ALKENES
Hydrocarbons
Aliphatic
Alkanes
Aromatic
Alkenes
Alkynes
Alkene Nomenclature
Alkenes
Alkenes are hydrocarbons that contain a
carbon-carbon double bond
also called "olefins"
characterized by molecular formula CnH2n
said to be "unsaturated"
Alkene Nomenclature
H2C
CH2
Ethene
or
Ethylene
(both are acceptable
IUPAC names)
H2C
CHCH3
Propene
(Propylene is
sometimes used
but is not an acceptable
IUPAC name)
Alkene Nomenclature
H2C
CHCH2CH3
1-Butene
1) Find the longest continuous chain that
includes the double bond.
2) Replace the -ane ending of the unbranched
alkane having the same number of carbons
by -ene.
3) Number the chain in the direction that gives
the lowest number to the doubly bonded
carbon.
Alkene Nomenclature
H2C
CHCHCH2Br
CH3
4) If a substituent is present, identify its position
by number. The double bond takes
precedence over alkyl groups and halogens
when the chain is numbered.
The compound shown above is
4-bromo-3-methyl-1-butene.
Alkene Nomenclature
H2C
CHCHCH2OH
CH3
4) If a substituent is present, identify its position
by number. Hydroxyl groups take
precedence over the double bond when the
chain is numbered.
The compound shown above is
2-methyl-3-buten-1-ol.
Alkenyl groups
methylene
H2C
vinyl
H2C
CH
allyl
H2C
CHCH2
isopropenyl
H2C
CCH3
Structure of Ethylene
bond angles:
H-C-H = 117°
H-C-C = 121°
bond distances:
C—H = 110 pm
C=C = 134 pm
Planar
Bonding in Ethylene
•
Side-by-side overlap of half-filled p
orbitals gives a  bond
Isomers
Isomers are different compounds that
have the same molecular formula.
H
CH2CH3
C
H
1-Butene
H3C
H
H
C
C
H
C
H3C
H
2-Methylpropene
CH3
C
H3C
H
H3C
C
C
H
cis-2-Butene
H
C
CH3
trans-2-Butene
Stereoisomers
H3C
CH3
C
H
H
H3C
C
C
H
cis-2-Butene
H
C
CH3
trans-2-Butene
Stereochemical Notation
cis (identical or
analogous substituents
on same side)
trans (identical or
analogous substitutents
on opposite sides)
Reactions of Alkenes
The characteristic reaction of alkenes
is addition to the double bond.
C
C
+
A—B
A
C
C
B
Hydrogenation of ethylene
H
C
H

+
C
H

H
H—H
H

H
C
C
H
H
H

H
exothermic H° = –136 kJ/mol
catalyzed by finely divided Pt, Pd, Rh, Ni
Mechanism of catalytic hydrogenation
B
Y
A
C
H
H
X
C
H
H
General equation for electrophilic addition
C
C
 –
+ E—Y
E
C
C
Y
When EY is a hydrogen halide
C
C
 –
+ H—X
H
C
C
X
Markovnikov’s Rule
When an unsymmetrically substituted
alkene reacts with a hydrogen halide,
the hydrogen adds to the carbon that
has the greater number of hydrogen
substituents, and the halogen adds to
the carbon that has the fewer hydrogen
substituents.
Markovnikov’s Rule
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH3
Br
(80%)
Example
Acid-Catalyzed Hydration of Alkenes
C
+
C
H—OH
reaction is acid
catalyzed; typical
hydration medium is
50% H2SO4-50% H2O
H
C
C
OH
Cyclopentene +Br2
Bromonium ion
–
trans-Stereochemistry in
vicinal dibromide
Epoxidation of Alkenes
O
C
C
+
RCOOH
peroxy acid
O
C
C
O
+
RCOH
Polymerization of alkenes
cationic polymerization
free-radical polymerization
coordination polymerization
Free-Radical Polymerization of Ethylene
H2C
CH2
200 °C
2000 atm
CH2
CH2
CH2
CH2
O2
peroxides
CH2
polyethylene
CH2
CH2
Free-Radical Polymerization of Propene
H2C
CHCH3
CH
CH
CH
CH
CH
CH
CH
H
CH3
H
CH3
H
CH3
H
polypropylene
H2C=CHCl  polyvinyl chloride
H2C=CHC6H5

polystyrene
F2C=CF2

Teflon
Classes of Dienes
Classification of Dienes
isolated diene
conjugated diene
C
cumulated diene
Nomenclature
(2E,5E)-2,5-heptadiene
(2E,4E)-2,4-heptadiene
C
3,4-heptadiene
Isolated diene
 bonds are
independent of
each other
p orbitals overlap
to give extended 
bond
encompassing
four carbons
Conjugated diene
Cumulated Dienes
C
C
C
cumulated dienes are less stable than
isolated and conjugated dienes
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
Y
Y
X
X
Alkynes
Hydrocarbons
Aliphatic
Alkanes
Aromatic
Alkenes
Alkynes
Nomenclature
Acetylene and ethyne are both acceptable
IUPAC names for HC CH
Higher alkynes are named in much the same
way as alkenes except using an -yne suffix
instead of -ene.
HC
CCH3
Propyne
(CH3)3CC
HC
CCH2CH3
1-Butyne
CCH3
4,4-Dimethyl-2-pentyne
Structure
linear geometry for acetylene
H
120 pm
C
C
106 pm
CH3
106 pm
121 pm
C
C
146 pm
H
H
106 pm
 Bonds in Acetylene
Each carbon is
connected to a
hydrogen by a
 bond. The two
carbons are connected
to each other by a
 bond and two  bonds.
Acidity of Acetylene andTerminal Alkynes
H
C
C
Reactions of Alkynes
Acidity
Hydrogenation
Metal-Ammonia Reduction
Addition of Hydrogen Halides
Hydration
Addition of Halogens
Ozonolysis