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Tema 1.9 Session2
 

Tema 1.9 Session2

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    Tema 1.9 Session2 Tema 1.9 Session2 Presentation Transcript

    • Mòdul MD2301 - Formació general; 564497 – Química orgànica i inorgànica MASTER ON DRUGS, COSMETICS AND FOOD QUALITY Dr. Antonio Delgado Universitat de Barcelona October 2009
    • 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.
        •  H o for the above reaction can be estimated from the relevant bond energies:
          •  H o = (DH o  bond + DH o A-B ) – (DH o C-A + DH o C-B )
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    • Catalytic hydrogenation
      • Hydrogenation takes place on the surface of a heterogeneous catalyst.
        • In the absence of a catalyst, hydrogenations of alkenes, although exothermic, do not spontaneously occur, even at high temperatures.
        • In the presence of a catalyst, the same hydrogenations proceed at a steady rate, even at room temperature.
        • The most frequently used catalysts for hydrogenation reactions are:
          • Palladium dispersed on carbon (Pd-C)
          • Collodial platinum (Adam’s catalyst, PtO 2 )
          • Nickel (Raney nickel, Ra-Ni)
    • Hydrogenation is a sterospecific process (syn addition)
    • 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
    • Bromination is a sterospecific process (anti addition)
    • A cyclic bromonium ion explains the stereochemistry
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      • The bromonium ion can be trapped by other nucleophiles (synthetic utility)
        • Bromination of cyclopentene using water as the solvent gives the vicinal bromoalcohol (bromohydrin).
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    • Electrophilic Addition of Hydrogen Halides to alkenes
    • This result is known as Markovnikov’s rule and is based on the stability of the carbocation formed by the addition of the proton
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    • Electrophilic hydration of alkenes: synthesis of alcohols by thermodynamic control
        • Electrophilic hydration results when an alkene is exposed to an aqueous solution of sulfuric acid (HSO 4 - is a poor nucleophile).
      Electrophilic hydration of alkenes
    • Nucleophilic addition at the carbonyl group
    • Nucleophilic addition at the carbonyl group
    • Nucleophilic addition of hydrides and Grignard reagents
        • Reactions between a primary amine and an aldehyde or ketone , in which two molecules are joined with the elimination of water, are called condensations .
      Nucleophilic addition of primary amines
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    • Enolates as nucleophiles (C-C bond formation)
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    • One molecule of aldehyde acts as the nucleophile and the other as the electrophile