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ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Hydrocarbons > Alkenes > Nomenclature of Alkenes

Nomenclature of Alkenes

Alkenes contain carbon-carbon double bonds and are unsaturated hydrocarbons with the molecular formula is CnH2n. This is also the same molecular formula as cycloalkanes. Alkenes are named by dropping the -ane ending of the parent and adding -ene.

Introduction

 The parent structure is the longest chain containing both carbon atoms of the double bond. The two carbon atoms of a double bond and the four atoms attached to them lie in a plane, with bond angles of approximately 120° A double bond consists of one sigma bond formed by overlap of sp2 hybrid orbitals and one pi bond formed by overlap of parallel 2 p orbitals

alkene7.png

The Basic Rules

For straight chain alkenes, it is the same basic rules as nomenclature of alkanes except change the suffix to "-ene."

i. Find the Longest Carbon Chain that Contains the Carbon Carbon double bond. If you have two ties for longest Carbon chain, and both chains contain a Carbon Carbon double bond, then identify the most substituted chain.

ii. Give the lowest possible number to the Carbon Carbon double bond.

  1. Do not need to number cycloalkenes because it is understood that the double bond is in the one position.
  2. Alkenes that have the same molecular formula but the location of the doble bonds are different means they are constitutional isomers.
  3. Functional Groups with higher priority:

iii. Add substituents and their position to the alkene as prefixes.  Of course remember to give the lowest numbers possible. And remember to name them in alphabetical order when writting them.

iv. Next is identifying stereoisomers. when there are only two non hydrogen attachments to the alkene then use cis and trans to name the molecule. 

 

 

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

v. On the other hand 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.

(You could think of Z as Zame Zide to help memorize it.)

 

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 th 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.

 

Here is a chart containing the systemic name for the first twenty straight chain alkenes.

Name Molecular formula
Ethene C2H4
Propene C3H6
Butene C4H8
 Pentene  C5H10
 Hexene  C6H12
 Heptene  C7H14
 Octene  C8H16
 Nonene  C9H18
 Decene  C10H20
 Undecene  C11H22
 Dodecene  C12H24
 Tridecene  C13H26
 Tetradecene  C14H28
 Pentadecene  C15H30
 Hexadecene  C16H32
Heptadecene C17H34
 Octadecene  C18H36
 Nonadecene  C19H38
 Eicosene  C20H40

Did you notice how there is no methene? Because it is impossible for a Carbon to have a double bond with nothing.

Geometric Isomers

Double bonds can exist as geometric isomers and these isomers are designated by using either the cis / trans designation or the modern E / Z designation.

cis Isomers

.The two largest groups are on the same side of the double bond.

trans Isomers

...The two largest groups are on opposite sides of the double bond.

E/Z nomenclature

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 1

Try to name the following compounds using both conventions...

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.

cycloalkane.jpg

Cyclopentene is an example of an endocyclic double bond.

Methylenecylopentane is an example of an exocyclic double bond.

Name the following compounds...

 

1-methylcyclobutene. The methyl group places the double bond. It is correct to also name this compound as 1-methylcyclobut-1-ene.

 

 

 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.

 

Try to draw structures for the following compounds...

 

  •  2-vinyl-1,3-cyclohexadiene

 

References

  1. 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...

 J

• Try to draw a structure for the following compound...

4-methyl-2-pentene J

Name the following structures:

alkene problem 2.jpg

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. alkene9.jpg

Contributors

  • S. Devarajan (UCD)

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Last Modified
12:11, 1 Jul 2014

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