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According to my Organic Chemistry Textbook:

Benzene can be reduced to 1,4-cyclohexadiene by treating it with an alkali metal (sodium, lithium, or potassium) in a mixture of liquid ammonia and an alcohol. This reaction is called the Birch reduction.

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Why the conjugated 1,3-cyclohexadiene is not formed in this reaction (as conjugated molecules are more stable)?

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According to the article on Birch Reduction on the Organic Chemistry portal:

The question of why the 1,3-diene is not formed, even though it would be more stable through conjugation, can be rationalized with a simple mnemonic. When viewed in valence bond terms, electron-electron repulsions in the radical anion will preferentially have the nonbonding electrons separated as much as possible, in a 1,4-relationship.

http://www.organic-chemistry.org/namedreactions/birch-reduction.shtm

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