Alkenes
.1
Introduction
.2
Nomenclature of Alkenes
.3
Physical Properties of Alkenes
.4
Preparation of Alkenes
.5
Reactions of Alkenes
Structure Of Alkenes
They are unsaturated hydrocarbons – made up
of C and H atoms and contain one or more C=C
double bond somewhere in their structures.
Their general formula is CnH2n - for non-cyclic
alkenes
Their general formula is CnH2n-2 - for cyclic
alkenes
2
SP2Hibridization:
Angle of SP2 bond =120o
Introduction
Functional group of alkenes:
The C = C double bond in ethene
Each carbon is trigonal and planar.
 Bonding: sp2 hybridization for 3 s-bonds to the three atoms bonded
to each carbon
 pz orbital for π-bond
 Typical C=C bond distance (i.e., 1.34 Å) shorter than C-C bond
distance (i.e., 1.54 Å)
 slightly shorter C-H distance than alkanes
Nomenclature of Alkenes
Nomenclature of Alkenes
1. Determine the stem name by selecting the longest
possible straight chain containing the C = C double bond
and use the ending ‘-ene’
2. Number the parent chain so as to include both carbon
atoms of the double bond, and begin numbering with the
end of the chain nearer the C = C double bond
3. Designate the position of the C = C double bond by
using the number of the first atom of the double bond
4. Designate the positions of the substituent's by using
the numbers obtained by application of rule 2
Nomenclature of Alkenes
Examples:
5. Indicated number of double bonds by prefixes (ene,
diene, triene, tetraene, etc.)
Examples
C H2
1
C H2
1
CH2
C H3
2
3
C
C H2
4
5
C H2
C H3
2- ethyl -1-pentene
2
3
4
CH2
CH
CH
5
CH2
1,4-pentadiene
1
CH2
CH3
2
C
3
CH
4
CH2
2- methyl-1,3-butadiene
Nomenclature of Alkenes
6. If two identical groups are present on the same side of
the C = C double bond, the compound is designated as
cis; if they are on opposite sides, the compound is
designated as trans.
e.g.
Nomenclature of Alkenes
Example
Give the IUPAC names for the following alkenes:
(a)
(b)
Solution:
(a) trans-3,4-dichlorohept-3-ene
(b) cis-3,4-dimethyloct-3-ene
Answer
Nomenclature of Alkenes
Check Point
Draw the structural formula for each of the following
alkenes:
(a) cis-hex-3-ene
(b) trans-2,3-dihydroxybut-2-ene
(c) cis-1,2-dichloroethene
(a)
(b)
(c)
Answer
Physical Properties of Alkenes
 Alkenes are non polar compounds.
 Insoluble in water.
 Soluble in non polar organic solvents.
 They are less dense than water.
 Range of physical states:
 ≤ 4 C's are gases
5 - 17 C's are liquids
 ≥ 18 C's are solids
 The alkenes has a boiling point which is a small number
of degrees lower than the corresponding alkanes.
13
Physical Properties of Alkenes
Name
Formula
Boiling
point (°C)
Melting Density at
20 °C
point
(°C)
(g cm-3)
Ethene
CH2 = CH2
-104
-169
—
Propene
CH3CH = CH2
-47.7
-185
0.514
But-1-ene
CH3CH2CH = CH2
-6.3
-185
0.595
Pent-1-ene
CH3(CH2)2CH = CH2
30
-165
0.641
Hex-1-ene
CH3(CH2)2CH = CH2
62.9
-140
0.673
cis-But-2-ene
CH3CH = CHCH3 (cis)
4
-139
0.621
trans-But-2-ene
CH3CH = CHCH3
(trans)
1
-106
0.604
2-Methylbut-1ene
CH3CH3C(CH3) = CH2
31
-138
0.650
Preparation of Alkenes
Elimination Reactions
Dehydrohalogenation
• Dehydrohalogenation is the elimination of a hydrogen
halide molecule from a haloalkane in presence of KOH
in alcohol
Preparation of Alkenes
Examples:
Preparation of Alkenes
Dehydrohalogenation of 2° and 3° haloalkanes can take place in
more than one way and a mixture of alkenes is formed
alc. KOH
CH3CH2CHClCH3  CH3CH = CHCH3 + CH3CH2CH = CH2
heat
2-chlorobutane
But-2-ene
But-1-ene
(80%)
(20%)
Note: the more highly substituted alkene is formed as major
product
Saytzeff s Rule: In every instance in which more than one alkene can
be formed, the major product is the alkene with the most alkyl
substituents attached on the double bonded carbon.
