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Alkenes. (chapter 31). Preparation. Industrial - cracking. Laboratory 1. Elimination Dehydrohalogenation of haloalkanes RX => alkene b. Dehydration of alkanols ROH => alkene Hydrogenation of alkynes. Physical properties. Chemical properties.

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Alkenes l.jpg

Alkenes

(chapter 31)


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Preparation

Industrial - cracking

  • Laboratory

  • 1. Elimination

    • Dehydrohalogenation of haloalkanes

    • RX => alkene

    • b. Dehydration of alkanols

    • ROH => alkene

  • Hydrogenation of alkynes


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Physical properties


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Chemical properties

Weaker  bond (Bond energy: C=C 611

C-C 346)

More reactive than alkanes.

  • electrons in C=C bond are easily

    polarized, acts as a source of electrons,

    attacked by electrophiles.

E+---N-


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+ -

+ E-N

C=C

C C

+ -

E N

+ -

E-N = H-Cl, H-Br, H-I, H-OSO3H,

H-OH (H3O+), Cl-Cl, Br-Br,

Br-OH, Cl-OH

Electrophilic Additions of Alkenes


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X OH

Hydrogensulphate

+ H-OSO3H

+ H-X

Hydrohalogenation

C=C

C C

C C

C C

C C

C C

H OSO3H

H X

H+

+ H2O

Hydration

H OH

+ X2

Halogenation

X X

Halohydrin

formation

+ X-OH

Electrophilic Additions of Alkenes


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+ -

+ H-X

C=C

C C

+ X-

C C

+

+ -

+

H

H

+ X-

C C

X

H

Mechanism of Addition reactions

Carbonium ion as intermediate (H-X, acidic reagents)

Two steps:


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Orientation of Addition reactions

CH3CH=CH2 + H-X => CH3CHXCH3 + CH3CH2CH2X

(major)

Markovnikov’s 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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CH3CH=CH2 + H-X

CH3-CH-CH3

CH3-CH2-CH2

(more stable)

(less stable)

+

+

X-

X-

CH3CH2CH2X

CH3CHXCH3

(major product)

(minor product)

Orientation of Addition reactions

(-R group has +inductive effect, stabilizes the carbocation.)


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CH3-CH2-CH2

+

H

CH3-CH-CH3

+

CH3CH2CH2X

CH3CH=CH2 + H-X

CH3CHXCH3

Reaction coordinate

Orientation of Addition reactions


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+ c. H2SO4

H OSO3H

H OH

(Alkyl Hydrogensulphate)

C=C

C C

C C

(Alkanol)

Electrophilic Additions of Alkenes

H2O

Uses:

Produce alkanol

Separate alkenes from alkanes


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Pt/Pd/Ni

CH3CH=CHCH3 + H2 CH3CH2CH2CH3

heat, pressure

H

C

H

C

Nickel

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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CH2-OOC(CH2)14CH3

CH-OO(CH2)7CH=CH(CH2)7CH3

CH2-OO(CH2)6(CH2CH=CH)3CH2CH3

CH2-OOC(CH2)14CH3

CH-OO(CH2)7CH=CH(CH2)7CH3

CH2-OO(CH2)16CH3

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

CH2=CH2

2 HCH=O

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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1. O3

e.g. X CH3CHO + CH3COCH3

2. Zn,H2O

Ozonolysis

By analysing the products from ozonolysis,

the position of the C=C bond in the alkene

molecule, and hence the structure can be

determined.

X: CH3CH=C(CH3)2


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Predict the structures of the following hydrocarbons

using the information:

  • OHC-(CH2)4-CHO (C6H10)

  • CH3CHO, OHC-CH2-CHO (C10H16)

Ozonolysis

Check Point 31-3


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Polymerization

O2,200-400oC

n CH2=CH2 (-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· + CH2=CH2  RO-CH2-CH2·

Chain Propagation

RO-CH2-CH2· + CH2=CH2 

RO-CH2CH2-CH2-CH2·

Chain 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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Poly(propene)

Ziegler-Natta Catalyst

nCH3-CH=CH2 (-CH-CH2-)n poly(propene)

CH3

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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Isotactic (Me all on same side)

CH3 H

CH3 H

CH3 H

CH3 H

CH3 H

CH3 H

CH3 H

H CH3

CH3 H

CH3 H

H CH3

H CH3

CH3 H

CH3 H

Syndiotactic (Me on alternate sides)

H CH3

Atactic (Me randomly distributed)


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Poly(phenylethene) or Polystyrene

peroxides

nC6H5-CH=CH2 (-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

Markovnikov’s 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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  • CH3 CH3

  • ?(Alkene) + ?(reagent) => CH3-C-CH2-C-CH3

  • H OH

CH3 CH3 CH3 CH3

Ans. CH3-C-CH=C-CH3 or CH3-C-CH2C=CH2

H H

Practice questions


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Ans. (CH3)2C-CH (CH3)

Cl I

Practice questions

2. (CH3)2C=CHCH3 + I-Cl => ?


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Practice questions

3. H2C=CHCF3 + HCl => ?

Ans. CH2ClCH2CH3


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