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alkene
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3.1 Lesson Outcomes3.2 Structure & Bonding in Alkenes3.4 Interesting of Alkenes IUPAC3.5 Nomenclature of Alkenes3.5 Physical Properties of Alkenes3.6 Preparation of Alkenes3.7 Chemical reaction of Alkenes
1
ALKENES3
• At the end of this topic, you should be able to:- Identify alkenes Explain structure and physical properties
of alkene Name and draw alkene by using IUPAC
nomenclature Write the equation for the preparation of
alkene. Write the equation for the reactions of
alkene. Describe the sources and uses of alkene
LEARNING OUTCOMES
STRUCTURE & BONDING OF
ALKENES
Alkenes are also called olefins.
Alkenes contain a carbon—carbon double bond.
General formula: CnH2n (for one double bond)
STRUCTURE & BONDING
The double bond consists of a bond and a bond. The bond is stronger
than the bond.
Each carbon is sp2 hybridized and trigonal planar, with bond angles of
approximately 120°.
STRUCTURE & BONDING
INTERESTING OF ALKENES
Ethylene : Prepared from petroleum by a process called cracking The most widely produced organic chemicals Starting material not only for polymer polyethylene, a widely used plastic, but also many other useful organic compounds..
INETERESTING OF ALKENES
INETERESTING OF ALKENES
INTERESTING OF ALKENES
INTERESTING OF ALKENES
INTERESTING OF ALKENES
INTERESTING OF ALKENES
INTERESTING OF ALKENES
INTERESTING OF ALKENES
Organic compounds containing carbon-carbon double bonds have been isolated from natural sources.
β-carotene(orange pigment in carrot)
Zingiberene(oil of ginger)
α-farnesene(found in the waxy
coating on apple skin
CH3
HCH2C
CH3
(R)-limonene(from oranges)
CH3
H CH3C
CH2
(S)-limonene(from lemons)
INTERESTING OF ALKENES
The Chemistry of VisionIn the rod and cone cells in the retina of the eye, retinal in rhodopsin is found “at rest” in the cis form. When it absorbs a photon (hν) of light, one of the π- bonds is broken, causing the molecule to rotate and lock into the trans form, which has a completely different shape. This starts a long chain of chemical processes which eventually results in a visual image in the brain. The trans-retinal molecule is then twisted back into the cis form by another enzyme. When you look directly at a very bright light, the “afterimage” that you see in front of your eyes is the result of a large amount of cis-retinal having been transformed into trans-retinal all at once; the enzymes take a little bit of time to go through and “reset” all of these molecules.
INTERESTING OF ALKENES
The Chemistry of Vision
INTERESTING OF ALKENES
IUPAC NOMENCLATURE OF ALKENES
Step 1: Identify and name the longest continuous chain of C atoms which contains
the double bond(s) (#C + -ene).
NOMENCLATURE OF ALKENES
NOMENCLATURE OF ALKENES
Step 2: Locate the double bond by the lower numbered carbon atom joined by the
double bond (e.g., 1-butene).
If the double bond has the same position starting from either end, use the positions of the substituents to determine beginning of
the chain.
Step 3: Write out the full name, numbering the substituents according to their position in the chain and list them in alphabetical order.
NOMENCLATURE OF ALKENES
Indicate the double bond by the number of the first alkene carbon.
NOMENCLATURE OF ALKENES
NOMENCLATURE OF ALKENES
Compounds with two double bonds are named as dienes by changing the “-ane” ending of the
parent alkane to the suffix “–adiene”.
Compounds with three double bonds are named as trienes
NOMENCLATURE OF ALKENES
In naming cycloalkenes, the double bond is located between C1 and C2. The ring is numbered
to give the first substituent the lower number.
NOMENCLATURE OF ALKENES
In naming cycloalkenes: If there is a substituent on one of the double bond carbons, it gets number
1.
NOMENCLATURE OF ALKENES
Compounds that contain both a double bond and a hydroxyl group are named as alkenols &
the chain (or ring) is numbered to give the OH group the lower number.
NOMENCLATURE OF ALKENES
28
trans-3-hexene
C CCH2CH3
H
H
H3CH2C
cis-3-heptene
C CCH2CH3
H
H3CH2CH2C
H
cis-3-methyl-3-heptene
C CCH2CH3
H
H3CH2CH2C
H3C
NOMENCLATURE OF ALKENES: CIS-TRANS
MOLECULE
trans-1,2-dimethylcyclohex-1-ene
cis-1,2-dimethylcyclohex-1-ene
PHYSICAL PROPERTIES OF
ALKENES
Alkenes exhibit only weak van der Waals interactions.
Low melting points and boiling points.
Melting & boiling points increase as the number of carbons increases because of increased surface area.
Soluble in organic solvents and insoluble in water.
Ethene, propene & butene - gases at room temperature.Alkenes are nonpolar compounds.
