH3 are the two higher priorities and they are on opposite sides.
vi. A hydroxyl group gets precedence over the double bond. Therefore alkenes containing alchol groups are called alkenols. And the prefix becomes --enol. And this means that now the alcohol gets lowest priority over the alkene.
vii. Lastly remember that alkene substituents are called alkenyl. Suffix --enyl.
Common names
Remove the -ane suffix and add -ylene. There are a couple of unique ones like ethenyl's common name is vinyl and 2-propenyl's common name is allyl. That you should know are...
- vinyl substituent H2C=CH-
- allyl substituent H2C=CH-CH2-
- allene molecule H2C=C=CH2
- isoprene
Endocyclic Alkenes
Endocyclic double bonds have both carbons in the ring and exocyclic double bonds have only one carbon as part of the ring.
Cyclopentene is an example of an endocyclic double bond.
Methylenecylopentane is an example of an exocyclic double bond.
Exercise \(\PageIndex{1}\)
Name the following compounds:
a:
b:
- Answer a
-
1-methylcyclobutene. The methyl group places the double bond. It is correct to also name this compound as 1-methylcyclobut-1-ene.
- Answer b
-
1-ethenylcyclohexene, the methyl group places the double bond. It is correct to also name this compound as 1-ethenylcyclohex-1-ene. A common name would be 1-vinylcyclohexene.
Exercise \(\PageIndex{1}\)
Name the following compounds:
a:
b:
- Answer a
-
1-methylcyclobutene. The methyl group places the double bond. It is correct to also name this compound as 1-methylcyclobut-1-ene.
- Answer b
-
1-ethenylcyclohexene, the methyl group places the double bond. It is correct to also name this compound as 1-ethenylcyclohex-1-ene. A common name would be 1-vinylcyclohexene.
Exercise \(\PageIndex{1}\)
Draw structures for the following 2-vinyl-1,3-cyclohexadiene
- Answer
-
References
- Vollhardt, Peter, and Neil E. Schore. Organic Chemistry: Structure and Function. 5th Edition. New York: W. H. Freeman & Company, 2007.
Problems
Try to name the following compounds...
1-pentene or pent-1-ene
2-ethyl-1-hexene or 2-ethylhex-1-ene
Try to draw structures for the following compounds...
- 2-pentene
CH3–CH=CH–CH2–CH3
- 3-heptene
CH3–CH2–CH=CH–CH2–CH2–CH3
b. Give the double bond the lowest possible numbers regardless of substituent placement.
• Try to name the following compound...
• Try to draw a structure for the following compound...
4-methyl-2-pentene J
Name the following structures:
v. Draw (Z)-5-Chloro-3-ethly-4-hexen-2-ol.
Answers
I. trans-8-ethyl-3-undecene
II. E-5-bromo-4-chloro-7,7-dimethyl-4-undecene
III. Z-1,2-difluoro-cyclohexene
IV. 4-ethenylcyclohexanol.
V. 
Contributors
- S. Devarajan (UCD)
William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook ofOrganicChemistry
- Jonathan Mooney (McGill University)
FAQs
Nomenclature of Alkenes? ›
The ene suffix (ending) indicates an alkene or cycloalkene. The longest chain chosen for the root name must include both carbon atoms of the double bond. The root chain must be numbered from the end nearest a double bond carbon atom.
How do you name alkenes with 3 double bonds? ›If more than one double bond is present, indicate their position by using the number of the first carbon of each double bond and use the suffix -diene (for 2 double bonds), -triene (for 3 double bonds), -tetraene (for 4 double bonds), etc.
What is the nomenclature of alkanes? ›Alkanes with five or more carbon atoms are named by adding the suffix -ane to the appropriate numerical multiplier, except the terminal -a is removed from the basic numerical term. Hence, C5H12 is called pentane, C6H14 is called hexane, C7H16 is called heptane and so forth.
What are the rules for naming alkenes and alkynes? ›Find the longest continuous chain of carbons containing the double or triple bond. The names of alkenes end with –ene, and the names of alkynes end with -yne. When there is more than one multiple bond, use numerical prefixes (diene, diyne, triene, triyne, etc.)
What is the rule for naming alkenes? ›- The ene suffix (ending) indicates an alkene or cycloalkene.
- The longest chain chosen for the root name must include both carbon atoms of the double bond.
- The root chain must be numbered from the end nearest a double bond carbon atom.
If both double bond and triple bond are present in the molecule and if there is a choice in numbering, then triple bond is given preference.
What are the 5 rules for naming alkanes? ›- Find and name the longest continuous carbon chain.
- Identify and name groups attached to this chain.
- Number the chain consecutively, starting at the end nearest a substituent group.
- Designate the location of each substituent group by an appropriate number and name.
The simplest are the alkenes, which are hydrocarbons which have carbon-carbon double bond functional groups and are unsaturated hydrocarbons with the molecular formula is CnH2n, which is also the same molecular formula as cycloalkanes.
What is the priority order of alkenes and alkynes? ›In naming hydrocarbons, alkenes gets position first priority as compared to alkynes so the correct name is pent-1-en-4-yne. But in numbering carbons, alkynes gets higher priority than alkenes.
What is the priority order of alkane alkene alkyne? ›According to IUPAC convention, Carboxylic Acids and their derivatives have the highest priority then carbonyls then alcohols, amines, alkenes, alkynes, and alkanes, Hence option (B) is correct.
What is nomenclature of alkane alkene and alkyne examples? ›
For alkanes the suffix '-ane' is used, for alkenes, the suffix '-ene' is used. Similarly the suffix 'yne' is used for alkynes. For example, \[C_{2}H_{6}\] is called ethane, \[C_{2}H_{4}\] is called ethene and \[C_{2}H_{2}\] is called ethyne.
What is the name of 3 double bonds? ›Compounds that contain two double bonds are classified as dienes, those with three as trienes, and so forth.
How do you name alkenes with double bonds? ›- Find the longest carbon chain that contains both carbons of the double bond.
- Start numbering from the end of the parent chain which gives the lowest possible number to the double bond. ...
- Place the location number of the double bond directly before the parent name.
When there are two triple bonds in the molecule, find the longest carbon chain including both the triple bonds. Number the longest chain starting at the end closest to the triple bond that appears first. The suffix that would be used to name this molecule would be –diyne.
Can an alkene have multiple double bonds? ›Alkenes with more than one double bond. In alkenes with more than one double bond, the suffix -ane in the corresponding alkane parent name is replaced with the suffix -adiene if there are two double bonds, and -atriene if there are three double bonds.