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Alkenes Group
Naming of Alkenes
Hydrocarbons with double bonds
make up the alkene family. Open
chain alkenes with one double bond
have the general formula, CnH2n,
where n equals the number of
carbon atoms.
1.
Alkenes are named by dropping the -ane
ending of the parent and adding -ene. The
parent structure is the longest chain
containing both carbon atoms of the double
bond.
Example:
a. CH2=CH—CH2—CH3 (1-butene/ but-1-ene)
b. CH3—CH=CH—CH3 (2-butene/ but-2-ene)
c. CH3—CH2—CH=CH2 (??)
d. CH3—CH2—C=CH2
CH3
(2-methyl-1-butene/ 2-methyl-but-1-ene)
Try to name the following compounds...
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Try to draw structures for the following
compounds...
2-pentene
3-heptene
2. 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
answer\6.mht
Common names that you should know are...
vinyl substituent H2C=CHallyl substituent H2C=CH-CH2allene molecule H2C=C=CH2
isoprene
3. 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.
· Try to name the following compounds...
answer\7.txt
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answer\8.txt
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4. Double bonds can exist as geometric isomers
and these isomers are designated by using the cis /
trans designation or the modern E / Z designation.
cis...The two largest groups are on the same side of the
double bond.
trans...The two largest groups are on opposite sides of the
double bond.
E/Z nomenclature
E = entgegan ("trans") Z = zusamen ("cis")
· Try to name the following compounds using both
conventions...
answer\9.txt
_________________________________________
answer\10.txt
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Properties of Alkene
• They are made from alkanes by cracking
• Because they have a double bond, they are called unsaturated.
(Alkanes have no double bonds, so are saturated)
• They are much more reactive than alkanes
C2H4 + H2 ---C3H6
The hydrogen just add on, so this is called an addition reaction
• Alkenes also do an addition reaction with steam, to form
compounds called alcohols
• The alkenes are highly flammable, and burn readily in air
C2H4 + 3O2 - 2CO2 + 2H2O
• But note that an alkene has a higher % of carbon than the
corresponding alkane does. So alkenes are more likely to
undergo incomplete combustion.
• Finally, alkene molecules can add on to each other to form
compounds with very long carbon chains. These compounds are
called polymers
C2H4 - -(CH2—CH2)-n
Physical Properties:
The physical properties of alkenes are comparable with alkanes.
The physical state depends on molecular mass. The simplest
alkenes, ethylene, propylene and butylene are gases. Linear
alkenes of approximately five to sixteen carbons are liquids,
and higher alkenes are waxy solids.
Chemical Properties:
Alkenes are relatively stable compounds, but are more reactive
than alkanes due to their double carbon-carbon bond. Although
stronger than the single carbon-carbon bond in alkanes, the
majority of the reactions of alkenes involve the rupture of this
double bond, forming two new single bonds.
Isomerism of Alkenes
Positional Isomers of Alkenes
The location of the double bond is unambiguous in propene (also known
as propylene), but two butenes are possible. These two isomers are
POSITIONAL ISOMERS. They have been called n-butene,
CH2=CHCH2CH3, and isobutene, H3C-CH=CHCH3, but are more commonly
and correctly called 1-butene and 2-butene. Two butadienes, which each
contain two double bonds, are known also: 1,2-butadiene, H2C=C=CHCH3,
and 1,3-butadiene, H2C=CHCH=CH2.
CH2=CHCH2CH3
H3C-CH=CHCH3
1-butene
2-butene
H2C=C=CHCH3
H2C=CHCH=CH2
1,2-butadiene
1,3-butadiene
Geometrical Isomer
Geometric isomers are a result of the rigidity of a molecule,
which prevents rotation of atoms about a bond. This rigidity
is usually due to a ring structure or a double bond. Such
rigidity is usually not present in singly bonded, open-chained
molecules.
cis-2-butene
trans-2-butene
Geometric Isomers
In those molecules in which the substituent groups are on the same side of
the double bond, the name is preceded by the prefix cis-. In those
molecules in which the groups are on opposite side of the double bond,
the name is preceded by the prefix trans-.