Birch reduction reduces aromatic compounds to isolated dienes. Substituents attached to the ring can affect the orientation of the double bonds.
Overview:
How exactly does Birch reduction work? Good news! It uses reagents very similar to those in a reaction you’ve already learned: dissolving metal reduction (AKA metal-ammonia reduction of alkynes). Before we cover the effects substituents have, let’s cover the basics. Birch reduction uses two equivalents of lithium or sodium metal, two equivalents an alcohol, and liquid ammonia. The only major difference between this reagent set and dissolving metal reduction is the presence of alcohol.
Mechanism:
The mechanism will look very similar to that of dissolving metal reduction, so strap in! The first step is sodium’s (or lithium’s) donation of an electron to the benzene, and that forms the radical anion. The resulting lone pair then pulls a hydrogen from the alcohol, resulting in a conjugated radical. Another equivalent of sodium donates an electron, and then the resulting lone pair pulls a hydrogen from another equivalent of alcohol. This mechanism produces an isolated diene, forgoing the more stable conjugated diene.
Birch reduction mechanism
Substituent Effects:
That’s all fine and dandy, but what happens when there are substituents on the benzene? Remember that benzene substituents can be divided into two categories: electron-donating groups (EDGs) and electron-withdrawing groups (EWGs). The methoxy group on anisole would be an EDG, and the chlorine on chlorobenzene would be an EWG. EDGs and EWGs will orient the double bonds differently. EDGs attach themselves to the diene, and EWGs
Birch reduction generic substituents
Above is the general reaction scheme with generic substituents. Below is the reaction scheme with toluene, aniline, nitrobenzene, and acetophenone.
Birch reduction specific examples
So, that’s about it! Good luck studying. Remember that I’ve got tons of videos on this topic and everything else you need in Organic Chemistry.
