The Birch Reduction:
This extremely useful reaction uses the alkali metal/ammonia solution again to partially reduce aromatic rings. The basic reaction scheme is as follows;
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However, the reaction becomes more interesting when substituents on the ring are involved – it seems that EWGs and EDGs cause a different arrangement of the final two double bonds:
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The key to understanding this is to look at where the anion would rather go (where it will most stabilised) in the first step – there are two choices;
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In scheme 1, the anion has to lie adjacent to the substituent, so clearly this scheme will be favoured if X can stabilise an anion i.e. if X is an EWG. In scheme 2 the anion does not have to lie adjacent to the substituent X, so clearly this scheme will be favoured if X would destabilise an anion adjacent to it i.e. if X is an EDG. So for example: