Tuesday, February 5, 2008

Explain this one to me...

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.

Any ideas?


Jordan said...

Could a phosphate ester be involved?

Liquidcarbon said...

Makes sense, the conditions are quite harsh and the ultimate product is thermodynamically more stable than cyclohexene.

Though, how much of methylcyclopentene was there?

Anonymous said...

That's the same mechanism that I thought of when I saw your question. I'm not sure how a phosphate ester could do it.
Great blog-chemistry and brewing are my favorites too.

Russ said...

Neat observation. I would check scifinder to see if it's been observed before, and if not, it'd be a cool project to follow up on. Maybe even get undergrads to do it. Get a few of the keener students to do the experiments with various substituted cyclohexanols and/or in deuterated H3PO4 and see what comes out.

Chemgeek said...

The ratio of cyclohexene to the cyclopentene was about 20:1 according to GC.

liquidcarbon may be right. 100°C is rather harsh.

My students will get their GC-MS reports back today. It will make for an interesting discussion. Chemistry in the book is always nice and clean. Any bench chemist will tell you otherwise.

Liquidcarbon said...

Hell yeah! Do they still teach that you can make alkynes by dehydrohalogenation of vinyl halides with "base"?

It's KNH2 @ 160'C in the real world - tonight we die in heck!

Liquidcarbon said...

Check refractive index:

cyclohexene 1.4465
1-methylcyclopentene 1.4347

Not sure whether refractometer can catch this difference... check pure cyclohexene as well. :)

milkshake said...

Carbocation insetrs into C-C bond instead of C-H bond.

The mechanism is correct, you see lots of dirty things happening with the carbocation chemistry. The fact that the primary carbocation is uphil does not mean that it will not happen if you push long enough - especially when the following step, H-shift to produce the tertiary cation is practically instantenious. You can make ethylene by dehydratation of ethanol with sulfuric acid under similar conditions.

the other anonymous said...

What temperature is injection port on your GC? Sparky in my lab in g-skool found that all kinds of fun chemistry can occur if the injector is too hot.

I like to think of it as the chemical equivalent of Vegas.

Anonymous said...

Could it be loss of water in the ionizing chamber of the MS and not actually a component of the product mixture?

Chemgeek said...

I don't think anything is happening in the GC or MS (though I can't be 100% certain). Cyclohexanol and cyclohexene standards come out clean on the GC-MS under identical conditions.

It isn't outside of the realm of possibilities that stuff is happening in the GC-MS somewhere.

rangermonk said...

Did you analyze the toluene? Ultra pure reagents/solvents are a must for GC/MS analysis. Many solvents while clean still have trace impurities from the manufacturing process. Are you doing direct injection? When was injection port maintenance last done? Reaction is most likely not in GC/MS, unless system is dirty. Stopped to read cause you homebrew. RDWHAHB

Chemgeek said...


good point. I didn't check the toluene.

The injector is "probably" clean. I put a new liner and septa etc.. in about two months ago. No guarantees though.