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I've done a student experiment using cyclohexanol to prepare cyclohexene.

We mix $10.5~\mathrm{g}$ cyclohexanol and $5~\mathrm{mL}$ $85\%$ $\ce{H3PO4}$ in a $50~\mathrm{mL}$ flask, then distill while heating.

I want to know during this process what kind of side reaction happens because I notice there is something light yellow in the flask. After reaction, the liquid remaining in the flask is red and upon it there is something dark green and muddy.

I have a guess that there is some phenol in the cyclohexanol (since cyclohexanol may be prepared from phenol), and that during reaction the phenol oxidized, producing 1,2-benzoquinone (which is red) and 1,4-benzoquinone (which is yellow). Is this idea reasonable? If it is, why does the red color appear later?

And about the dark green thing: can it be produced by some kind of carbonized organic matter? I have been told that if the $\ce{H3PO4}$ and cyclohexanol were not well mixed, there would be some cyclohexanol carbonization.

(Can I ask one more question?) We are required to compare the chemical properties of cyclohexene and cyclohexane. So we reacted them with $\ce{Br2}/\ce{CCl4}$ respectively, under light or covered by aluminium foil. Should it be different when cyclohexene reacts under light or covered by aluminium foil? I notice the result is slightly different; the reactions in the light become a little opaque.

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  • $\begingroup$ Hmm, it's strange - have done everything right? $\endgroup$
    – Mithoron
    May 7, 2015 at 15:12
  • $\begingroup$ @Mithoron I don't know... but other classmates also get something like this. However, most of them only get something light yellow. Only some get red. I know our reagent may not be that pure - they want to save cost. $\endgroup$
    – Asydot
    May 7, 2015 at 15:29
  • $\begingroup$ Light yellow should be ok - cyclohexene - it's not purified so it's slightly colored. Othrwise you probably did sth wrong. $\endgroup$
    – Mithoron
    May 7, 2015 at 15:40
  • $\begingroup$ Cyclohexene gets probably polymerized $\endgroup$
    – Maurice
    Oct 18, 2021 at 20:07

1 Answer 1

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The acid-catalyzed dehydration of alcohols involves a carbocation intermediate, which can act as a Brønsted acid; the loss of a proton yields the desired cyclohexene. Furthermore, the carbocation may undergo other side reactions. In particular, the carbocation may be subject to a reaction with other molecules to give substitution products; or the carbocation may undergo carbocation rearrangement.

The coloured residue of the distillation is probably the result of cationic polymerization of cyclohexene. This reaction in combination with rearrangements may yield various oligomers.

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