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Alkenes and Electrophilic Addition

Alkenes and Electrophilic Addition. Preparation of Alkenes. A. Industrial preparation. Cracking. Prepared by the cracking of alkanes of high molecular masses Give alkenes of low molecular masses. 600 o C. 2CH 3 CH 3  CH 2 = CH 2 + 2CH 4.

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Alkenes and Electrophilic Addition

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  1. Alkenes and Electrophilic Addition New Way Chemistry for Hong Kong A-Level 3A

  2. Preparation of Alkenes New Way Chemistry for Hong Kong A-Level 3A

  3. A. Industrial preparation Cracking • Prepared by the cracking of alkanes of high molecular masses • Give alkenes of low molecular masses New Way Chemistry for Hong Kong A-Level 3A

  4. 600 oC 2CH3CH3 CH2 = CH2 + 2CH4 2CH3CH2CH3 CH3CH = CH2 + CH2 = CH2 + CH4 + H2 600 oC Cracking e.g. New Way Chemistry for Hong Kong A-Level 3A

  5. B. Synthetic preparation Elimination Reactions • Involve removal of atoms or groups of atoms from adjacent carbon atoms in the reactant molecule • Formation of a double bond between carbon atoms New Way Chemistry for Hong Kong A-Level 3A

  6. 1. Intramolecular Dehydration of Alcohols • Removal of a water molecule from a reactant molecule • By heating the alcohols in the presence of a dehydrating agent. • E.g. Alumina(Al2O3), conc. H2SO4, • conc. H3PO4 • Give alkenesand water as the products New Way Chemistry for Hong Kong A-Level 3A

  7. 1. Intramolecular Dehydration of Alcohols New Way Chemistry for Hong Kong A-Level 3A

  8. 1. Intramolecular Dehydration of Alcohols • Experimental conditions (i.e. temperature and concentration of concentrated sulphuric acid) • is closely related to the structure of the individual alcohol. New Way Chemistry for Hong Kong A-Level 3A

  9. 1. Intramolecular Dehydration of Alcohols • Primary alcohols generally required concentrated sulphuric acid and a relatively high temperature New Way Chemistry for Hong Kong A-Level 3A

  10. 1. Intramolecular Dehydration of Alcohols • Secondaryalcohols are intermediate in reactivity • Tertiary alcohols dehydrate under mild conditions (moderatetemperature and dilute sulphuric acid) New Way Chemistry for Hong Kong A-Level 3A

  11. > > Tertiary alcohol Secondary alcohol Primaryalcohol 1. Intramolecular Dehydration of Alcohols • The relative ease of dehydration of alcohols generally decreases in the order: New Way Chemistry for Hong Kong A-Level 3A

  12. Intramolecular vs intermolecular Substitution New Way Chemistry for Hong Kong A-Level 3A

  13. Intramolecular dehydration is favoured at higher temperatures because it involves breaking of strong C – H bonds. New Way Chemistry for Hong Kong A-Level 3A

  14. Q.29(a) New Way Chemistry for Hong Kong A-Level 3A

  15. Q.29(b) New Way Chemistry for Hong Kong A-Level 3A

  16.  Q.29(c) New Way Chemistry for Hong Kong A-Level 3A

  17. 1. Intramolecular Dehydration of Alcohols • Secondary and tertiary alcohols may dehydrate to give a mixture of alkenes • The more highly substituted alkene is formed as the major product New Way Chemistry for Hong Kong A-Level 3A

  18. 2. Dehydrohalogenation of haloalkanes • Elimination of a hydrogen halide molecule from a haloalkane • By heating the haloalkane in an alcoholic solution of KOH New Way Chemistry for Hong Kong A-Level 3A

  19. C2H5OH is a co-solvent for both RX and OH New Way Chemistry for Hong Kong A-Level 3A

  20. 2. Dehyhalogenation of haloalkanes e.g. New Way Chemistry for Hong Kong A-Level 3A

  21. 2. Dehyhalogenation of haloalkanes • Dehydrohalogenation of secondary or tertiary haloalkanes can take place in more than one way • A mixture of alkenes is formed New Way Chemistry for Hong Kong A-Level 3A

  22. Q.30(a) New Way Chemistry for Hong Kong A-Level 3A

  23. Q.30(b) New Way Chemistry for Hong Kong A-Level 3A

  24. > > Primaryhaloalkane Tertiary haloalkane Secondary haloalkane 2. Dehyhalogenation of haloalkanes • The ease of dehydrohalogenation of haloalkanes decreases in the order: New Way Chemistry for Hong Kong A-Level 3A

