The Acid Test for Dehydration

This year we varied the catalytic acid used to dehydrate 2-methylcyclohexanol. The standard procedure for us has used commercial concentrated phosphoric acid 85% (w/w) aqueous solution which has a molarity of about 14.8 M. 2.5 mL acid (0.037 moles) is combined with 11.5 g (0.10 moles) 2-methyl-1-cyclohexanol. This year we also tried sulfuric acid and methanesulfonic acid. Sulfuric acid is sold commercially as 98% solution which has a molarity of 18.4 M. For the first lab both the sulfuric acid and methanesulfonic acid were diluted to achieve the same molarity as the phosphoric acid. For subsequent labs both the sulfuric acid and methanesulfonic acid were diluted 1:1 (v/v) with water. The pKas of the three acids vary considerably. sulfuric acid, methanesulfonic acid, and phosphoric acid have pKa values (first ionization) of  -4, -2, and 1.9 respectively. I was concerned that the strong acids would degrade the substrate more than dehydration but that did not seem to occur. In fact, the results were surprisingly similar. The table below suggests that only 14.7 M H2SO4 creates a significant change in product ratios. The biggest difference is that the sulfuric and methanesulfonic at 14.8 M resulted in very little unreacted alcohol being distilled over. With phosphoric acid, about 15% of the recovered distillate is unreacted alcohol.

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  1. […] fourth synthesis project started with the dehydration of cyclohexanol. We had performed the dehydration of 2-methylcyclohexanol during the Fall semester. The resulting cyclohexene was oxidized by hydrogen peroxide in the […]

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