1 / 30

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.

tyne
Download Presentation

Alkenes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related