The fully delocalized π electron system of the benzene ring remains intact during electrophilic aromatic substitution reactions. However, in the Birch reduction, this is not the case. In the Birch reduction, benzene, in the presence of sodium metal in liquid ammonia and methyl alcohol, produces a nonconjugated diene system. This reaction provides a convenient method for making a wide variety of useful cyclic dienes.
The production of the less stable nonconjugated diene instead of the more stable conjugated diene occurs because the reaction is kinetically controlled rather than thermodynamically controlled.
In general, reactions that aren't easily reversible are kinetically controlled because equilibrium is rarely established. In kinetically controlled reactions, the product with the lowest‐energy transition state predominates. Reactions that are easily reversible are thermody namically controlled, unless something occurs that prevents equilibrium. In thermodynamically controlled reactions, the lowest‐energy product predominates.