Tema 1.9 Session2

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