Alkenes

1
Alkenes

• (chapter 31)

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Preparation
Industrial - cracking

• Laboratory
• 1. Elimination
• Dehydrohalogenation of haloalkanes
• RX gt alkene
• b. Dehydration of alkanols
• ROH gt alkene
• Hydrogenation of alkynes

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Physical properties
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Chemical properties

• Weaker ? bond (Bond energy CC 611
• C-C 346)
• More reactive than alkanes.
• electrons in CC bond are easily
• polarized, acts as a source of electrons,
• attacked by electrophiles.

E?---N?-
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Electrophilic Additions of Alkenes
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Electrophilic Additions of Alkenes
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Mechanism of Addition reactions
Carbonium ion as intermediate (H-X, acidic
reagents)
Two steps
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Orientation of Addition reactions
CH3CHCH2 H-X gt CH3CHXCH3 CH3CH2CH2X
(major)
Markovnikovs rule In addition of HX to
alkenes, hydrogen adds to the doubly-bonded
carbon that has the greater number of hydrogen
already attached to it.
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Orientation of Addition reactions
(-R group has inductive effect, stabilizes the
carbocation.)
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Orientation of Addition reactions
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Electrophilic Additions of Alkenes
H2O
Uses Produce alkanol Separate alkenes from
alkanes
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Catalytic Hydrogenation
Transition metals are able to adsorb hydrogen on
to their surface to form metal-hydrogen
bond. The alkene molecule then reacts with these
adsorbed hydrogen. The lowered activation energy
makes the reaction goes faster.
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Catalytic Hydrogenation
Heterolytic catalyst
Exothermic
Stereochemistry The two H atoms are added from
the same side of the ?-bond of the alkene
molecule. (syn or cis-addition)
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Hardening of oils - Margarine
Margarine is made from vegetable oils by the
hydrogenation of double bonds in the
oil. Hydrogenation converts liquid oils
(polyunsaturated fats) into semi-solid fats
(partially saturated fats).
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Hardening of oils - Margarine
3 H2
vegetable oil
Powdered Ni catalyst, 420K and 5 atm. pressure
margarine
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Link
Check point 31-2
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Ozonolysis
1. O3
CH2CH2
2 HCHO
2. Zn,H2O
Step 1 Oxidation Step 2 Hydrolysis by adding
water, zinc is used to prevent H2O2
from oxidizing the aldehydes.
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Ozonolysis
By analysing the products from ozonolysis, the
position of the CC bond in the alkene molecule,
and hence the structure can be determined.
X CH3CHC(CH3)2
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Ozonolysis
Check Point 31-3
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Polymerization
O2,200-400oC
n CH2CH2 ? (-CH2CH2-)n
Poly(ethene)
1500 atm
n 700 800 Molar mass 20000 - 25000
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Polymerization
Free radical mechanism Chain Initiation
RO-OR ? 2RO (organic peroxide) RO
CH2CH2 ? RO-CH2-CH2 Chain Propagation
RO-CH2-CH2 CH2CH2 ? RO-CH2CH2-CH2-CH2 Ch
ain Termination 2 RO-(CH2CH2)m-CH2-CH2 ?
RO-(CH2CH2)m-CH2-CH2-CH2-CH2-(CH2CH2)m-OR
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Low Density poly(ethene) LDPE Condition high
pressure, 1500 atm, 200oC. Consists of mainly
irregularly packed, branched chain polymers.
Properties highly deformable, low tensile
strength and low m.p.
(105oC) Uses plastic bags, wrappers, squeeze
bottles.
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High Density poly(ethene) HDPE Condition lower
pressure (2-6 atm), 60oC. Ziegler-Natta Catalyst
(ionic mechanism). Consists of regularly packed,
linear polymers with extensive crystalline region.
Uses Rigid articles such as refrigerator ice
trays, buckets, crates.
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More rigid than HDPE. Regular structure, -CH3
group arranged on one side (isotactic) of the
polymer chain.
Uses Crakes, kitchenware food containers,
fibres for making hard-wearing carpets.
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(No Transcript)
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Poly(phenylethene) or Polystyrene
peroxides nC6H5-CHCH2 ?
(-CH-CH2-)n reflux in
kerosene C6H5
Stiffer than poly(ethene), greater Van der
Waals force due to the benzene rings. Uses
Toys, cups, refrigerator parts.
Expanded polystyrene for packaging, heat
and sound insulation.
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Past AL papers
Markovnikovs rule and Mechanism of
Electrophilic addition 90II Q.8 (4b) 91 I
Q.1 (6a) 93II Q.9 (20b) 94 I Q.3 (21b) 96II
Q.9 (37b)
Polymerisation of alkenes 93I (17) 94II Q.9
(26b) 95I (29) 97I Q.4 (38c)
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Practice questions
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Practice questions
2. (CH3)2CCHCH3 I-Cl gt ?
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Practice questions
3. H2CCHCF3 HCl gt ?
Ans. CH2ClCH2CH3