  25. The relative stabilities of alkenes decrease in the order: New Way Chemistry for Hong Kong A-Level 3A

  26. Relative Stability of Alkenes in Terms of Enthalpy Changes of Hydrogenation • Hydrogenation of alkenes is exothermic • From enthalpy changes of hydrogenation •  predict the relative stabilities of alkenes New Way Chemistry for Hong Kong A-Level 3A

  27. Enthalpy changes of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene New Way Chemistry for Hong Kong A-Level 3A

  28. Relative Stability of Alkenes in Terms of Enthalpy Changes of Hydrogenation • The pattern of the relative stabilities of alkenes determined from the enthalpy changes of hydrogenation: New Way Chemistry for Hong Kong A-Level 3A

  29. Addition Reactions Hydrogenation of alkynes • Alkenes can be prepared by hydrogenation of alkynes •  Depend on the conditions and the catalyst employed New Way Chemistry for Hong Kong A-Level 3A

  30. Hydrogenation • Lindlar’s catalyst is metallic palladium(Pd) deposited on calcium carbonate • further hydrogenation of the alkenes formed can be prevented New Way Chemistry for Hong Kong A-Level 3A

  31. Reactions of Alkenes An Introduction New Way Chemistry for Hong Kong A-Level 3A

  32. Alkenes are more reactive than alkanes • Undergoes addition reaction rather than substitution New Way Chemistry for Hong Kong A-Level 3A

  33. Presence of C=C double bond • C=C double bond is made up of a  bond and a  bond • Addition reactions only involve breaking of weaker  bonds of alkenes New Way Chemistry for Hong Kong A-Level 3A

  34. The electrons of the  bond are •  diffuse in shape •  less firmly held by the bonding carbon nuclei Susceptible to the attack by electrophiles New Way Chemistry for Hong Kong A-Level 3A

  35. Electrophiles : - Electron-deficient species Attack electron-rich center e.g. C=C bond Examples : Cations : H+, Br+, R+,… (lead to heterolysis) Free radicals : H, Cl, R,…(lead to homolysis) New Way Chemistry for Hong Kong A-Level 3A

  36. All have lone pairs for donating to the reaction sites Nucleophiles : - Electron-rich species Attack electron-deficient site e.g. carbonyl carbon, C=O Examples : anions : OH, Br, RO,… molecules : H2O, ROH, NH3 All lead to heterolytic fissions New Way Chemistry for Hong Kong A-Level 3A

  37. Reactions of Alkenes Examples New Way Chemistry for Hong Kong A-Level 3A

  38. Catalytic Hydrogenation • Alkenes react with hydrogen in the presence of metal catalysts (e.g. Ni, Pd, Pt) to give alkanes Lower temperatures can be used with Pd or Pt New Way Chemistry for Hong Kong A-Level 3A

  39. Catalytic Hydrogenation e.g. cis-addition, refer to notes on ‘chemical kinetics’, pp.36-37) New Way Chemistry for Hong Kong A-Level 3A

  40. Catalytic Hydrogenation Under mild conditions, C=O and benzene ring are unaffected. New Way Chemistry for Hong Kong A-Level 3A

  41. Q.31 New Way Chemistry for Hong Kong A-Level 3A

  42. A / B New Way Chemistry for Hong Kong A-Level 3A

  43. A / B New Way Chemistry for Hong Kong A-Level 3A

  44. * * * * * * C New Way Chemistry for Hong Kong A-Level 3A

  45. Partial hydrogenation Complete hydrogenation Application : - hardening of plant oils Plant oil (polyunsaturated liquid with low m.p.) Margarine (soft unsat’d solid with higher m.p.) Animal fat (hard sat’d solid with still higher m.p.) New Way Chemistry for Hong Kong A-Level 3A

  46. Catalytic Hydrogenation • Fats and oils are organic compounds called triglycerides •  triesters formed from glycerol and carboxylic acids of long carbon chains New Way Chemistry for Hong Kong A-Level 3A

  47. Catalytic Hydrogenation • Saturated fats •  solids at room temp •  usually come from animal sources •  long carbon chains are zig-zag and easily packed New Way Chemistry for Hong Kong A-Level 3A

  48. Catalytic Hydrogenation • Unsaturated oils •  liquids at room temp •  usually come from plant sources •  lower m.p. due to cis-arrangement (kinked shape) New Way Chemistry for Hong Kong A-Level 3A

  49. Catalytic Hydrogenation • Fats are stable towards oxidation by air • More convenient to handle and store New Way Chemistry for Hong Kong A-Level 3A

  50. Catalytic Hydrogenation • Advantages: • higher m.p.  ideal for baking • turning rancidmuch less readily than unsaturated oils New Way Chemistry for Hong Kong A-Level 3A

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