In my organic lab this week my students did the classic elimination reaction of cyclohexanol to form cyclohexene. The reaction is easy.
The cyclohexanol is mixed with some phosphoric acid and refluxed for a bit and the cyclohexene product is distilled. A second distillation with a toluene chaser is used to purify the product. I analyzed the products with GC-MS. The GC-MS showed what one would expect a 2nd year Organic lab student to produce: cyclohexene, toluene, and unreacted cyclohexanol.
However, in some product mixtures I also saw a significant amount of 1-methylcyclopentene. The reaction of that is as follows:
My question is, "How does that form?"
Here's a possible mechanism that I think is highly unlikely due to the primary carbocation.
This mechanism moves the atoms to the correct final position, but I find the ring contraction to form a primary carbocation unlikely. Maybe I'm wrong.
Other things: I check the starting material. It was 99.8% cyclohexanol. In the reaction dicyclohexyl ether forms. I'm not sure if this could assist in the rearrangement.