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Exercise: Birch reduction 2

Exercise

Which of the following statements are right?

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Catalytic hydrogenations of benzene and benzene derivatives usually do not yield completely hydrogenated products.

The Birch reduction is an example of a one-electron transfer mechanism.

Catalytic hydrogenations of benzene and benzene derivatives usually yield completely hydrogenated products. In contrast, a selective partial reduction can be achieved by a reaction with a one-electron transfer mechanism.

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Exercise

Which statements describe steps of the mechanism of the Birch reduction?

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The first step of the Birch reduction is a one-electron transfer into an antibonding π orbital of the aromatic system, which yields a radical anion.

A cyclopentadienyl anion is formed by protonation of the radical anion through ethanol.

The radical anion is protonated by ethanol, yielding a cyclohexadienyl radical.

By an additional one-electron transfer, the resonance-stabilized allyl radical is converted into a cyclohexadienyl anion that is subsequently protonated by ethanol.

By an additional one-electron transfer, the resonance-stabilized vinyl radical is converted into a cyclohexadienyl anion that is subsequently protonated by ethanol.

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Exercise

The final protonation yields ...

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... the thermodynamically more stable, conjugated 1,3-cyclohexadiene.

... the 1,4-cyclohexadiene and not the thermodynamically more stable, conjugated 1,3-cyclohexadiene.

... only the thermodynamically more stable, conjugated 1,4-cyclohexadiene.

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Exercise

In case of substituted benzene derivatives, the substituents control the position of the 1,4-double bonds of the product of a Birch reduction. Select the right statements:

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Electron-donating substituents (e.g. alkyoxy or alkyl groups) are located at a triple bond of the product.

Electron-donating substituents (e.g. alkyoxy or alkyl groups) are located at a double bond of the product.

Electron-withdrawing substituents, such as carboxyl and amide groups, are located at one of the sp3-hybridized carbons in the ring of the product.

Electron-withdrawing substituents, such as carboxyl and amide groups, are located at one of the sp2-hybridized carbons in the ring of the product.

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