Watered Down Dehydration

We are continuing our investigation of the acid catalyzed dehydration of 2-methyl-1-cyclohexanol. This year we compared diluted phosphoric acid to the concentrated phosphoric acid (48% w/w aqueous solution) that we have been routinely using. The procedure calls for 11.5 grams of alcohol and 2.5 mL of concentration H3PO4 which may be a large excess of this purported “catalyst.” We tried 1.5 mL of H3PO4 with 1 mL of water and 0.5 mL of acid with 2.0 mL of water. GC FID analysis of the alkene products showed that there was little difference between the concentrated acid and the dilutions in the product distribution. In particular the first dilution seemed to behave very much like the concentrated acid in the way the reaction occurred and in terms of yield. The second dilution did look different than the other in two in that the reaction mixture had thick vapors at the beginning of boiling and the boiling action tended to foam and bump a lot. The yields of alkene certainly seemed lower for the second dilution. What seems to happen is that unreacted alcohol is distilled over along with water and alkenes. Overall, the amount of alcohol in the distillate seems to depend somewhat on how fast the distillation occurs and is not directly proportional to the amount of acid used. This reaction is amazing stable in terms of product distribution.


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