December 03, 2021, 02:28:18 PM
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Topic: A classic student lab: Dehydration of methylcyclohexanols question  (Read 2440 times)

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Offline Cesium-137

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Last week my lab section began the classic undergrad experiment, dehydrating methylcyclohexanols. For the sake of simplicity, everyone dehydrated 4-methylcyclohexanol to produce 4-methylcyclohexene. The procedure is straight forward, add your alcohol and 85% phosphoric acid to a roundbottom, set up for fractional distillation, and heat. As the products are produced (water and alkene) they distill over, shifting equilibrium to make more products. The boiling point of the alkene is 103-105 C and an interesting question came to me while observing students run their reaction. Nearly every student found that they began collecting distillate at around 80C and the temperature never really rose much higher than that. When students went to purify their alkene using a simple distillation (following their initial reaction/distillation) it distilled over at 103-105. I was baffled, and the best explanation I could come up with was one involving the vapor pressure in the reaction flask being high enough to push the alkene over at 80C, even though that is well below its BP. The alkene comes over at its normal BP in the following distillation because there is no reaction occuring and thus the vapor pressure much be lower for an unknown reason to me. Does this make sense? Does anyone have experience with this particular lab, or a similar one? Im sure 4-methylcyclohexene doesnt azeotrope with water so Im stumped on this one! Is my explanation along the right track?? Thanks for the expertise!

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