Alkenes: Organic Chemistry CHM 207
Alkenes: Organic Chemistry CHM 207
Alkenes: Organic Chemistry CHM 207
CHM 207
CHAPTER 3:
ALKENES
Nomenclature
Nomenclature
Structures
Structures and
and
physical
physical
properties
properties
Addition
Addition
Preparation
Preparation of
of
alkenes
alkenes
Subtopics Combustion
Combustion
Reactions
Reactions of
of
alkenes
alkenes
Oxidation
Oxidation
Unsaturation
Unsaturation
tests
tests
Polimerization
Polimerization
Uses
Uses
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
This chain
contains 6 C
contains 8
atoms
C atoms
wrong correct
4 3 2 1
4-ethyl-1-octene
NEW IUPAC NAMES
Placing numbers (location of double bond) before the
part of the name –ene.
Example:
1 2 3 4 1 2 3 4 5 6
CH2 C CH2 CH3 CH3 C C CH2 CH2 CH3
H H H
Old naming system: 1-butene Old naming system: 2-hexene
New naming system: but-1-ene New naming system: hex-2-ene
CH3
1 2 3 4
CH2 C C CH3
H H
Old naming system: 3-methyl-1-butene
New naming system: 3-methylbut-1-ene
A compound with more than one double bond.
Two double bond: diene
1 2
8 3
7 4
6 5
IUPAC names: 1,3, 5, 7-cyclooctatetraene
new IUPAC names: cycloocta-1,3,5,7-tetraene
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.
CH=CH2
CH2 CHCHCH2CH CH2
3-methylenecyclohexene IUPAC name: 3-vinyl-1,5-hexadiene
New IUPAC name: 3-vinylhexa-1,5-diene
CYCLOALKENES
Contains C=C in the ring
• Nomenclature of cycloalkenes:
- Similar to that alkenes
- Number the cycloalkane so that the double bond is between C1 and
C2 and so that the first substituent has as low a number as possible.
* Double bond always between C1 and C2.
6 1
CH3 5 6 2 CH2CH3
5 1 4 1
4 2 5 3
3 2 4
3
1-methylcyclohexene 1,5-dimethylcyclopentene IUPAC name: 2-ethyl-1,3-cyclohexadiene
New IUPAC name: 2-ethylcyclohexa-1,3-diene
NOMENCLATURE OF cis-trans ISOMERS
H3 C CH2CH3 H3C H
C C C C
H H H CH2CH3
cis-2-pentene trans-2-pentene
- relatively nonpolar.
i. Dehydration of alcohols
conc. H2SO4
R-CH2-CH2-OH R-CH=CH2 + H2O
Dehydrohalogenation of haloalkanes
CH3CH-CH-CH2 alcohol CH3CH2CH=CH2
KOH CH3CH=CH-CH3
reflux
H Br H 2-butene
1-butene
2-bromobutane (major product)
REACTIVITY OF ALKENES
Alkenes are more reactive than alkanes because:
iii) π bond will broken, each carbon atom becomes an active site
which can form a new covalent bond with another atom. One π
bond is converted into 2 σ bonds.
Reaction of Alkenes
REACTIONS OF ALKENES
i) Addition reaction:
a) Addition of hydrogen (Catalytic hydrogenation)
b) Addition of halogens
- In inert solvent
- In water / aqueous medium
c) Addition of hydrogen halides
d) Addition reaction with concentrated sulfuric acid: hydration of
alkenes
e) Addition reaction with acidified water (H3O+): hydration of
alkenes
ii) Combustion of alkenes
iii) Oxidation:
a) Epoxidation
b) Hydroxylation
c) Ozonolysis
i) Addition reaction
a) Addition of hydrogen (Catalytic hydrogenation):
- hydrogenation: addition of H to a double bond and triple bond to
yield saturated product.
- alkenes will combine with hydrogen in the present to catalyst to
form alkanes.
Pt or Pd
C C H H o C C
25-90 C
H H
- Plantinum (Pt) and palladium (Pd) – Catalysts
- Pt and Pd: temperature 25-90oC
- Nickel can also used as a catalyst, but a higher temperature of 140oC –
200oC is needed.
EXAMPLES:
Pt
H2C CH2 H2 H3C CH3
low pressure ethane
ethylene
Pt
CH3CH2CH2CH2CH CH2 H2 CH3CH2CH2CH2CH2CH3
low pressure
hexene hexane
b) Addition of halogens:
i) In inert solvent:
- alkenes react with halogens at room temperature and in dark.
