chapter 3.1-alkene (1).pdf
TRANSCRIPT
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CHAPTER 3: UNSATURATED HYDROCARBON ALKENES
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Unsaturated Hydrocarbon
Hydrocarbons that do not contain the maximum amount of hydrogen
NOT all carbon atoms have four single covalent bonds
One or more double bonds or triple bonds between carbon atoms
Alkenes
hydrocarbons that contain double covalent bonds
General formula for alkenes with one double bond is CnH2n
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Alkenes are nonpolar
show trends in properties similar to those of alkanes in boiling points and physical states.
EX:
-farnesene, a solid at room temperature, is found in the natural wax covering of apples
Properties of Alkenes
H3C
C
HC
CH2
H2C
C
HC
CH2
HC
C
HC
CH2
CH3 CH3 CH3
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Rules similar to naming alkanes
parent hydrocarbon
longest continuous chain of carbon atoms that contains the double bond
Number so the double bond has the lowest number
Systematic Names of Alkenes
pentene
H2C CH2C
H2C CH3
H2C CH3
hexane
H2C CH2C
H2C CH3
H2C CH3
not:
1-pentene
H2C CH2C
H2C CH3
H2C CH3
1 2 3 4 5
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IUPAC RULES
RULE 1. Select the longest continuous carbon chain that contains a double bond.
This chain
contains 6
carbon atoms
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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 is the longest
continuous chain.
Select it as the parent
compound.
Name the parent
compound octene.
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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.
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IUPAC RULES
This end of the chain is closest to the double bond. Begin numbering here.
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The name of the parent compound is 1-octene.
IUPAC RULES
8
7
4 3 2 1
6
5
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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.
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IUPAC RULES
This is an ethyl group.
8
7
4 3 2 1
6
5
The ethyl group is attached to carbon 4.
4
4-ethyl-1-octene
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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.
CH2 C C CH2H H
IUPAC names: 1,3-butadiene 1,3,5-heptatriene
new IUPAC names: buta-1,3-diene hepta-1,3,5-triene
1 2 3 4
CH3 C C C C C CH2
12347 6 5
H H H H H
1 2
3
47
6 5
8
IUPAC names: 1,3, 5, 7-cyclooctatetraene
new IUPAC names: cycloocta-1,3,5,7-tetraene
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CYCLOALKENES
Contains C=C in the ring
cyclopropene cyclobutene cyclohexenecyclopentene
Nomenclature of cycloalkenes:
- Similar to that alkenes
General formula CnH2n-2 where n =3,4,5..
CH31
2
34
5
6
1
23
4
5
1-methylcyclohexene 1,5-dimethylcyclopentene
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1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene.
2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent.
6-Ethyl-1-methylcyclohexene CH3
CH2CH3
1
2 3
4
5 6
NAMING CYCLOALKENES
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Number substituted cycloalkenes in the way that gives the carbon atoms of the double bond the 1 and 2 positions. That also gives the substituent groups the lower numbers at the first point of difference.
5
1
4
2
3
CH3 2
3
1
4
6
5
CH3CH3
1-methylcyclopentene 3,5-dimethylcyclohexene
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NOMENCLATURE OF cis-trans
ISOMERS
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
C CH3C
H
CH2CH3
H
C CH3C
H
H
CH2CH3
cis-2-pentene trans-2-pentene
geometric isomers (diastereomers)
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C=C are called vinyl carbons
If either vinyl carbon is bonded to two equivalent groups, then no geometric isomerism exists.
CH3CH=CHCH3 CH3CH2CH=CH2
YES NO
CH3 (CH3)2C=CHCH3 CH3CH=CCH2CH3 NO YES
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Confusion about the use of cis- and trans-. According to IUPAC rules it refers to the parent chain.
cis-
????????
C C
H
H3C CH2CH3
CH3
C C
H3C
H Br
Cl
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E/Z system is now recommended by IUPAC for the designation of geometric isomerism.
