Alkane

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EXAMPLE 1

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.)

EXAMPLE 2

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 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 3

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.

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