- the halogens is usually dissolved in an inert solvent such as
dichloromethane (CH2Cl2) and tetrachloromethane (CCl4).
- Iodine will not react with alkenes because it is less reactive than
chlorine and bromine.
- Fluorine is very reactive. The reaction will produced explosion.
inert solvent
C C X X C C
X X
X X = halogen such as Br2 or Cl2
Inert solvent = CCl4 or CH2Cl2
EXAMPLES:
* the red-brown colour of the bromine solution will fade and the
solution becomes colourless.
Br
CCl4
Br2
Br
cyclohexene 1,2-dibromocyclohexane
CCl4 Cl Cl
CH3CH=CH2 Cl2 CH3CH CH2
propene 1,2-dichloropropane
b) Addition of halogens:
- same as bromine
Br2 (aq) + H2O(l) HBr(aq) + HOBr(aq)
H X
C C HX C C
alkene haloalkane
EXAMPLES:
H-I I
cyclopentene iodocyclopentane
Br
CH3CH=CHCH3 + H-Br CH3CH2CHCH3
2-butene 2-bromobutane
MARKOVNIKOV’S RULE
There are 2 possible products when hydrogen halides react with an
unsymmetrical alkene.
It is because hydrogen halide molecule can add to the C=C bond in two
different ways.
H H H H
CH3 C C H H-I CH3 C C H
H I
1-iodopropane
H H H H
CH3 C C H H-I CH3 C C H
I H
2-iodopropane
(major product)
Markovnikov’s rules:
- the addition of HX to an unsymmetrical alkene,
the hydrogen atom adds to the carbon atom (of
the double bond) that already has the greater
number of hydrogen atoms.
Mechanism of electrophilic addition reactions:
- C=C : electron rich part of the alkene molecule
- Electrophiles: electron-seeking
Step 1: Formation of carbocation.
Attack of the pi bond on the electrophile to form carbocation.
δ-
C C δ+
E Y C C Y-
E
carbocation
Step 2: Rapid reaction with a negative ion.
The negative ion (Y-) acts as nucleophile and attacks the positively
charged carbon atom to give product of the addition reaction.
C C Y- C C
E E Y
ADDITION OF HYDROGEN HALIDES TO
UNSYMMETRICAL ALKENES AND MARKOVNIKOV’S
RULE
CH3CHCH2
3 2 1 H Cl
CH3CH=CH2 HCl
1-chloropropane
Propene
CH3CHCH2
Cl H
2-chloropropane
(major product)
according to Markovnikov's
rules
d) Addition reaction with concentrated sulfuric acid: hydration of
alkenes
or
H OH
H2O H +
C C C C
alkene alcohol
Markovnikov’s rule is apply to the addition of a water molecule
across the double bond of an unsymmetrical alkene.
For examples:
CH3 CH3
+
CH2 H OH H
CH3 C CH3 C CH2
25oC
2-methylpropene
OH H
tert-butyl alcohol
CH3CHCH3
CH3CHCH3 +
O H H
OH
H H+ = catalyst
ANTI-MARKOVNIKOV’S RULE: FREE RADICAL
ADDITION OF HYDROGEN BROMIDE
When HBr is added to an alkene in the absence of peroxides it obey
Markovnikov’s rule.
When HBr (not HCl or HI) reacts with unsymmetrical alkene in the
presence of peroxides (compounds containing the O-O group) or
oxygen, HBr adds in the opposite direction to that predicted by
Markovnikov’s rule.
The product between propene and HBr under these conditions is 1-
bromopropane and not 2-bromopropane.
peroxide
CH3CH=CH2 HBr CH3CH2CH2Br
1-bromopropane
(major product)
anti-Markovnikov's orientation
Anti-Markovnikov’s addition:
examples:
H2O2, -OH
CH3CH=CH2 B 2H 6 CH3CHCH2-OH
propene propanol
CH3 CH3
H2O2, -OH
B2H6 CH3CHCHCH3
CH3 CH C CH3
2-methyl-2-butene OH
3-methyl-2-butanol
ii) Combustion of alkenes
O O H
O
Cl O
CH3 C O O H C O OH O
MCPBA
o
O
CH2CI2, 25 C
cycloheptene 1,2-epoxycycloheptane
b) Hydroxylation of alkenes
Hydroxylation:
- Converting an alkene to a glycol requires adding a hydroxyl
group to each end of the double bond.
