Wikipedia mentions that alkynes can be produced from geminal dihalides through the Wurtz reaction. However, I am unsure of the reaction mechanism for this reaction. I have not been able to find any sources which show the mechanism for this reaction. Does anyone have any idea how the reaction is likely to occur?
@tox123: You have generated two radicals on the same carbon by a two-electron reductive α-elimination (1 --> 2). So far, so good. In other words, you have created a carbene (2). Carbenes can rearrange to alkenes, NOT alkynes. Think of the two orbitals on the carbenoid carbon as one bearing two-electrons (negative) and the other one vacant (positive) (3). When a vicinal hydride migrates to the vacant orbital, one has a negative charge on one carbon and a positive charge on the vicinal carbon (4). Structures 4 and 5 are identical. If dichloride 1 is treated with base, one gets a double elimination to an alkyne. There is no change in oxidation state. Therefore, dichloride 1 cannot give an alkyne via a 2-electron reduction.
So, as wikipedia says:
The Wurtz reaction occurs through a free radical mechanism.
One of these electrons binds with an already paired electron on the other carbon forming an alkene with free electrons on both sides of the double bond (this should also form a free hydrogen atom, which is not pictured below):
These electrons then pair with one another to form an alkyne (freeing another hydrogen atom, which will react with the first to form hydrogen gas):
As a disclaimer, I don't know if this is the exact mechanism this is just an educated guess on my part, but because the Wurtz reaction has a free radical mechanism, this seems the most obvious way of this occurring.