Posted by: Mark Foreman | February 11, 2016

Elimination from an alkyl halide

Dear Reader,

Something unexpected happened today when I was teaching, a student instead of drawing out the E2 mechanism did something else. Now normally the base attacks the hydrogen on the carbon next to the one bearing the leaving group. Here is the correct mechanism for you.

E2 reaction of a base with 1,1,1-trichloroethane ("methyl chloroform")

E2 reaction of a base with 1,1,1-trichloroethane (“methyl chloroform”)

Now instead of doing the easy E2 this student had drawn the formation of a carbene according to the following mechanism. Now the carbene synthesis is not a common reaction, I suspect that this is because the carbene has a much higher energy than that of the alkene. So I think that if a choice exists between a molecule doing an alkene synthesis by a E2 and a carbene synthesis then it is very likely that the alkene synthesis will occur instead of the carbene synthesis. Here for your delight is a carbene synthesis from chloroform.

Carbene synthesis from chloroform

Carbene synthesis from chloroform

It might look good but keep in mind that carbene formation is quite a rare event.

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