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Alkenes

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

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  1. Alkenes (chapter 31)

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

  3. Physical properties

  4. 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-

  5. + - + 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

  6. 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

  7. + - + 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:

  8. 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.

  9. 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.)

  10. CH3-CH2-CH2 + H CH3-CH-CH3 + CH3CH2CH2X CH3CH=CH2 + H-X CH3CHXCH3 Reaction coordinate Orientation of Addition reactions

  11. + 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

  12. 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.

  13. 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)

  14. 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).

  15. 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

  16. Link Check point 31-2

  17. 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.

  18. 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

  19. 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

  20. Polymerization O2,200-400oC n CH2=CH2 (-CH2CH2-)n Poly(ethene) 1500 atm n = 700 – 800 Molar mass 20000 - 25000

  21. 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

  22. 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.

  23. 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.

  24. 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.

  25. 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)

  26. 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.

  27. 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)

  28. 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

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

  30. Practice questions 3. H2C=CHCF3 + HCl => ? Ans. CH2ClCH2CH3

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