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Halogenation of Alkanes

Radical Bromination in Benzylic Position

Since the whole aromatic system takes part in the resonance stabilization, benzyl radicals are even more stabilized than allyl radicals. The diagram illustrates the spin density distribution (illustration) in a benzyl radical and shows that the unpaired electron is distributed among the benzylic and aromatic carbon atoms in ortho and para position.

Fig.1
Resonance stabilization of a benzylic system.
Fig.2
Distribution of the spin density in a benzylic system.

The high stability of benzyl radicals can be shown clearly in compounds in which the benzylic carbon carries more than one phenyl group. The triphenylmethyl radical, for example, is so stable that, in a solution at room temperature, it is at equilibrium with a dimer even if the radical makes up only two percent of the equilibrium mixture.

The mechanism of the bromination of alkylated aromatic compounds in benzylic position resembles the allylic bromination of alkenes. The benzyl radical is formed by abstracting a benzylic hydrogen atom with the help of a bromine radical. Subsequently, the reaction between bromine and the benzyl radical yields not only the benzyl bromide, but a new bromine radical as well. This bromine radical then abstracts one benzylic hydrogen from another benzyl radical, so that the chain reaction may proceed. The bromine radicals required in the initial stage of the reaction, as well as after chain terminations, are delivered by the reaction between HBr and NBS (N-bromosuccinimide).

Fig.3
Mechanism of the benzylic bromination with NBS.

Benzylic bromination, which occurs under illumination and boiling heat with the possible aid of a radical former, such as NBS, represents a radical mechanism. In contrast, the bromination of the aromatic nucleus, which requires no boiling heat but a lewis acid catalyst instead, follows an ionic mechanism.

Fig.4
Radical side-chain bromination and ionic bromination of the aromatic nucleus.

Exercises: Radical bromination in benzylic position

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