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Exercises: Radical bromination in allylic position

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Fig.1

Exercise

Which reaction conditions must be applied to prevent the formation of vicinal dihalogenides in the reaction between alkenes and halogens? Which reaction occurs under these conditions instead?

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In order to prevent the formation of vicinal dihalogenides in the reaction between alkenes and halogens, the halogen must be applied in high concentration.

In order to prevent the formation of vicinal dihalogenides in the reaction between alkenes and halogens, the halogen must be applied in very low concentration.

Radical substitution is promoted by a low halogen concentration. With a low halogen concentration, radical substitution exceeds the polar addition to the double bond, especially when it may occur in allylic or benzylic position.

Radical substitution is promoted by a high halogen concentration. With a high halogen concentration, radical substitution exceeds the polar addition to the double bond, especially when it may occur in allylic or benzylic position.

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Exercise

Which reagent is commonly used for allylic brominations?

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A suspension of N-bromosuccinimide (NBS) in chloroform.

A suspension of N-borosuccinimide (NBS) in carbon tetrachloride.

A suspension of N-bromosuccinimide (NBS) in carbon tetrachloride.

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Exercise

Which of the following statements are correct?

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Usually, since the chlorine radical is less reactive than the bromine radical, radical brominations are less selective in comparison to radical chlorinations.

The selectivity of the allylic halogenation depends not only on the stability of the intermediately formed radical, but also on the reactivity of the halogen.

Usually, since the chlorine radical is more reactive than the bromine radical, radical brominations are more selective in comparison to radical chlorinations.

The selectivity of the allylic halogenation depends only on the stability of the intermediately formed radical and not on the reactivity of the halogen.

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