I noted briefly in discussing why Birch reduction of benzene gives 1,4-cyclohexadiene
(diagram below) that the geometry of the end-stage pentadienyl anion was distorted in the presence of the sodium cation to favour this product. This distortion actually has some pedagogic value, and so I elaborate this here.
In a time when large (molecules) are considered beautiful (or the corollary that beauty must be big), it is good to reflect that small molecules may teach us something as well. Take ethane. Is there anything left which has not been said about it already? Well, consider the reaction below, in which two hydrogen atoms mutually hop from one carbon to the other.
The mechanism of the Birch reduction. Sequel to benzene reduction. is licensed by Henry Rzepa under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.