Tema 1.9 Session2
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Transcript of Tema 1.9 Session2
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Mòdul MD2301 - Formació general; 564497 – Química orgànica i inorgànica
MASTER ON DRUGS, COSMETICS AND FOOD QUALITYMASTER ON DRUGS, COSMETICS AND FOOD QUALITY
Dr. Antonio DelgadoUniversitat de BarcelonaOctober 2009
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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.
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Ho for the above reaction can be estimated from the relevant bond energies:
Ho = (DHo bond + DHo
A-B) – (DHoC-A + DHo
C-B)
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Catalytic hydrogenation
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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, PtO2)•Nickel (Raney nickel, Ra-Ni)
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Hydrogenation is a sterospecific process(syn addition)
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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
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Bromination is a sterospecific process(anti addition)
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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
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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
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Electrophilic hydration results when an alkene is exposed to an aqueous solution of sulfuric acid (HSO4
- is a poor nucleophile).
Electrophilic hydration of alkenes
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Nucleophilic addition at the carbonyl group
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Nucleophilic addition at the carbonyl group
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Nucleophilic addition of hydrides and Grignard reagents
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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