1. Use the sequence rules to assign the higher priority * to the two groups attached to each vinyl carbon.
2. * * *
*
(Z)- zusammen (E)- entgegen
together opposite
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C C
H
H3C CH2CH3
CH3
C C
H3C
H Br
Cl
*
* *
*
(Z)-
(E)-
-
C C
H
H3C CH2CH3
CH3
C C
H3C
H Br
Cl
*
* *
*
(Z)-3-methyl-2-pentene
(3-methyl-cis-2-pentene)
(E)-1-bromo-1-chloropropene
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CH3
CH3CH2 CHCH2CH3
/
C = C 3-ethyl-5-methyl-3-heptene
/
CH3CH2 H (NOT a geometric isomer)
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SYNTHESIS OF ALKENES
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SYNTHESIS OF ALKENES
DeHydration
Removal of Water
DeHydrohalogenation
Removal of Hydrogen Halides
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SYNTHESIS OF ALKENES
Alkenes can be prepared in the following ways:
i) Dehydration of alcohols
conc. H2SO4
R-CH2-CH2-OH R-CH=CH2 + H2O
Loss of H and OH from adjacent carbons. Acid catalyst is necessary. (H2SO4 / H3PO4) The reactions are carried out at high temperature; depending on type of alcohols used.
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SYNTHESIS OF ALKENES
Alkenes can be prepared in the following ways:
ii) Dehydrohalogenation of haloalkanes
*CH3CH2O-Na+ in
ethanol
R-CH2-CH2-X
(reflux)
R-CH=CH2 + HX
*Potassium ethoxide
Loss of H and halogen (X) from an alkyl halide
In the presence of strong base in solvent
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Saytzeff rule:
- A reaction that produces an alkene would favour the formation of an alkene that has the greatest number of substituents attached to the C=C group.
CH3CH2-CH-CH3OH
H+
H+
CH3CH=CH-CH3 + H2O
CH3CH2-CH=CH2 + H2O
2-butanol2-butenemajor product
1-butene
CH3CH-CH-CH2
BrH H
KOH CH3CH=CH-CH3 CH3CH2CH=CH2alcohol
reflux
2-bromobutane2-butene
(major product)1-butene
Dehydration of alcohols
Dehydrohalogenation of haloalkanes
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REACTION OF ALKENES
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REACTION OF ALKENES
Hydrogenations Addition of Hydrogen
Halogenation
Addition of Halogen
Hydrohalogenation
Addition of Hydrogen Halides
Hydration
Addition of Water
Ozonolysis
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Reaction of Alkenes
Primarily reactions involve the double bond
The key reaction of double bond is addition reaction (Breaking the bond and adding something to each carbon)
+ A - B
A B
The major alkene reactions include additions of hydrogen (H2),halogen ( CI2 or Br2), water (HOH) or hydrogen halides (HBr or HCI)
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Why do alkenes undergo addition reactions?
Addition Reactions
Carbon-carbon double bonds in alkenes are reactive.
readily undergoes addition reactions
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In an addition reaction, carbon-carbon double bonds become single bonds. This means that an unsaturated hydrocarbon becomes a saturated organic compound.
+
Unsaturated
hydrocarbon
Saturated organic
compound
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(1) Catalytic hydrogenation:
- hydrogenation: addition of hydrogen to a double bond and triple bond to yield saturated product.
- alkenes will combine with hydrogen in the present to catalyst to form alkanes.
C C H H C C
H H
Pt or Pd
25-90oC
- 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.
Reaction of Alkenes
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H2C CH2 H2Pt
CH3CH2CH2CH2CH CH2 H2Pt
H3C CH3
CH3CH2CH2CH2CH2CH3
EXAMPLES:
ethylene ethanelow pressure
low pressurehexene hexane
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(2) 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.
C C X X C C
X X
inert solvent
X X = halogen such as Br2 or Cl2Inert solvent = CCl4 or CH2Cl2
Reaction of Alkenes
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EXAMPLES:
C C
HH
H H Br Br
Br2
Br
Br
CCl4
CH3CH=CH2 Cl2CCl4
CH3CH
Cl
CH2
Cl
C C
Br
H H
Br
H H
inert solvent (CCl4)
ethene1,2-dibromoethane
* the red-brown colour of the bromine solution will fade and the
solution becomes colourless.
cyclohexene1,2-dibromocyclohexane
propene 1,2-dichloropropane
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(3) Addition of hydrogen halides:
- Addition reaction with electrophilic reagents.
