Tema 1.9 Session2


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Tema 1.9 Session2

  1. 1. Mòdul MD2301 - Formació general; 564497 – Química orgànica i inorgànica MASTER ON DRUGS, COSMETICS AND FOOD QUALITY Dr. Antonio Delgado Universitat de Barcelona October 2009
  2. 2. Session 2 Addition reactions . Electrophilic adittion to alkenes. Catalytic hydrogenation. Electrophilic addition of halogens. Electrophilic addition of hydrogen halides. Hydration of alkenes. Mechanistic considerations and synthetic applications. Nucleophilic addition and nucleophilic “addition-elimination” at the carbonyl group . Reactivity of ketones and carboxylic acid derivatives. The enolate as nucleophile: the aldol reaction. Synthetic applications.
  3. 3. <ul><ul><li> H o for the above reaction can be estimated from the relevant bond energies: </li></ul></ul><ul><ul><ul><li> H o = (DH o  bond + DH o A-B ) – (DH o C-A + DH o C-B ) </li></ul></ul></ul>
  4. 5. Catalytic hydrogenation
  5. 6. <ul><li>Hydrogenation takes place on the surface of a heterogeneous catalyst. </li></ul><ul><ul><li>In the absence of a catalyst, hydrogenations of alkenes, although exothermic, do not spontaneously occur, even at high temperatures. </li></ul></ul><ul><ul><li>In the presence of a catalyst, the same hydrogenations proceed at a steady rate, even at room temperature. </li></ul></ul><ul><ul><li>The most frequently used catalysts for hydrogenation reactions are: </li></ul></ul><ul><ul><ul><li>Palladium dispersed on carbon (Pd-C) </li></ul></ul></ul><ul><ul><ul><li>Collodial platinum (Adam’s catalyst, PtO 2 ) </li></ul></ul></ul><ul><ul><ul><li>Nickel (Raney nickel, Ra-Ni) </li></ul></ul></ul>
  6. 7. Hydrogenation is a sterospecific process (syn addition)
  7. 8. Electrophilic addition of halogens to alkenes The reaction with bromine results in a color change from red to colorless , which is sometimes used as a test for unsaturation
  8. 9. Bromination is a sterospecific process (anti addition)
  9. 10. A cyclic bromonium ion explains the stereochemistry
  10. 12. <ul><li>The bromonium ion can be trapped by other nucleophiles (synthetic utility) </li></ul><ul><ul><li>Bromination of cyclopentene using water as the solvent gives the vicinal bromoalcohol (bromohydrin). </li></ul></ul>
  11. 14. Electrophilic Addition of Hydrogen Halides to alkenes
  12. 15. This result is known as Markovnikov’s rule and is based on the stability of the carbocation formed by the addition of the proton
  13. 17. Electrophilic hydration of alkenes: synthesis of alcohols by thermodynamic control
  14. 18. <ul><ul><li>Electrophilic hydration results when an alkene is exposed to an aqueous solution of sulfuric acid (HSO 4 - is a poor nucleophile). </li></ul></ul>Electrophilic hydration of alkenes
  15. 19. Nucleophilic addition at the carbonyl group
  16. 20. Nucleophilic addition at the carbonyl group
  17. 21. Nucleophilic addition of hydrides and Grignard reagents
  18. 22. <ul><ul><li>Reactions between a primary amine and an aldehyde or ketone , in which two molecules are joined with the elimination of water, are called condensations . </li></ul></ul>Nucleophilic addition of primary amines
  19. 28. Enolates as nucleophiles (C-C bond formation)
  20. 30. One molecule of aldehyde acts as the nucleophile and the other as the electrophile