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CH118K Dehydration - Normally we need to worry about other...

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Michael Tam Chris Johnson 5/1/2011 Dehydration Dehydration is a reversible reaction in which acid catalyzes the formation of an alkene from an alcohol. In order to shift the equilibrium toward the products, we separate the products from the reactants via fractional distillation. This is possible because the products always have a lower boiling point than the alcohol group. This is an example of LeChatlier’s principle. In this reaction, we need to worry about the competing substitution reaction. If we choose HCl as our catalyst, the Cl ion could trap the carbocation and yield an undesired byproduct, as well as consuming the acid catalyst. We therefore use sulfuric acid, which is a poor nucleophile. This way the product formed by the substitution reaction reionizes under the reaction conditions.
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Unformatted text preview: Normally, we need to worry about other products forming. We can’t do anything about it normally, but because we are dehydrating cyclohexanol, the only structure that can result is cyclohexene. Note: the expected yield is 2.05mL, not 2.5mL because the molecular weights of the product are lower than the molecular weight of the reactant. Bromine test was positive, the Bromine was consumed, and hence there was unsaturation. IR had no OH peaks; the finger print region is identical to the one on p.363. Lab was ultimately, successful. Sources of error include presence of the low BP azeotrope, which we can’t separate because it has a lower BP than both compounds, not drying the IR thouroughly, and contamination (any alcohols in the glassware will produce an OH peak in the IR....
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