- Alkenes react with hydrogen halides (in gaseous state or in aqueous solution) to form addition products.
- The hydrogen and halogen atoms add across the double bond to form haloalkanes (alkyl halides).
- General equation:
C C C C
H X
HX
alkene haloalkane
- Reactivity of hydrogen halides : HF < HCl < HBr < HI
Reaction of Alkenes
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* Reaction with HCl needs a catalyst such as AlCl3
H2C CH2 HClAlCl3
CH3CH2Cl
H-I
CH3CH=CHCH3 + H-Br
I
CH3CH2CHCH3
Br
EXAMPLES:
cyclopentene iodocyclopentane
2-butene 2-bromobutane
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MARKOVNIKOVS 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.
C C
H
HCH3
H
H-I
C C
H
HCH3
H
H-I
C C
H
HCH3
H
H I
C C
H
HCH3
H
I H
1-iodopropane
2-iodopropane
(major product)
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Markovnikovs rules:
- the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms.
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(5) Addition reaction with acidified water (H3O+): hydration of
alkenes
Hydration: The addition of H atoms and OH groups from water molecules to a multiple bond.
Reverse of the dehydration reaction.
Direct hydration of ethene:
- passing a mixture of ethene and steam over phosphoric (v) acid (H3PO4) absorbed on silica pellets at 300
oC and a pressure of 60 atmospheres.
- H3PO4 is a catalyst.
CH2=CH2 H2OH3PO4
CH3CH2OH(g) (g)300
oC, 60 atm
(g)
ethene ethanol
C C H2O C C
H OH
alkene alcohol
H+
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Markovnikovs rule is apply to the addition of a water molecule across the double bond of an unsymmetrical
alkene.
For examples:
CH3 C CH2
CH3
H OH H+
CH3CH=CH2 + H2O CH3CHCH3
OH
CH3 C CH2
CH3
OH H
25oC
2-methylpropene
tert-butyl alcohol
propene
2-propanol
H+
H+ = catalyst
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(5) Ozonolysis:
- The reaction of alkenes with ozone (O3) to form an ozonide, followed by hydrolysis of the ozonide to produce aldehydes and /or ketone.
- Widely used to determine the position of the carbon-carbon double bond.
- Ozonolysis is milder and both ketone and aldehydes can be recovered without further oxidation.
OZONOLYSIS OF ALKENES
C C
R
R
R'
H
O3 C
O O
CO R'
H
R
R
(CH3)2S
C O
R
RCO
R'
Hozonide ketone aldehyde
or H2O, Zn/H+
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EXAMPLES:
H
OCH3CH3O
H
H
O
O
OCH3
H
O
CH3O
O
H
O
H
Oi) O3
ii) (CH3)2S3-nonene
i) O3
ii) (CH3)2S
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Ethylene and propylene are the largest-volume industrial organic chemicals.
Used to synthesis a wide variety of useful compounds.
USES OF ALKENES
CH3 C
O
OH
CH2 CH2
CI CI
Cl2C C
H
H
H
H
CH3 C
O
H
O2
C C
CIH
H H
CH3 CH2
OH
NaOH
C C
H H
HH
H+
H2O
CH2 CH2
OHOH
O
H2C CH2
n
polyethylene
polymerize
acetaldehyde
oxidize
oxidize
acetic acid
ethylene ethylene dichloride
vinyl chloride
H2O
catalyst
Ag catalystethylene oxide
ethylene glycol ethanol
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The most popular plastic.
Uses:
i) Grocery bags
ii)Shampoo bottles
iii)Children's toy
iv)Bullet proof vests
v)Film wrapping
vi)Kitchenware
POLYETHENE (PE)
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POLYVINYL CHLORIDE (PVC)
C C
H H
CIH C C
H
H
CI
H
C
H
H
C
CI
H
C
H
H
C
CI
Hnvinyl chloride
polymerize
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.
uPVC or Rigid PVC is used in the building industry as a low-maintenance material.
Ceiling tiles.
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USES 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.
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In short, cracking is used to produce:
Petrol (fuel) Short-chain alkenes
(starting materials for
making ethanol and
plastics)
Hydrogen (fuel and raw material
for Haber process)
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