Alkenes

Alkenes (chapter 31)

Preparation Industrial - cracking Laboratory 1. Elimination a. Dehydrohalogenation of haloalkanes RX => alkene b. Dehydration of alkanols ROH => alkene 4. Hydrogenation of alkynes

Physical properties Alkene

m.p. oC

b.p. oC

density

CH2=CH2

-169.4

-102.4

0.610

CH2=CHCH3

-185.0

-47.7

0.610

CH2=CHCH2CH3

-185.0

-6.5

0.643

Cis-CH3CH=CHCH3

-139.0

3.7

0.621

Trans-CH3CH=CHCH3

-106.0

2.9

0.604

CH2CH=C(CH3)2

-140.7

-6.6

0.627

CH2=CH(CH2)2CH3

-138.0

30.1

0.643

Chemical properties Weaker π bond (Bond energy: C=C 611 C-C 346) More reactive than alkanes.

Eσ+---Nσ-

π electrons in C=C bond are easily polarized, acts as a source of electrons, attacked by electrophiles.

Electrophilic Additions of Alkenes

C=C

δ+ δ-

δ+ δ-

δ+ δ-

+ E-N

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 C=C

+ H-X

+ H-OSO3H + H2O + X2

H

+

C C

Hydrohalogenation

H X C C

Hydrogensulphate

H OSO3H Hydration C C H OH C C

Halogenation

X X + X-OH

C C X OH

Halohydrin formation

Mechanism of Addition reactions Carbonium ion as intermediate (H-X, acidic reagents) Two steps: C=C

δ+ δ-

C C H

+

δ+ δ-

+ H-X

C C +

+ X-

H

C C H X

+ X-

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.

Orientation of Addition reactions CH3CH=CH2 + H-X

CH3-CH-CH3

(more stable)

+

X-

CH3-CH2-CH2 +

(less stable)

X-

CH3CHXCH3

CH3CH2CH2X

(major product)

(minor product)

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

Orientation of Addition reactions

H

CH3-CH2-CH2 + CH3-CH-CH3 + CH3CH2CH2X CH3CH=CH2 + H-X CH3CHXCH3

Reaction coordinate

Electrophilic Additions of Alkenes C=C

+ c. H2SO4

C C H OSO3H (Alkyl Hydrogensulphate) H2O

C C H OH (Alkanol)

Uses: Produce alkanol Separate alkenes from alkanes

Catalytic Hydrogenation Pt/Pd/Ni CH3CH=CHCH3 + H2

heat, pressure

H H

C C

Nickel

CH3CH2CH2CH3

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.

Catalytic Hydrogenation Heterolytic catalyst Exothermic Stereochemistry: The two H atoms are added from the same side of the π-bond of the alkene molecule. (syn or cis-addition)

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).

Hardening of oils - Margarine CH2-OOC(CH2)14CH3 + 3 H2 CH-OO(CH2)7CH=CH(CH2)7CH3 CH2-OO(CH2)6(CH2CH=CH)3CH2CH3 vegetable oil Powdered Ni catalyst, 420K and 5 atm. pressure

CH2-OOC(CH2)14CH3 CH-OO(CH2)7CH=CH(CH2)7CH3 CH2-OO(CH2)16CH3 margarine

Link

Check point 31-2

Ozonolysis

CH2=CH2

1. O3

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.

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. e.g. X

1. O3 2. Zn,H2O

CH3CHO + CH3COCH3

X: CH3CH=C(CH3)2

Ozonolysis Predict the structures of the following hydrocarbons using the information: • OHC-(CH2)4-CHO (C6H10) • CH3CHO, OHC-CH2-CHO (C10H16) Check Point 31-3

Polymerization

n CH2=CH2

O2,200-400oC

→

1500 atm

(-CH2CH2-)n Poly(ethene) n = 700 – 800 Molar mass 20000 - 25000

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

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.

High Density poly(ethene) HDPE Condition: lower pressure (2-6 atm), 60oC. ZieglerNatta Catalyst (ionic mechanism). Consists of regularly packed, linear polymers with extensive crystalline region.

Uses: Rigid articles such as refrigerator ice trays, buckets, crates.

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.

Isotactic (Me all on same side) CH3 H CH3 H CH3 H CH3 H CH3 H

Syndiotactic (Me on alternate sides) CH3 H H

CH3 CH3 H H

CH3 CH3 H

Atactic (Me randomly distributed) CH3 H CH3 H H

CH3 CH3 H H

CH3

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.

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)

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

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

Practice questions 3. H2C=CHCF3 + HCl => ?

Ans. CH2ClCH2CH3