PHYSICAL PROPERTIES OF ALKENES
PREPARATION/SYNTHESIS OF ALKENES
X Y
Dehydration of alcohols:X = H; Y = OH
Dehydrohalogenation of alkyl halides:X = H; Y = Br, Cl
C CC C + X Y
SYNTHESIS OF ALKENES: ELIMINATION
REACTIONS
Heating an alcohol with excess concentrated H2SO4 at 170°C, or with excess concentrated phosphoric acid,
H3PO4 at 225°C
1. DEHYDRATION OF ALCOHOLS
· Loss of H and OH from adjacent carbons.· Acid catalyst is necessary.
H OH
C CC C + H2O
Examples
OHconc. H3SO4 + H2O
CH3CH2CH2OHconc. H3SO4 CH3CH=CH2
+ H2O
1. DEHYDRATION OF ALCOHOLS
Haloalkanes undergo elimination reaction when refluxed with purely alcoholic
solutions of potassium hydroxide, KOH to produce an alkene.
2. DEHYDROHALOGENATION OF ALKYL
HALIDES
H X
C CC C + HX
Reagent: NaOCH3 in methanol, or KOH in ethanol
Examples
CH3CH2CH2Br + KOH/ethanol reflux CH3CH=CH2 + KBr + H2O
CH3CCH3 + KOH/ethanol reflux CH3CH=CH2 + KCl + H2OCH3
Cl
CH3
2. DEHYDROHALOGENATION OF ALKYL
HALIDES
REACTION OF ALKENES
The characteristic reaction of alkenes is addition: the bond is broken and two new bonds are formed.
Alkenes react with Electrophiles
REACTION OF ALKENES
Alkenes undergo ELECTROPHILIC ADDITION reactions –
the C=C (C double bonds) are broken to form C-C bond (single bonds).
REACTION OF ALKENES
Addition reactions• Hydrogenation• Halogenation• Hydration• Hydrohalogenation• Halohydrin formation
Oxidation reactions• Ozonolysis• Hydroxylation with KMnO4 (room temp) • Oxidation cleavage of alkenes with
acidic KMnO4
ADDITION REACTIONS
ADDITION OF HYDROGEN (HYDROGENATION)
The addition of hydrogen to an alkene can be carried out by
passing hydrogen & the alkene, both in the gaseous state, over a metal catalyst.
General reaction
Example
C C + H HNi or Pt C C
H H
C CH
H
H
H+ H H
Ni or Pt C CH
H
H
H
H
H
ethene ethane
Hydrogenation of double bonds – used to convert unsaturated edible oil as
to solid fats like margarine.
1
ADDITION OF HYDROGEN (HYDROGENATION)
Example
1
CH2=CH2 Ni / H2 CH3-CH3
CH3-C C-CH2CH3
CH3CH3
Ni / H2
CH3
2H2Ni
CH3
(2 mole C=C requires 2 mole of H2 gas)
H3C
CH3
H
CH3
H
ADDITION OF HYDROGEN (HYDROGENATION
Margarine Manufacturing Process
1
ADDITION OF HALOGEN (HALOGENATION)
Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal
dihalide.
2
ADDITION OF HALOGEN (HALOGENATION)
Decolorisation of bromine solution – test-tube reaction to detect a double bond.
When an alkene is bubbled through a solution of hexane or tetrachlomethane (CCl4), the
orange color of bromine is rapidly decolorised.
CH2=CH2 + Br2/CCl4 CH2CH2
Br Br
room temperature
orange
colorless
2
ADDITION OF HALOGEN (HALOGENATION)2
CH3-CH2-CH=CH2 CH3-CH2-CH-CH2Br
Cl2 / CClH2C CH2
Cl Cl
Br2 / CCl4
Br
CH2 CH3
2Br2 / CCl4Br
Br
Br
Br
CH2CH3
CH2=CH2
HYDROHALOGENATION: ELECTROPHILIC ADDITION OF HX
Two bonds are broken in this reaction: 1. The weak bond of the alkene and the HX bond2. Two new bonds are formed: one to H and one to
X.Because the electrophilic H end of HX is attracted to the electron-rich double bond, these reactions are
called electrophilic additions.
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HYDROHALOGENATION
Addition of hydrogen halide to unsymmetrical alkenes Markonikov’s
RuleWith an unsymmetrical alkanes, HX can add to the double bond to give to positional isomers.
Markovnikov’s rule : The addition of HX to an unsymmetrical alkene, the H atom
adds to the carbon atom of the double bond with the greatest
number of carbon atoms
3
HYDROHALOGENATION
Addition of hydrogen halide to unsymmetrical alkenes Markonikov’s
Rule
3
HYDROHALOGENATION
Addition of hydrogen halide to unsymmetrical alkenes Markonikov’s
RuleAn unsymmetrical carbon-carbon double bond is one whose 2 carbon atoms hold unequal number of hydrogen atoms.
