Formation of an Alkene by Alcohol Dehydration
Balanced Chemical Equation for the Main Reaction
The acid-catalyzed dehydration of secondary and tertiary alcohols involves non-isolable carbocation intermediaries. In the first step of the of the reaction mechanism below, a phosphoric acid catalyst adds a proton to the oxygen atom of the alcohol to form an oxonium ion. The OH is converted to a better leaving group as the positive charge on the oxygen weakens the carbon-oxygen bond. The carbon-oxygen bond breaks during heating giving water an unstable carbocation. Without a nucleophile, the carbocation intermediate loses a proton from a carbon and forms a carbon-carbon double bond from the carbon-hydrogen electrons. The formation of a carbocation intermediate from the loss of a leaving group classifies this as an Elimination Reaction. Table of Reagents
Pertinent Safety Information
The phosphoric acid in this lab is a corrosive acid. Caution must be used to not get the acid in contact with skin or clothes. Gloves must be worn while pouring it. If there is a spill, wash the area with large amounts of water. Cyclohexanol, and especially cyclohexene, are very flammable liquids. Do not use an open flame as a source of heat with these substances. Outline of the Procedure
Add 20 mL of cyclohexanol and 5 mL of 85% phosphoric acid to a 50-mL round-bottom flask along with 1 or two boiling chips Swirl the flask gently to thoroughly mix the layers and incorporate this into a simple distillation column Heat the flask until distillation begins and slowly distills the mixture, controlling the heating so that the temperature of the distillation vapor does not go above 100-105 degrees C. Cool the receiver in an ice bath to reduce the loss of distillate to evaporation Continue the distillation until only 5-10 mL of high-boiling residue is left in the flask Allow the flask to cool and...
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