Hydroxylation reagents:
i) Osmium tetroxide (OsO4)
ii)Potassium permanganate (KMnO4)
C C OsO4 H2O2 C C
(or KMnO4, -OH) OH OH
glycol
Examples:
KMnO4 (aq), OH-
CH2 CH2 CH2 CH2 MnO2
cold, dilute
ethene OH OH
1,2-ethanediol
Ozonolysis:
- The reaction of alkenes with ozone (O3) to form an ozonide, followed by
hydrolysis of the ozonide to produce aldehydes and /or ketone.
i) O3 H O
3-nonene ii) (CH3)2S
O H
H H
iv) Polymerization of alkenes
Polymer: A large molecule composed of many smaller repeating units
(the monomers) bonded together.
CI H CI H CI H Cl H Cl
H CI H
C C C C C C C C C C C C
H H H H H H H H H H H H
n
vinyl chloride poly(vinyl chloride)
SOME OF THE MOST IMPORTANT ADDITION POLYMERS
CH3
KMnO4/H+ CH3
CH3 C CH2
CH3 C O CO2 + H 2O
2-methylpropene
propanone
(ketone)
REACTIONS OF ALKENES WITH HOT, ACIDIFIED
KMnO4
R R' R R'
KMnO4/H+
C C R'' C C H
R'' H OH OH
R R' R R'
C O O C C O
R'' R'' O C
OH H
ketone acid ketone aldehyde
+
KMnO4/H
R CH=CH2 R COOH + CO2 + H2O
Example:
+
KMnO4/H O HO
C C
O
4-methyl-4-octene 2-pentanone butanoic acid
ii) Reaction of alkenes with bromine
- A solution of bromine in inert solvent (CH2CI2 or CCI4) and dilute
bromine water are yellow in colour.
- The solution is decolorised when added to alkenes or organic
compounds containing C=C bonds.
CH2CI2
C C Br2 C C
Br Br
H2O
C C Br2(aq) C C C C
OH Br Br Br
Uses of Alkenes
i) PE
ii) PVC
iii) ethanol
USES OF ALKENES
Ethylene and propylene are the largest-volume industrial organic
chemicals.
Used to synthesis a wide variety of useful compounds.
H H
C C O O
oxidize
H H n CH3 C OH
CH3 C H
polyethylene acetaldehyde acetic acid
polymerize
oxidize
O H H
O2 Cl2 CH2 CH2
H2C CH2 Ag catalyst C C
ethylene oxide H H CI CI
ethylene ethylene dichloride
+
H H2O NaOH
H2O catalyst
CH2 CH2 CH3 CH2 H CI
OH OH OH H C C H
ethylene glycol ethanol vinyl chloride
i) POLYETHENE (PE)
The most popular plastic.
Uses:
i) Grocery bags
ii)Shampoo bottles
iii)Children's toy
iv)Bullet proof vests
v)Film wrapping
vi)Kitchenware
ii) POLYVINYL CHLORIDE (PVC)
H H polymerize H CI H CI H CI
H C C CI C C C C C C
vinyl chloride H H H H n H H
poly(vinyl chloride)
PVC, "vinyl"
USES OF PVC:
Clothing
- PVC fabric has a sheen to it and is waterproof.
- coats, shoes, jackets, aprons and bags.
As the insulation on electric wires.
Producing pipes for various municipal and industrial applications. For
examples, for drinking water distribution and wastewater mains.
As a composite for the production of accessories or housings for
portable electronics.
used in the building industry as a low-maintenance material.
Ceiling tiles.
iii) ETHANOL
USE OF ETHANOL:
Motor fuel and fuel additive.
As a fuel to power Direct-ethanol fuel cells (DEFC) in order to produce
electricity.
As fuel in bipropellant rocket vehicles.
In alcoholic beverages.
An important industrial ingredient and use as a base chemical for other
organic compounds include ethyl halides, ethyl esters, diethyl ether,
acetic acid, ethyl amines and to a lesser extent butadiene.
Antiseptic use.
An antidote.
Ethanol is easily miscible in water and is a good solvent. Ethanol is less
polar than water and is used in perfumes, paints and tinctures.
Ethanol is also used in design and sketch art markers.
Ethanol is also found in certain kinds of deodorants.