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Introduction to Benzene's Chemistry

Introduction to Benzene's Chemistry: Birch Reduction

Catalytic hydrogenation of benzene is either practically impossible or extremely complicated to acquire. However, if benzene is treated with sodium that has been dissolved in liquid ammonia in the presence of some alcohol, the reduction product 1,4-cyclohexadiene is obtained. In contrast to catalytic hydrogenations, only partial hydrogenation and not complete hydrogenation, which would yield the corresponding alkane, occurs in this case. The reaction, called Birch reduction, has been named after the Australian chemist Arthur Birch. The Birch reduction's mechanism resembles the mechanism of alkynes' reaction with sodium in liquid ammonia.

  • The transfer of an electron from sodium or a so-called solvated electron, respectively, to an antibonding π molecular orbital of benzene yields a resonance-stabilized radical anion.
  • Protonation by an alcohol leads to a resonance-stabilized radical.
  • An additional electron transfer results in a resonance-stabilized anion.
  • This is then converted into the final product through protonation.
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