ALKENES
• Also called olefins
• Contain at least one carbon-carbon double bond
(C=C)
• General formula, CnH2n (n=2,3,…)
• Classified as unsaturated hydrocarbons
(compound with double or triple carbon-carbon
bonds that enable them to add hydrogen atoms.
• sp2-hybridized
• For example:
C2H4 - ethylene
CH2
CH2
Naming Alkenes
IUPAC RULES
RULE 1. Select the longest continuous
carbon chain that contains a double bond.
This chain
contains 6
carbon atoms
RULE 2. Name this compound as you would an
alkane, but change –ane to –ene for an alkene.
This chain
contains 8
carbon atoms
This isthe
theparent
longest
Name
continuous octene.
chain.
compound
Select it as the parent
compound.
RULE 3. Number the carbon chain of the
parent compound starting with the end nearer to
the double bond. Use the smaller of the two
numbers on the double-bonded carbon to
indicate the position of the double bond. Place
this number in front of the alkene name.
IUPAC RULES
This end of the chain is closest to the
double bond. Begin numbering here.
IUPAC RULES
The name of the parent compound is
1-octene.
4
3
2
1
5
6
7
8
RULE 4. Branched chains and other groups are
treated as in naming alkanes. Name the
substituent group, and designate its position on
the parent chain with a number.
IUPAC RULES
The
is attached to carbon 4.
Thisethyl
is an group
ethyl group.
4
3
2
1
5
6
7
8
4-ethyl-1-octene
• A compound with more than one double bond.
- Two double bond: diene
- Three double bond: triene
- Four double bond: tetraene
* Numbers are used to specify the locations of the double bonds.
1
2
3
CH2 C
H
4
C CH2
H
7
6
CH3 C C
H H
IUPAC names: 1,3-butadiene
new IUPAC names: buta-1,3-diene
1
2
8
3
7
4
6
5
5
IUPAC names: 1,3, 5, 7-cyclooctatetraene
new IUPAC names: cycloocta-1,3,5,7-tetraene
4
3
C
H
C
H
2
C
H
1,3,5-heptatriene
hepta-1,3,5-triene
1
CH2
ALKENES AS SUBSTITUENTS
• Alkenes names as substituents are called alkenyl groups.
• Can be named systematically as ethenyl, propenyl, etc. or
by common names such as vinyl, ally, methylene and
phenyl groups.
CH2
methylene group
(methylidene group)
-CH=CH2
vinyl group
(ethenyl group)
-CH2-CH=CH2
allyl group
(2-propenyl group)
CH=CH2
CH2 CHCHCH2CH CH2
3-methylenecyclohexene
3-vinyl-1,5-hexadiene
3-vinylhexa-1,5-diene
CYCLOALKENES
• Contains C=C in the ring
cyclopropene
cyclobutene
cyclohexene
cyclopentene
• Nomenclature of cycloalkenes:
- Similar to that alkenes
- Carbons atoms in the double bond are designated
C1 and C2
6
5
4
CH3
1
2
3
1-methylcyclohexene
5
1
4
3
2
1,5-dimethylcyclopentene
NOMENCLATURE OF cis-trans
ISOMERS
H3C
CH2CH3
C
H
H3C
C
C
H
cis-2-pentene
H
H
C
CH2CH3
trans-2-pentene
•cis – two particular atoms (or groups of atoms) are
adjacent to each other
•trans – the two atoms (or groups of atoms) are
across from each other
PHYSICAL PROPERTIES OF ALKENES
•
Boiling points and densities:
-Most physical properties of alkenes are similar to
those alkanes.
-Example: the boiling points of 1-butene, cis-2-butene,
trans-2-butene and n-butane are close to 0oC.
- Densities of alkenes: around 0.6 or 0.7 g/cm3.
-Boiling points of alkenes increase smoothly with
molecular weight.
-Increased branching leads to greater volatility and
lower boiling points.
• Polarity:
- relatively nonpolar.
-insoluble in water but soluble in non-polar
solvents such as hexane, gasoline, halogenated
solvents and ethers.
- slightly more polar than alkanes because:
i) electrons in the pi bond is more polarizable
(contributing to instantaneous dipole moments).
ii) the vinylic bonds tend to be slightly polar
(contributing to a permanent dipole moment).