C Cpropene
H3C
H H
H
carbon atom with the greatest number of hydrogen atom
unsymmetrical double bond
Example
2-bromobutane
CH3CH2CH=CH2 + HBr CH3CH2CHCH3
Br
CH3
+ HClCH3Cl
1-chloro-1-methylcyclohexane
3
HYDROHALOGENATION3
CH3-CH2-CH-CH3
HCl CH3-CH2Cl(one product)
CH3-CH=CH-CH3 HBr CH3-CH-CH2-CH3
Br(one product)
CH3-CH2-CH=CH2 HCl
Cl
CH3-CH2-CH2-CH2-Cl
(major)
(minor)
CH2=CH2
HYDROHALOGENATION3
with HBr peroxide/ ROOR , the major product is ANTI-MARKOVNIKOV.
The H from HBr will be bonded to carbon double bond is bonded directly to less
hydrogens atoms.
CH3-CH2-CH=CH2
HBr
Peroxideor
ROOR
CH3-CH2-CH2-CH2Br
(major)
CH3HBr
Peroxideor
ROOR
CH3
Br(major)
HALOHYDRIN FORMATION
Treatment of an alkene with a halogen X2 and H2O forms a halohydrin by addition of the elements of X and OH to the double bond.
4
HALOHYDRIN FORMATION
When ethene is bubbled into bromine water, the reddish-brown water is rapidly decolorised and
2-bromoethanol is produced.
CH2=CH2 + Br2/H2O CH2CH2
Br OH
room temperature
orange
colorless
Unsymmetrical alkenes: the preferred product has the electrophile X+ bonded to the carbon with more H
atom, and the nucleophile (H2O) bonded to the carbon with less hydrogen
4
HALOHYDRIN FORMATION
Although the combination of Br2 and H2O effectively forms bromohydrins from alkenes,
other reagents can also be used.Bromohydrins are also formed with N-
bromosuccinimide (NBS) in aqueous DMSO [(CH3)2S=O].
In H2O, NBS decomposes to form Br2, which then goes on to form a bromohydrin by the same
reaction mechanism.
4
HYDRATION: ELECTROPHILIC ADDITION OF WATER
Hydration is the addition of water to an alkene to form an alcohol.
Markovnikov’s rule holds.
5
HYDRATION: ELECTROPHILIC ADDITION OF WATER
5
HYDRATION: ELECTROPHILIC ADDITION OF WATER
5
FIVE ADDITION REACTIONS OF
CYCLOHEXENE
OXIDATION REACTIONS
OZONOLYSIS OF ALKENES1
1st step - Reaction of alkene with ozone to form ozonide.
2nd step - hydrolysis of ozonide to form aldehyde and ketone.
+ O3
C CO
O O
C C
C O CO+H2O, Zn
R R’
R”RH
H
R’
R”
ozonide
aldehyde ketone
CLEAVAGE OF DOUBLE BOND
OZONOLYSIS OF ALKENES1
examples
H
O2
acetaldehyde
O3, Zn
H2O, H+
O3, Zn
H2O, H+H
O+
acetaldehyde
O
propan-2-one
O3, Zn
H2O, H+
O
O
CLEAVAGE OF DOUBLE BOND
REACTION WITH HOT CONC. KMnO4/H+2
H2C C CH3
H
conc. KMnO4/ H+
CH H
O
+ CH
O
CH3
HCOOH
O
C CH3HO
O
CO2 & H2O
(methanal) aldehyde
carboxylic acidIf the product is methanal, it will oxidised to HCOOH and then
form carbon dioxide and water.
If the product is other aldehyde containing more than one carbon atom, it will oxidised to carboxylic acid.
CLEAVAGE OF DOUBLE BOND
OXIDATION WITH COLD DILUTE KMnO4/H+
An alkene will react with dilute KMnO4 in acidic/alkaline solution to give a diol.
CH2=CH2 + [O] + H2O C C
H
OH
H
OH
H
H
Ethane-1,2-diol
dilute MnO4-/H+
room temperature
When ethene is added to dilute KMnO4 in alkaline conditions at room temperature, the purple color of
the KMnO4 is decolorized, producing ethane-1,2,-diol and a brown precipitate of manganese (VII)
oxide.
purple
3
REACTION WITH COLD DILUTE KMnO4/H+3
C C
H
H
H
H
KMnO4 / H+
cold C C
H
H
OH
H
H
OH
CH3
CH3 KMnO4 / H+
cold
CH3
CH3
OH
OH
TRY THIS!!!
Q1: Base on the reaction below, draw the structure of the organic
products formed.
Q2:
An alkene X, containing 8 carbon atoms reacts with hot acidified potassium manganate (VII) solution to form propanoic acid and pentanoic acid. Draw the two forms of which exist as cis-trans isomers.
The end
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