CH3
CH3
Br
+ KOH
H
CH3
EtOH
+
∆
1-methyl cyclohexene
Major
3 -methyl cyclohexene
Minor
Preparation of Alkenes
Dehydration of Alcohols
Dehydration is the removal of a water molecule from a
reactant molecule, in the presence of Mineral acids (H2SO4,
H3PO4)
The presence of H2SO4 to prevent the reversible reaction
Preparation of Alkenes
The experimental conditions of dehydration depend on the
structures of alcohols
e.g.
Preparation of Alkenes
Addition Reactions
Hydrogenation
• hydrogenation of alkynes using Lindlar’s catalyst
produces alkenes
• prevent further hydrogenation of the alkenes formed to
alkanes
Reactions of Alkenes
Alkenes are more reactive than alkanes
Reason: presence of the C = C double bond
Energetically
favourable!!
 alkenes undergo addition reactions and the reactions
are exothermic
π-bonds weaker than σ-bonds
Reactions of Alkenes
Electrophilic Addition Reactions
Addition of Hydrogen Bromide
Addition of hydrogen bromide to C = C double bond
yields a bromoalkane
Reactions of Alkenes
Examples:
Reactions of Alkenes
Propene reacts with HBr to give 2-bromopropane
(major product) and 1-bromopropane (minor product)
The formation of two possible products can be explained
by the reaction mechanism.
Reactions of Alkenes
Markownikoff’s rule states that in the addition of HX to an
unsymmetrical alkene, the hydrogen atom adds to the carbon
atom of the carbon-carbon double bond that already has the
greater number of hydrogen atoms.
Example:
Reactions of Alkenes
2-bromopropane is the major product because the more stable
secondary carbocation is formed in the first step
Reactions of Alkenes
Addition of Bromine
Alkenes react rapidly with Br2 in 1,1,1-trichloroethane
at room temperature and in the absence of light
e.g.
Reactions of Alkenes
The behaviour of alkenes towards Br2 in CH3CCl3 is a useful
test for the presence of carbon-carbon multiple bonds
Add Br2 in
CH3CCl3 to excess
alkene
The reddish brown
colour of Br2 is
decolourized
Reactions of Alkenes
Addition of Bromine Water
In an aqueous solution of Br2, the following equilibrium exists
HOBr
HBr + Br2 + H2O
Bromic(I) acid
The bromine atom bears a partial positive charge while
the oxygen atom bears a partial negative charge
∵ oxygen is more electronegative than bromine
Reactions of Alkenes
When bromic(I) acid reacts with alkenes, bromohydrin
is formed
e.g.
Reactions of Alkenes
Addition of SulphuricAcid
Alkenes react with cold and concentrated H2SO4 to
form alkyl hydrogensulphates
e.g.
Reactions of Alkenes
The large bulky –OSO3H group makes the alkyl hydrogensulphate
very unstable. Two possible further reactions take place:
1. Regeneration of alkenes
2. Production of alcohols
Reactions of Alkenes
Catalytic Hydrogenation
In the presence of metal catalysts (e.g. Pt, Pd or Ni), H2 is
added to each atom of C = C double bond to form an alkane
e.g.
Addition of H2O: Hydration
 Only one product is possible from the addition of H2O in
presence of acids as catalysts to symmetrical alkenes such as
ethene and cyclohexene. Symmetrical akenes
A
A
A
+
A
H2 O
H
A
A
A
A
H
OH
OH
+
H2 O
H
CH3
H3C
H
 However, addition reactions to unsymmetrical alkenes will result in
the formation of Markovonikov’s product preferentially.
Unsymmetrical akenes
H
H
35
CH3
+
H2O
OH
CH3
Addition of HCN
A
A
+
A
A
+
HCN
H
A
A
A
A
H
CN
CN
H3C
CH3
+
+
H
HCN
H3C
CH3
H
H
+
+
CH3
HCN
H
CN
CH3
36
Reaction of Alkenes
Check Point 30-3
(b) What is the major product of each of the following
reactions?
(i)
(ii)
(b) (i)
(ii)
Answer
Reaction of Alkenes
Check Point
(c) Give the reaction products for the following reactions:
(i) CH3CH = CH2 + H2 
Ni
conc. H2SO4
(ii) CH3CH = CHCH3 
(iii) CH3CH = CHCH3 + Br2 
Answer
(c) (i) CH3CH2CH3
(ii)
(iii)
Reactions of Alkenes
Ozonolysis
Ozonolysis is a widely used method for locating the double
bond of an alkene
(unstable)
The unstable ozonide is reduced directly by treatment with Zn and
H2O
Reactions of Alkenes
Polymerization
Polymers: Compounds that consist of very large molecules
made up of many repeating units
Monomer: Each repeating unit
Polymerization:The reaction by which monomers are joined
together
Addition polymerization: alkene monomers are joined together
without the elimination of small
molecules
Addition polymer: The polymer produced by addition
polymerization
Reactions of Alkenes
Poly(ethene)
Monomer: ethene
Depending on the conditions, two kinds of poly(ethene) are
formed