Author Topic: Birch Mechanism -bbman  (Read 4449 times)

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Birch Mechanism -bbman
« on: April 19, 2000, 09:45:00 AM »

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Author  Topic:   Birch Mechanism 
Junior Member   posted 12-14-1999 12:26 PM          
I'm trying to understand the mechanism of the Brich reduction of PSE to Meth. I found a regerence which suggests that the mechanism procedds as an elimination with a double bond formed between the ring and the alpha carbon, followed by hydrogenation across the double bond. In this reaction Ammonium CHloride is added to quench the reaction. My question is if this is the mechanism, can one get a di-phenyl compound formed due to the free electron in the para position to the alpha carbon?
Here is a description of the mechanism:
" The dissolved lithium metal gives up an electron to the ring and forms a radical anion. The hydroxyl group is eliminated, forming a double bond between the ring and the alpha carbon. A second atopm of Lihium gives up an electron to the radical, and the resulting carbanion is protonated by the ammonium chloride. Through keto-enol tauterism, methamphetamine is formed as the more stable species".

rev drone
Member   posted 12-15-1999 07:49 AM          
Not likely. The free radical intermediate is rather short-lived, with the more stable carbanion being quickly formed in the electron-rich medium of the dissolved metal solution.
Or something like that.

-the good reverend drone

rev drone
Member   posted 12-15-1999 07:57 AM          
Oh, I forgot to add the other reason why: I think you're imagining two molar equivalents of the short-lived free radical intermediates pairing up to form a sigma bond between them. However, one free radical attacking another free radical with the same electron properties isn't going to be very likely -- I've honestly never seen an example of any reaction mechanism in organic chemistry fitting that description.
-the good reverend drone

Niels Bohr
Member   posted 12-15-1999 12:29 PM          
I thought that the Birch reaction was one where nobody has really figured out the mechanism of how it takes place? I think that is what my old O-Chem instructor told me.